1

The influence of reproductive timing on white spruce seed escape

and red squirrel hoarding

Devan W. Archibald

Department of Natural Resource Sciences,

McGill University, Macdonald Campus,

Montréal, Québec, Canada

June 2011

A thesis submitted to McGill University in partial fulfillment

of the requirements of the degree of Master of Science

© Devan W. Archibald 2011

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Abstract

This thesis evaluates how reproductive timing influences red squirrel hoarding of white

spruce cones, from the perspective of both the trees and the squirrels. This was

accomplished for white spruce by assessing the degree of intra-annual reproductive

synchrony exhibited by individual trees relative to others in the population and the

amount of cones (i.e. seeds) escaping red squirrel predation. In two years with vastly

different cone production at the population level, individual white spruce trees exhibiting

higher levels of intra-annual reproductive synchrony had enhanced seed escape from red

squirrels, leading to positive directional selection on this trait in the lower cone year. In

red squirrels, we used behavioural observations of radio-collared individuals to assess

how variation in the temporal separation of breeding and hoarding seasons, across four-

years of varying cone production, affects cone hoarding behaviour. Hoarding behaviour

was more affected by cone levels than reproductive timing and under high cone levels

both activities were successfully combined. However, males and females used different

hoarding strategies that were consistent with differences in the timing of reproductive

demands, indicating that although overall hoarding behaviour was driven by resource

levels, the timing of reproduction may be a factor in gender differences. The seasonal

scheduling of reproduction in white spruce trees appears to be an important component of

seed escape from red squirrels, and although the timing of reproduction may promote

gender differences in hoarding behaviour of red squirrels, it is less important than cone

production to overall hoarding behaviour, allowing red squirrels flexibility in their

reproductive timing relative to hoarding.

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Résumé

Cette thèse évalue comment la phénologie reproductive influence la collecte de cône

d‟épinette blanche par l‟écureuil roux, de la perspective de l‟arbre et de l‟écureuil. Chez

l‟épinette blanche, nous avons évalué le degré de synchronie reproductive intra-annuelle

de chaque arbre relatif à la population et le nombre de cône (graines) qui échappe à la

collecte des écureuils roux. Pour deux années avec des productions de cône très

différentes au niveau populationnel, les épinettes blanches davantage synchronisées intra-

annuellement dans leur phénologie reproductive ont davantage de graines qui échappent

aux écureuils, menant vers une sélection directionnelle positive sur ce trait lors d‟une

année de faible production de cône. Chez l‟écureuil roux, au cours de quatre années avec

des productions de cône très différentes, nous avons utilisé des observations

comportementales pour évaluer comment la variation dans la séparation temporelle entre

les saisons de reproduction et de la collecte affecte le comportement de collecte.

L‟activité de collecte de cône est affectée davantage par le niveau de production annuelle

de cône que par la phénologie de reproduction de l‟écureuil et, avec un haut niveau de

production de cône, les deux activités peuvent être combinées avec succès. Cependant,

les males et les femelles utilisent des stratégies différentes selon leur différente

phénologie de demande reproductive, indiquant que malgré que le niveau de ressource

soit déterminant sur l‟activité de collecte, la phénologie de reproduction pourrait être un

facteur dans la différence entre les sexes. La phénologie de reproduction de l‟épinette

blanche semble être une composante importante de la survie des graines face aux

écureuils roux, et malgré que la phénologie de reproduction puisse promouvoir une

différence dans l‟activité de collecte entre les sexes, cela est moins important que la

production de cône pour l‟ensemble de l‟activité de collecte, ce qui permet à l‟écureuil

roux d‟être flexible dans sa phénologie reproductive par rapport à la collecte de cône.

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Contribution of authors

This thesis is presented as two chapters, each intended for publication. For each

chapter the candidate was responsible for developing the research questions,

experimental design, field work, data management and analysis, interpretation and

writing.

Chapter one is co-authored by Andrew McAdam, Stan Boutin, Quinn Fletcher,

and Murray Humphries. Andrew McAdam provided logistical assistance with the

fieldwork, and was responsible for white spruce data collection in 2010. He also

provided input on the study design and comments on the manuscript. Stan Boutin

provided logistical assistance with the field work, provided input on the study design and

early versions of the manuscript. Quinn Fletcher provided input on the study design and

comments on the manuscript. Murray Humphries provided input on the study design,

assistance with logistics, and comments on the manuscript.

Chapter two is co-authored by Andrew McAdam, Stan Boutin, Quinn Fletcher,

and Murray Humphries. Andrew McAdam provided logistical assistance with the

fieldwork, input on the study design and comments on the manuscript. Stan Boutin

provided logistical assistance with the field work, provided input on the study design and

comments on the manuscript. Quinn Fletcher provided input on the study design,

assistance with data management for three of the four years of red squirrel hoarding

behaviour data, and comments on the manuscript. Murray Humphries provided input on

the study design, assistance with logistics, and comments on the manuscript.

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Acknowledgements

I would like to thank my supervisor, Murray Humphries for his patience, support,

guidance, consistent availability and positive attitude. This work received input from the

Humphries lab, and numerous lab-mates helped me gain the skill-set required to complete

this thesis and provided the friendship to flourish (Amy, Emily, Elise, Guillaume,

Jeremy, Jason, Karine, Marianne, Nicholas, Paul, Quinn, and Sébastien). Specifically,

Guillaume was always there to answer my statistical and IT questions, Manuelle was

always willing to help and always knew the answer, Quinn taught me how to use excel,

access, and R after I realized I knew nothing, and was always there for a squirrel chat.

Other members of the Natural Resource Sciences department at Macdonald Campus

provided feedback in the initial stages, as well as more final versions, and the members of

Journal Club helped keep my mind stimulated and thinking about my research from other

perspectives.

Field work was in collaboration with the Kluane Red Squirrel Project (KRSP) and

thus I benefited from the wisdom of Stan Boutin and Andrew McAdam who were

involved in this research from start to finish, and who provided field resources and

assistance which was greatly aided by the coordination of Ainsley Sykes. I benefited

from the collective squirrel lore of all „squirrelers‟ involved with KRSP and thank

everyone for their hard work. In particular my fellow graduate student Ben was

instrumental in assisting me with research design questions on the fly in the field, and

provided excellent comradery and advice during my stint as field crew supervisor.

Meghan and Quinn made learning how to conduct metabolic measurements on red

squirrels fun. I owe an enormous thank you to everyone involved in focal observations,

Emily, Julia, Manuelle, and last but definitely not least Kristin who had squirrels in view

for over 147 hours and likely spent more time entering data! An additional thank you is

required for Tasha for counting cones in 2010 and all the other volunteers involved in this

exciting endeavour. I am grateful for to the Yukon Government and the Alsek

Renewable Resource Council for their permission to conduct field work in the Yukon,

and this project was funded by an NSERC Discovery grant to MMH, as well as a NSERC

CGS scholarship and NSTP grant to DWA.

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In addition to her assistance in the field, my wife Kristin deserves a second thank

you for her patience in never tiring of hearing my squirrel theories. This thesis greatly

benefited from her knowledge of red squirrels, and her moral support in every stage of

the process.

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Table of Contents

Abstract ...................................................................................................................2

Résumé ....................................................................................................................3

Contribution of authors .........................................................................................4

Acknowledgements ................................................................................................5

Table of Contents ...................................................................................................7

List of Tables ..........................................................................................................9

List of Figures .......................................................................................................10

General Introduction and Literature Review ...................................................12

Granivory ...........................................................................................................12

Mast seeding ......................................................................................................13

Food hoarding ....................................................................................................14

North American red squirrels ............................................................................15

White spruce ......................................................................................................16

Kluane Red Squirrel Project ..............................................................................17

Research Objectives ...........................................................................................18

Literature Cited ..................................................................................................20

Chapter 1: Swamping seed predators in number and time: within-season synchrony

of a masting conifer enhances seed escape .........................................................26

Abstract ..............................................................................................................27

Introduction ........................................................................................................28

Methods .............................................................................................................30

Study area .......................................................................................................30

White spruce cone counts ..............................................................................30

White spruce intra-annual reproductive synchrony .......................................31

Red squirrel seed predation and hoarding behavioral observations ...............32

Statistical analysis ..........................................................................................33

Results ................................................................................................................34

Discussion ..........................................................................................................35

Literature Cited ..................................................................................................40

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Tables .................................................................................................................47

Figures ...............................................................................................................48

Connecting Statement ..........................................................................................49

Chapter 2: Reproductive and resource constraints on food hoarding in male and

female red squirrels. ............................................................................................50

Abstract ..............................................................................................................51

Introduction ........................................................................................................52

Methods .............................................................................................................54

Study area .......................................................................................................54

Hoarding behavioral observations .................................................................55

Estimation of total number of cones clipped, hoarded, and the propensity to larder-

hoard ..............................................................................................................56

Reproductive timing .......................................................................................56

Statistical analysis ..........................................................................................57

Results ................................................................................................................59

Reproductive completion and resource levels ...............................................59

Total number of cones clipped and hoarded ..................................................59

Propensity to larder-hoard ..............................................................................60

Hoarding time allocation during concurrent reproductive activity ................60

Discussion ..........................................................................................................62

Literature Cited ..................................................................................................66

Tables .................................................................................................................71

Figures ...............................................................................................................72

General Conclusions ............................................................................................75

Literature Cited ..................................................................................................77

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List of Tables

General Introduction and Literature Review

Table 1. Hypotheses that have been suggested to explain mast seeding or fruiting,

adapted from Kelly (1994)……………………………………………………………25

Chapter 1

Table 1. Standardized directional (β‟) and stabilizing/disruptive (γ’) selection gradients

calculated for intra-annual reproductive synchrony and the number of cones produced by

white spruce during a non-mast (2009; n=212) and mast (2010; n=206) year. Separate

models were used to assess linear and non-linear selection gradients within each year.

Bold font indicates significance estimated with jackknife tests (Mitchell-Olds and Shaw

1987). Stabilizing/disruptive selection gradients have been doubled to accurately

represent their strength (Stinchcombe et al. 2008)……………………………………...47

Chapter 2

Table 1. Range and median date of reproductive completion for adult red squirrels

included in the study in 2002, 2003, 2005, and 2010 with white spruce average cone

index (ln transformed cone count; mean ± SE) calculated from annual cone counts

conducted prior to red squirrel harvesting (n = 167-171 trees per year). Days until

hoarding season was calculated by subtracting the median date last reproductively active

from August 16th

, the approximate date when hoarding activity initiates (Fletcher et al.

2010)……………………………………………………………………………………71

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List of Figures

Chapter 1

Figure 1. Seasonal decline in the number of closed cones on white spruce trees resulting

from red squirrel cone clipping activities as well as cone opening. Average number of

closed cones (closed triangles) and average number of open cones (open circles) per tree

in the study (non-mast; n=607, mast; n=292) plotted with the average observed and

modeled red squirrel cone clipping (dashed line) rate per day (non-mast; n=21, mast;

n=9) throughout the autumn of a non-mast (2009) and (2010) mast year. Cone count

rounds occurred over more than one day but are plotted on median date of each round.

Cone number values represent means ± SE and clipping activity values are means…48

Chapter 2

Figure 1. Total number of cones clipped (A), hoarded (B) and the proportion of hoarded

cones that were larder-hoarded (C) by adult male and female red squirrels over four study

years with varying cone availability and separation of reproduction and hoarding. All

models contained significant sex and year effects with different letters indicating

significant differences found between years using post hoc testing. None of the models

contained significant interactions between year and sex. Values are means ± SE…….72

Figure 2. The proportion of time adult female red squirrels spent feeding (A) and in the

nest (B) varied as a quadratic function of days postpartum, but time spent conducting

cone hoarding-related activities (C) varied linearly with days postpartum during autumn

2010, after accounting for the appropriate fit of Julian date (time spent feeding; linear,

time spent in the nest; linear, time spent hoarding; quadratic). Parturition dates ranged

from Jul 12 to Aug 24 (median; Aug 9). Data were analyzed using generalized linear

mixed models with squirrel identity as a random factor, but plotted values are raw data

representing means ± SE for each day postpartum……………………………………..73

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Figure 3. The proportion of time adult female (closed circles) and male (open squares)

red squirrels spent conducting cone hoarding-related activities throughout autumn 2010

varied non-linearly with Julian date and was not significantly different between the sexes.

The dashed grey line indicates the proportion of study females that were yet to pass the

mid-point of lactation (proportion less than 45 days postpartum). Data were analyzed

using a generalized linear mixed model with squirrel identity as a random factor, but

plotted values are raw data representing means ± SE for each day…………………….74

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General Introduction and Literature Review

The global diversity of animals depends critically on resources provided by

plants, the major primary producers energizing the planet (Price 2002). Plant-animal

interactions likely played an important role in shaping the diversity of both groups

(Ehrlich and Raven 1964, Bascompte and Jordano 2007). Plants and animals interact in

numerous ways, both mutualistic (e.g. pollination and seed dispersal) and antagonistic

(e.g. seed predation) (Herrera and Pellmyr 2002). Herbivorous animals consist of

members of a variety of invertebrate and vertebrate groups and consume a variety of

plant parts, with no plant tissues escaping their attention (Crawley 1983).

Granivory

The plant tissue possessing the highest energetic content per gram is seeds

(Robbins 1983). Seeds are the fertilized ovules of flowering plants and consist of an

embryo with food-storage organs surrounded by a protective seed coat (Hulme and

Benkman 2002). Numerous animals have become specialized to feed mainly or

exclusively on seeds and are termed granivores, or seed-predators (Hulme and Benkman

2002). Granivory differs from other forms of herbivory in that this high quality food

source is provided in discrete packets often with low perish-ability, but with the challenge

that it is only available for brief periods of time that can be highly unpredictable

(Crawley 2000).

Seed predators have been identified as having considerable impact on plant seed

populations due to high predation rates, often as high as 50 to 100 percent of available

seed (Crawley 2000). Seed predation is thought to play a pivotal role in the regeneration,

colonization ability, and spatial distribution of plants (Hulme and Benkman 2002).

Additionally, it has been suggested that seed predators act as agents of natural selection

that influence seed traits (Smith 1970, Hulme 1998, Benkman 1999, Benkman and

Parchman 2009) and production strategies (Silvertown 1980, Ruhren and Dudash 1996,

Curran and Leighton 2000). Seed predator-plant interactions are not always antagonistic,

they can also be mutualistic. Seed predators can be dispersal agents when seeds remain

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viable after ingestion and digestion, or if cached seeds are not recovered (Howe and

Smallwood 1982, Jordano 2000, Hulme 2002, Vander Wall et al. 2005).

Within granivores it is useful to differentiate between pre and post-dispersal seed

predation (Crawley 2000, Hulme 2002, Hulme and Benkman 2002). From the plants

perspective, the costs of defence from pre-dispersal seed predation can be borne by the

parent plant, whereas the costs of defence from post-dispersal seed predation lie solely

with the individual seed (Crawley 2000). Pre-dispersal seed predators can exploit

spatially and temporally aggregated resources and can use searching cues based on the

parent plants, whereas post-dispersal seed predators must search for inconspicuous items

scattered in an often cryptic background at lower densities (Crawley 2000). Most pre-

dispersal seed predators are small, sedentary, specialist feeders and often insects. In

contrast, most post-dispersal seed predators are larger, more mobile, and generalist

herbivorous birds or mammals (Crawley 2000, Hulme 2002, Hulme and Benkman 2002).

Mast seeding

Masting is a forestry term that has taken on a rather precise ecological meaning

(Crawley 2000). Mast seeding is the synchronous intermittent production of large seed

crops in perennial plants (Kelly and Sork 2002). The term originates from a German

word for fattening livestock on abundant seed crops, and thus years with high seed

abundance are called mast years (Kelly and Sork 2002). Kelly (1994) separates mast

seeding into three types based mainly on the level of seed production in inter-mast years:

strict masting, with no seeds produced in non-mast years, normal masting, where plants

produce seed in non-mast years but it is markedly lower than mast levels, and putative

masting, where seed crops vary greatly but there is little evidence of switching between

mast and non-mast years and no evidence it is due to anything other than environmental

variation.

There are at least eight hypotheses suggested to explain mast seeding (Table 1,

Kelly 1994). Of these, three are frequently supported as ultimate causes: predator

satiation, wind pollination, and environmental prediction. The environmental prediction

hypothesis proposes that plants can predict which years will be best for seedlings. This

hypothesis lacks empirical support in relation to climate variation, but is well

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documented in mast events triggered by fire (Kelly 1994). After a fire, enhanced nutrient

availability and reduced competition are favorable for seedling establishment. However,

this explanation applies only to fire prone habitats and thus is less general than the other

two explanations (Kelly 1994). The wind pollination hypothesis proposes that masting

increases the chances of successful pollination in wind pollinated plants. This hypothesis

has received support in a range of species and many of the well known masting species

are wind pollinated (Kelly 1994). The most widely known explanation for masting is the

predator satiation hypothesis (Kelly 1994). Large seed crops satiate seed predators and

thus destroy a lower percentage of the crop. This hypothesis has been well supported in

the literature, but depends on the functional response of the seed predators (Kelly 1994).

As with any form of reproductive synchrony, there may be multiple causes in any given

case (Ims 1990), and masting may result from interactions between the abiotic and biotic

environment.

Food hoarding

Vander Wall (1990) defines food hoarding as “the handling of food to conserve it

for future use” (p. 1). Food hoarding animals have the capacity to control the availability

of food in space and time (Vander Wall 1990). The benefits of this strategy may include

improving the chances of surviving a period of food shortage, allowing an animal to

optimize feeding and foraging time with regard to other activities, improving an animal‟s

competitive status when foraging for limited resources, or ensuring a continuous supply

of food to young in the reproductive season (provisioning, under the above definition is

also considered hoarding; Vander Wall 1990). A variety of arthropods, mammals and

birds hoard food, including members with herbivorous, omnivorous and carnivorous diets

(Vander Wall 1990). Familiar examples include nut storage by tree squirrels (Sciurus

sp.), and honey storage by honey bees (Apis mellifera).

Food hoarding animals distribute hoarded food in a variety of ways, ranging from

highly clumped to highly dispersed (Vander Wall 1990). The end points of this spectrum

of cache-dispersion patterns have been termed larder-hoarding, caching all items in a

central location, and scatter-hoarding, caching one to a few items widely spaced

throughout an area (Vander Wall 1990). Larder-hoards are attractive resources for other

15

foragers due to the high concentration of food. As a result, they tend to be placed in

protectable areas where they can be defended by the hoarder, which is usually vigilant

and well equipped to defend its larder-hoard (Vander Wall 1990). Scatter-hoards are

much less attractive and their dispersed nature makes them difficult to defend. Their

protection comes from their inconspicuousness and the hoarder often appears inattentive

(Vander Wall 1990). Inter-specific differences in food hoarding patterns and pilferage

ability have been suggested as mechanisms promoting the coexistence of similar species

(Jenkins and Breck 1998, Leaver and Daly 1998, Price et al. 2000).

Most animals can be categorized as either larder or scatter-hoarders, but some

species use a combination of strategies (Vander Wall 1990, Brodin 2010). Species that

use a combination of larder and scatter-hoarding include some kangaroo rats

(Dipodomys), chipmunks (Tamias), wood-mice (Apodemus), flying squirrels

(Glaucomys), the red fox (Vulpes vulpes), the white footed mouse (Peromyscus

leucopus), the red-headed woodpecker (Melanerpes erythrocephalus) and the red squirrel

(Tamiasciurus hudsonicus) (Vander Wall 1990, Hurly and Lourie 1997). Intra-specific

variation in hoarding behaviour has begun to receive more attention as researchers

attempt to identify factors associated with the use of different food storing strategies

(Daly et al. 1992, Clarke and Kramer 1994, Leaver and Daly 1998, Preston and Jacobs

2001, Leaver 2004, Tsurim and Abramsky 2004, Murray et al. 2006, Jenkins In press).

Factors identified to influence the propensity to larder or scatter-hoard in species that use

mixed strategies include the value of the food (Leaver and Daly 1998, Leaver 2004),

distance from the central-larder that food is encountered (Daly et al. 1992, Tsurim and

Abramsky 2004), the make-up of the competitive environment (Murray et al. 2006), the

pilferage of hoards (Preston and Jacobs 2001), the gender of the hoarder (Jenkins In

press), and the age and reproductive status of the hoarder (Clarke and Kramer 1994).

North American red squirrels

North American red squirrels are small (200-250g), arboreal and diurnal rodents

found throughout the majority of Canada‟s forests and those of the northern United States

of America (USA), and further south along the Rocky Mountains (Steele 1998). In

northern areas of their range red squirrels feed primarily on conifer seeds of white spruce

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(Picea glauca, Boutin et al. 2006). They hoard up to 20,000 conifer cones in autumn,

either burying them in a central-larder or midden, or scattering them throughout their

mutually exclusive territories, appearing to use a deliberate mix of both behaviours

(Smith 1968, Hurly and Lourie 1997, Steele 1998). These food stores aid overwinter

survival and are used to fuel reproduction in spring (Smith 1968, Steele 1998). Large

cone crops in one year lead to earlier parturition dates and higher juvenile growth rates in

the following year (Boutin et al. 2006).

Red squirrels are promiscuous (Lane et al. 2008), and in the northern areas of

their range females usually only attempt one litter per season, except during white spruce

mast years when they may attempt a second litter in late summer just prior to new cone

availability (Boutin et al. 2006). After a gestation period of about 33 days (Steele 1998),

females give birth to about 3 offspring (range; 1-7) usually in late March to early June

(McAdam et al. 2007). Offspring emerge from their nest about 50 days later, and are

weaned at about 70 days postpartum (Humphries and Boutin 1996, McAdam et al. 2007).

Juvenile overwinter survival is largely dependent upon obtaining a territory with a

midden, and occasionally a female will bequeath part or all of her territory to her

offspring (Price 1992, Boutin et al. 1993, Price and Boutin 1993, Berteaux and Boutin

2000, Boutin et al. 2000). Predation on red squirrels is largely by northern goshawk

(Accipiter gentilis), lynx (Lynx canadensis), great-horned owl (Bubo virginianus), and

coyote (Canis lantrans) (Stuart-Smith and Boutin 1995).

White spruce

White spruce has a transcontinental distribution in northern coniferous forests of

North America, being found throughout Canada and regions of the northeastern USA

(Nienstaedt and Zasada 1990). Within these regions it is able to grow under diverse soil

conditions, but it is generally more demanding than other coexisting conifers in achieving

its best development (Nienstaedt and Zasada 1990).

White spruce is monoecious, with female buds generally concentrated in the top

of the crown and the male buds in the middle to lower crown (Eis and Inkster 1972,

Nienstaedt and Zasada 1990). Reproductive buds are differentiated at the time annual

shoot growth ceases, the year before flowering and seed dispersal (Owens and Molder

17

1977). Female receptivity and pollen shedding occur at the same time over a three to five

day period generally in May, June or July, varying with region and climate (Owens and

Molder 1979, Nienstaedt and Zasada 1990). White spruce are wind pollinated and

adverse weather such as frost or rain can severely inhibit promising seed crops

(Nienstaedt and Zasada 1990). Self pollination can occur, but as a consequence viable

seed set is greatly reduced (Fowler and Park 1983, Connell et al. 2006).

Fertilization occurs about three weeks after pollination and cones attain maximum

water content and size in late June or July (Nienstaedt and Zasada 1990). However, the

majority of embryo growth occurs after cones attain full size (Nienstaedt and Zasada

1990). Cotyledons appear in mid to late July and embryo development is completed in

late August (Nienstaedt and Zasada 1990). The maturation process continues after

embryo development is completed as cone dry weight continues to increase until about

two weeks prior to cone opening (Cram and Worden 1957, Nienstaedt and Zasada 1990).

Cone maturation stage appears to vary both within and among trees (Cram and Worden

1957). Cone opening coincides with moisture contents of 45 to 70 percent, and specific

gravities of 0.6 to 0.8 (Cram and Worden 1957, Nienstaedt and Zasada 1990). Seed

dispersal may be impacted by the weather. Cool wet weather can delay cone opening, or

even cause open cones to close again, with drier weather re-opening them (Nienstaedt

and Zasada 1990). Seeds are wind dispersed and peak seed fall usually occurs in mid-

September, with minor seed fall earlier in August and later into autumn (Waldron 1965,

Dobbs 1976, Nienstaedt and Zasada 1990).

White spruce is a mast seeding species (Lamontagne and Boutin 2007). The

interval between excellent cone crops varies regionally, and among local sites, from

between two to six years in good areas, to between 10 to 12 years in less favorable ones

(Nienstaedt and Zasada 1990, Lamontagne and Boutin 2007). Mast years may be

triggered by hot, dry summers at the time of bud differentiation (Nienstaedt and Zasada

1990) and are always followed by years with little to no cone production.

Kluane Red Squirrel Project

The Kluane Red Squirrel Project (KRSP) is an ongoing multidisciplinary long-

term ecological research project investigating the ecology, evolution, and energetics of

18

red squirrels. Field work associated with this project is conducted near Kluane Lake in

the southwestern Yukon Canada (61 º N, 138 º W). The project started in the late 1980‟s

with Dr. Stan Boutin from the University of Alberta. Since then it has grown into a

collaborative project with three principal investigators (Dr. Stan Boutin, University of

Alberta; Dr. Andrew McAdam, University of Guelph; Dr. Murray Humphries, McGill

University), and several other collaborators at universities in Canada and abroad. To

date, the project has contributed 57 peer reviewed publications.

Populations of red squirrels and the annual white spruce cone production in the

region have been monitored extensively and continuously since 1987. All individual red

squirrels in the study populations are known, and individually marked with ear tags and

colour markers at birth or first capture (for general description see McAdam et al. 2007).

Behavioural observations and live trapping allow individuals to be followed throughout

their lifetime. Annual monitoring of female reproduction, and use of DNA sampling to

determine paternity, has allowed the creation of an extensive pedigree (McFarlane et al.

2010) . White spruce cone production on each of the study areas is assessed annually

using either binoculars or digital photographs to count cones on trees in July or August of

each year (LaMontagne et al. 2005, Lamontagne and Boutin 2007).

Research Objectives

In this thesis I explore how red squirrel hoarding of white spruce cones is

influenced by the reproductive timing of both white spruce trees (Chapter 1) and red

squirrels (Chapter 2).

Despite the obvious inter-annual reproductive synchrony in mast seeding or

fruiting plants, there is a lack of information about the extent and selective implications

of intra-annual reproductive synchrony in these plants (Rathcke and Lacey 1985, Kelly

1994). In chapter one, I explore the intra-annual reproductive synchrony of individual

white spruce trees, how it influences seed escape from red squirrel predation, and the

evolutionary implications for white spruce.

Hoarding and lactation have been shown to be the most energetically demanding

times of the year for red squirrels (Fletcher 2011). When reproduction is not adequately

separated from hoarding, there could be a trade-off between these activities that may or

19

may not be alleviated by high resource levels. In chapter two, I explore the cone

clipping, hoarding, and larder-hoarding levels of male and female red squirrels in four

years with varying resource availability and seasonal separation of reproduction from

hoarding.

20

Literature Cited

Bascompte, J. and P. Jordano. 2007. Plant-animal mutualistic networks: the architecture

of biodiversity. Annual Review of Ecology, Evolution, and Systematics 38:567-

593.

Benkman, C. W. 1999. The selection mosaic and diversifying coevolution between

crossbills and lodgepole pine. American Naturalist 153:S75-S91.

Benkman, C. W. and T. L. Parchman. 2009. Coevolution between crossbills and black

pine: The importance of competitors, forest area and resource stability. Journal of

Evolutionary Biology 22:942-953.

Berteaux, D. and S. Boutin. 2000. Breeding dispersal in female North American red

squirrels. Ecology 81:1311-1326.

Boutin, S., K. W. Larsen, and D. Berteaux. 2000. Anticipatory parental care: acquiring

resources for offspring prior to conception. Proceedings of the Royal Society of

London Series B-Biological Sciences 267:2081-2085.

Boutin, S., Z. Tooze, and K. Price. 1993. Post-breeding dispersal by female red squirrels

(Tamiasciurus hudsonicus): the effect of local vacancies. Behavioral Ecology

4:151-155.

Boutin, S., L. A. Wauters, A. G. McAdam, M. M. Humphries, G. Tosi, and A. A.

Dhondt. 2006. Anticipatory reproduction and population growth in seed predators.

Science 314:1928-1930.

Brodin, A. 2010. The history of scatter hoarding studies. Philosophical Transactions of

the Royal Society B: Biological Sciences 365:869-881.

Clarke, M. F. and D. L. Kramer. 1994. Scatter-hoarding by a larder-hoarding rodent:

intraspecific variation in the hoarding behaviour of the eastern chipmunk, Tamias

striatus. Animal Behaviour 48:299-308.

Connell, L. M., A. Mosseler, and O. P. Rajora. 2006. Impacts of forest fragmentation on

the reproductive success of white spruce (Picea glauca). Canadian Journal of

Botany 84:956-965.

Cram, W. H. and H. A. Worden. 1957. Maturity of white spruce cones and seed. Forest

Science 3:263-269.

21

Crawley, M. J. 1983. Herbivory : the dynamics of animal-plant interactions. Blackwell

Scientific, Oxford.

Crawley, M. J. 2000. Seed predators and plant population dynamics. Pages 167-182 in M.

Fenner, editor. Seeds : the ecology of regeneration in plant communities. CABI

Publishing, Wallingford.

Curran, L. M. and M. Leighton. 2000. Vertebrate responses to spatiotemporal variation in

seed production of mast-fruiting dipterocarpaceae. Ecological Monographs

70:101-128.

Daly, M., L. F. Jacobs, M. I. Wilson, and P. R. Behrends. 1992. Scatter hoarding by

kangaroo rats (Dipodomys merriami) and pilferage from their caches. Behavioral

Ecology 3:102-111.

Dobbs, R. C. 1976. White spruce seed dispersal in central British Columbia. The Forestry

Chronicle 52:225-228.

Ehrlich, P. R. and P. H. Raven. 1964. Butterflies and plants: a study in coevolution.

Evolution 18:586-608.

Eis, S. and J. Inkster. 1972. White spruce cone production and prediction of cone crops.

Canadian Journal of Forest Research 2:460-466.

Fletcher, Q. F. 2011. The role of energy expenditure in resource acquisition and energy

allocation in free-ranging North American red squirrels (Tamiasciurus

hudsonicus). PhD Thesis. McGill University, Montreal.

Fowler, D. P. and Y. S. Park. 1983. Population studies of white spruce. I. Effects of self-

pollination. Canadian Journal of Forest Research 13:1133-1138.

Herrera, C. M. and O. Pellmyr. 2002. Plant-animal interactions: an evolutionary

approach. Blackwell Science, Oxford; Malden, MA.

Howe, H. F. and J. Smallwood. 1982. Ecology of seed dispersal. Annual Review of

Ecology and Systematics 13:201-228.

Hulme, P. E. 1998. Post-dispersal seed predation and seed bank persistence. Seed Science

Research 8:513-519.

Hulme, P. E. 2002. Seed-eaters: seed dispersal, destruction and demography. Pages 257-

273. CABI Publishing, Wallingford.

22

Hulme, P. E. and C. W. Benkman. 2002. Granivory. Pages 132-154 in C. M. Herrera and

O. Pellmyr, editors. Plant-animal interactions: an evolutionary approach.

Blackwell Science, Oxford; Malden, MA.

Humphries, M. M. and S. Boutin. 1996. Reproductive demands and mass gains: a

paradox in female red squirrels (Tamiasciurus hudsonicus). Journal of Animal

Ecology 65:332-338.

Hurly, T. A. and S. A. Lourie. 1997. Scatterhoarding and larderhoarding by red squirrels:

size, dispersion, and allocation of hoards. Journal of Mammalogy 78:529-537.

Ims, R. A. 1990. The ecology and evolution of reproductive synchrony. Trends in

Ecology & Evolution 5:135-140.

Jenkins, S. H. In press. Sex differences in repeatability of food-hoarding behaviour of

kangaroo rats. Animal Behaviour In Press, Corrected Proof.

Jenkins, S. H. and S. W. Breck. 1998. Differences in food hoarding among six species of

Heteromyid rodents. Journal of Mammalogy 79:1221-1233.

Jordano, P. 2000. Fruits and frugivory. Pages 125-165 in M. Fenner, editor. Seeds: the

ecology of regeneration in plan communities, 2nd

edition. CABI, Wallingford.

Kelly, D. 1994. The evolutionary ecology of mast seeding. Trends in Ecology &

Evolution 9:465-470.

Kelly, D. and V. L. Sork. 2002. Mast seeding in perennial plants: why, how, where?

Annual Review of Ecology and Systematics 33:427-447.

Lamontagne, J. M. and S. Boutin. 2007. Local-scale synchrony and variability in mast

seed production patterns of Picea glauca. Journal of Ecology 95:991-1000.

LaMontagne, J. M., S. Peters, and S. Boutin. 2005. A visual index for estimating cone

production for individual white spruce trees. Canadian Journal of Forest Research

35:3020-3026.

Lane, J. E., S. Boutin, M. R. Gunn, J. Slate, and D. W. Coltman. 2008. Female multiple

mating and paternity in free-ranging North American red squirrels. Animal

Behaviour 75:1927-1937.

Leaver, L. and M. Daly. 1998. Effects of food preference on scatter-hoarding by

kangaroo rats (Dipodomys merriami). Behaviour 135:823-832.

23

Leaver, L. A. 2004. Effects of food value, predation risk, and pilferage on the caching

decisions of Dipodomys merriami. Behavioral Ecology 15:729-734.

McAdam, A. G., S. Boutin, A. K. Sykes, and M. M. Humphries. 2007. Life histories of

female red squirrels and their contributions to population growth and lifetime

fitness. Ecoscience 14:362-369.

McFarlane, S. E., J. E. Lane, R. W. Taylor, J. C. Gorrell, D. W. Coltman, M. M.

Humphries, S. Boutin, and A. G. McAdam. 2010. The heritability of multiple

male mating in a promiscuous mammal. Biology Letters 7:368-371.

Murray, A. L., A. M. Barber, S. H. Jenkins, and W. S. Longland. 2006. Competitive

environment affects food-hoarding behavior of Merriam's kangaroo rats

(Dipodomys merriami). Journal of Mammalogy 87:571-578.

Nienstaedt, H. and J. C. Zasada. 1990. Picea glauca (Moench) Voss white spruce. Pages

165-185 in R. M. Burns and B. H. Honkala, editors. Silvics of North America.

Vol. 1. Conifers. U.S. Department of Agriculture Agricultural Handbook. U.S.

Department of Agriculture, Washington, D.C.

Owens, J. N. and M. Molder. 1977. Bud development in Picea glauca. II. Cone

differentiation and early development. Canadian Journal of Botany 55:2746-2760.

Owens, J. N. and M. Molder. 1979. Sexual reproduction of white spruce (Picea glauca).

Canadian Journal of Botany 57:152-169.

Preston, S. D. and L. F. Jacobs. 2001. Conspecific pilferage but not presence affects

Merriam's kangaroo rat cache strategy. Behavioral Ecology 12:517-523.

Price, K. 1992. Territorial bequeathal by red squirrel mothers: a dynamic model. Bulletin

of Mathematical Biology 54:335-354.

Price, K. and S. Boutin. 1993. Territorial bequeathal by red squirrel mothers. Behavioral

Ecology 4:144-150.

Price, M. V., N. M. Waser, and S. McDonald. 2000. Seed caching by Heteromyid rodents

from two communities: implications for coexistence. Journal of Mammalogy

81:97-106.

Price, P. W. 2002. Species interactions and the evolution of biodiversity. Pages 3-25 in C.

M. Herrera and O. Pellmyr, editors. Plant-animal interactions: an evolutionary

approach. Blackwell Science, Oxford; Malden, MA.

24

Rathcke, B. and E. P. Lacey. 1985. Phenological patterns of terrestrial plants. Annual

Review of Ecology and Systematics 16:179-214.

Robbins, C. T. 1983. Wildlife feeding and nutrition. Academic Press, New York.

Ruhren, S. and M. R. Dudash. 1996. Consequences of the timing of seed release of

Erythronium americanum (Liliaceae), a deciduous forest Myrmecochore.

American Journal of Botany 83:633-640.

Silvertown, J. W. 1980. The evolutionary ecology of mast seeding in trees. Biological

Journal of the Linnean Society 14:235-250.

Smith, C. C. 1970. The coevolution of pine squirrels (Tamiasciurus) and conifers.

Ecological Monographs 40:349-371.

Smith, M. C. 1968. Red squirrel responses to spruce cone failure in interior Alaska. The

Journal of Wildlife Management 32:305-317.

Steele, M. A. 1998. Tamiasciurus hudsonicus. Mammalian Species 586:1-9.

Stuart-Smith, A. K. and S. Boutin. 1995. Behavioural differences between surviving and

depredated juvenile red squirrels. Ecoscience 2:34-40.

Tsurim, I. and Z. Abramsky. 2004. The effect of travel costs on food hoarding in gerbils.

Journal of Mammalogy 85:67-71.

Vander Wall, S. B. 1990. Food hoarding in animals. University of Chicago Press,

Chicago.

Vander Wall, S. B., K. M. Kuhn, and M. J. Beck. 2005. Seed removal, seed predation,

and seconday dispersal. Ecology 86:801-806.

Waldron, R. M. 1965. Cone production and seed fall in a mature white spruce stand.

Forestry Chronicle 41:314-329.

25

Tables

Table 1. Hypotheses that have been suggested to explain mast seeding or fruiting,

adapted from Kelly (1994).

Hypothesis Explanation

Predator satiation Large seed crops satiate seed predators and

decreases in predator abundance may occur in low

seed years

Wind pollination Increases pollination success in wind pollinated

plants

Environmental prediction Mast years timed to anticipate favorable conditions

for reproduction or establishment

Resource matching Plants vary reproductive effort to match available

resources

Animal pollination Increases pollination success in animal pollinated

plants

Animal dispersal Increases dispersal in animal dispersed plants

Accessory costs High accessory costs of reproduction make small

reproductive efforts less efficient per seed

Large seed size Selection for larger seed size increases contrast

between high and low seed years

26

Chapter 1: Swamping seed predators in number and time: within-season

synchrony of a masting conifer enhances seed escape

Authors:

Devan W. Archibald (email: devan.archibald@mail.mcgill.ca)

Natural Resource Sciences, Macdonald campus, McGill University

21 111 Lakeshore Drive, Ste-Anne-de-Bellevue, QC, H9X 3V9, Canada

Andrew G. McAdam (email: macadam@uoguelph.ca)

Department of Integrative Biology, University of Guelph,

50 Stone Road East, Guelph, ON, N1G 2W1, Canada

Stan Boutin (email: stan.boutin@ualberta.ca)

Department of Biological Sciences, University of Alberta

Z1109 Biological Sciences Building, Edmonton, AB, T6G 2R3, Canada

Quinn Fletcher (email: quinn.fletcher2@mail.mcgill.ca)

Department of Biological Sciences, University of Alberta

Z1109 Biological Sciences Building, Edmonton, AB, T6G 2R3, Canada

Murray M. Humphries (email: murray.humphries@mcgill.ca)

Natural Resource Sciences, Macdonald campus, McGill University

21 111 Lakeshore Drive, Ste-Anne-de-Bellevue, QC, H9X 3V9, Canada

27

Abstract

Predator satiation resulting from inter-annual synchrony has been widely documented in

masting plants, but how reproductive synchrony within a year influences seed escape is

poorly understood. We evaluated whether the intra-annual reproductive synchrony of

individual white spruce trees (Picea glauca) increased seed escape from their primary

pre-dispersal seed predator (North American red squirrels; Tamiasciurus hudsonicus). In

both a mast and non-mast year, seed escape tended to be enhanced by producing many

cones as well as by producing cones that matured synchronously relative to other trees in

the population. This led to significantly positive selection differentials for intra-annual

reproductive synchrony in both years, but after also accounting for the number of cones

produced, natural selection favoring increased synchrony was significantly different from

zero only in the non-mast year. Thus, maximizing number and minimizing time both

contribute to conifer seed escape, but their relative importance varies between mast and

non-mast years.

28

Introduction

The reproductive stages of many plants and animals are synchronized among

individuals within populations (reviewed in Ims 1990a). Synchrony is common in

seasonal environments because reproduction is often clustered during times of the year

that are most favorable to offspring survival. However, reproductive synchrony is often

far greater than would be expected from environmental seasonality alone (Hughes and

Richard 1974, Rutberg 1987, Sinsch 1988), suggesting that ecological and socio-

biological processes may influence the temporal pattern of reproduction (Ims 1990a).

Darling (1938) first suggested that reproductive synchrony might serve an anti-predatory

function, and this remains the most general and widely cited adaptive explanation for

reproductive synchrony (Ims 1990b). Among the three mechanisms suggested (see

Rutberg 1987, Ims 1990b), the sudden mass appearance of prey in a vulnerable stage,

which satiates predators due to handling time constraints, has been proposed as an

explanation for reproductive synchrony in birds (Darling 1938, Robertson 1973, Findlay

and Cooke 1982), mammals (Estes 1976, Rutberg 1987, Odonoghue and Boutin 1995),

and to explain mast fruiting or seeding in plants (Janzen 1971, Silvertown 1980, Kelly

1994, Kelly and Sork 2002).

Mast seeding is the synchronous and highly variable inter-annual seed production

by a population of plants (Kelly 1994). This reproductive strategy results in a pattern of

episodic mast years, in which a superabundance of seeds is produced, followed by non-

mast years where little or no seeds are produced. The predator satiation hypothesis is a

prominent, ultimate explanation for this inter-annual reproductive synchrony (Kelly and

Sork 2002). It proposes that more seeds are produced than can be consumed in mast

years and predators starve and may decline in abundance in the intervening non-mast

years. Reduced seed predation in mast years is a basic prediction of the predator satiation

hypothesis, and has been confirmed in numerous plant species with a variety of seed

predators (reviewed in Kelly and Sork 2002).

Inter-annual synchrony is the defining feature of the masting strategy, but, to date,

studies have ignored the potential importance of intra-annual synchrony to seed escape

(Rathcke and Lacey 1985), despite a number of studies in other non-masting plants

29

examining the importance of within-season synchrony in flowering or fruiting to

pollination success and seed escape (for a review see Elzinga et al. 2007). In masting

species, high inter-annual synchrony will be most effective in satiating seed predators

when combined with high intra-annual synchrony, such that in a year of abundant seed

production, seeds from all trees are vulnerable to predation during the same, short time

interval. Not only the amount of seeds produced, but also the timing of their seasonal

availability may, therefore, be an important fitness determinant for masting species.

White spruce (Picea glauca) is a mast seeding tree (LaMontagne and Boutin

2007) that satiates its dominant pre-dispersal seed predator, the North American red

squirrel (Tamiasciurus hudsonicus) during mast years (Fletcher et al. 2010). White

spruce is a wind dispersed conifer species with a transcontinental distribution in North

America (Nienstaedt and Zasada 1990), much of which overlaps with that of the red

squirrel (Steele 1998). The red squirrel is the dominant pre-dispersal seed predator of

North American conifers (Benkman et al. 1984, Benkman et al. 2003, Peters et al. 2003,

Benkman and Siepielski 2004). They defend mutually exclusive territories within which

they remove (cut or clip from the trees) and hoard thousands of conifer cones in autumn

(Steele 1998). Neither immature nor open cones are hoarded by red squirrels (Fletcher et

al. 2010). Therefore, in order to escape pre-dispersal seed predation by red squirrels

white spruce cones must remain on trees from maturation to opening, a vulnerable period

that lasts about two weeks (Cram and Worden 1957). The timing of cone maturation for

individual white spruce trees relative to the timing of other trees in the population may

have important consequences for their seed escape from red squirrels, but has not been

previously investigated.

Here we examined the degree of intra-annual reproductive synchrony of

individual white spruce trees and its influence on seed escape to dispersal in both a mast

and non-mast year. We hypothesized that high-levels of intra-annual reproductive

synchrony in white spruce trees could enhance seed escape by satiating red squirrel

hoarding efforts. We, therefore, predicted that white spruce trees whose cones matured

more synchronously with others in the population would have increased seed escape to

dispersal than those with less reproductive synchrony. Since pre-dispersal seed predation

can affect natural selection on a variety of plant traits (Kolb et al. 2007, Parachnowitsch

30

and Caruso 2008), we explored the evolutionary implications of our findings by

measuring the strength of directional selection (Lande and Arnold 1983) on intra-annual

reproductive synchrony in white spruce during these two episodes of selection, and the

repeatability of intra-annual reproductive synchrony and cone production between years.

We also concurrently monitored red squirrel cone clipping activities to determine how it

varied with cone maturation phenology.

Methods

Study area

This study was conducted in the autumns of 2009 (non-mast year) and 2010 (mast

year) on three study areas (approximately 40ha each) located near Kluane National Park

in southwestern Yukon, Canada (61°N, 138°W). Two of the study areas were located

across the Alaska Highway from each other. In these areas the red squirrel population

and annual white spruce cone production have been monitored continuously since 1988

(Boutin et al. 2006, McAdam et al. 2007). The third area was less than 500 meters away,

has historically been studied intermittently, and was included in the first year of the study

only. All sites were located in a glacial valley that is composed of boreal forest

dominated by white spruce with a willow (Salix spp.) understory. Red squirrels are

present in this area at an average density of 1.5-2.8 squirrels/ha (Boonstra et al. 2001),

and are the dominant vertebrate pre-dispersal seed predator of white spruce as seed

predation by white-winged crossbills (Loxia leucoptera) is rare and irruptive (Smith and

Folkard 2001). For a more detailed description of the general ecology of the study site

see Krebs, Boutin, and Boonstra (2001).

White spruce cone counts

To assess cone production, pre-dispersal predation, and cone opening we

conducted repeated cone counts of individually marked trees (non-mast; n=607 trees,

mast; n=292 trees) on red squirrel territories (non-mast; n=24 territories, mast; n=14

territories). Red squirrels defend mutually exclusive territories with a central food store

where they cache cones that they have clipped from the surrounding trees (Steele 1998).

31

Sampled trees were located within 30m of a red squirrel‟s central food cache, or midden,

which is similar to the average radius of a red squirrel territory in this area (LaMontagne

and Boutin 2007). Specifically, two perpendicular 60m transects were randomly oriented

through the centre of each midden. All trees larger than 5cm diameter at breast height

within 1m of either side of both transects were sampled. This diameter at breast height

approximates the age at which white spruce become cone-bearing in this region

(LaMontagne and Boutin 2007).

The numbers of closed and open cones were measured seven times for each tree

during the cone-hoarding season. In the non-mast year (2009) these counts were

performed between Aug 15 and Oct 2. In the mast year (2010) counts were performed

between Aug 7 and Oct 7. Each of the seven rounds of cone counts lasted between one

and six days (median = 3). The length of time between successive counts of the same

tree ranged from six to 16 days (median = 8). For each count, the total numbers of closed

cones and open cones visible on the top 3 m of one side of each sampled tree were

counted using binoculars. All counts for each tree were made from the same flagged

location. If more than 100 cones were visible, a digital photo was taken (6.0 megapixels)

and cones were counted from the image (LaMontagne et al. 2005). The technique has

been previously shown to correlate strongly with the total number of cones on the entire

tree (LaMontagne et al. 2005).

White spruce intra-annual reproductive synchrony

We assumed that the timing of cone opening reflected phenological patterns of

maturation and used the timing of cone opening as proxy for reproductive timing. White

spruce cone maturation cannot be assessed without removing cones from the trees

(Nienstaedt and Zasada 1990), and since we were interested in cone removal by red

squirrels we did not remove cones ourselves. Cram and Worden (1957) found that a

white spruce cone matures about two weeks prior to opening, indicating opening may be

a signal of maturation. Cone opening is also a function of specific gravity and lowering

water content (Cram and Worden 1957). Therefore, environmental factors may play a

role in opening synchronicity that could be absent in maturation synchrony. But,

different trees and cones likely have varying degrees of susceptibility to opening under

32

the same weather conditions due to being in differing phenological stages, so differences

in the timing of cone opening should be reflective of differences in the timing of cone

maturation.

To assess the degree of intra-annual reproductive synchrony exhibited by

individual white spruce trees with others in the population we used Mahoros (2002) index

for flowering synchrony and adapted it to cone opening (Eq‟n 1). There are numerous

methods for assessing fruiting and flowering synchrony in plants (Elzinga et al. 2007).

We chose the method of Mahoro (2002) because it is one of the only methods that

considers the relative numbers of mature fruit or open flowers on each individual

throughout the season, and how that compares to the relative numbers of mature fruit or

flowers of all the other individuals in the population, which are two criteria that are most

important when investigating the consequences of phenological patterns (Freitas and

Bolmgren 2008). Other indices examine the duration of flowering or fruiting, ignoring

the importance of the number of open flowers or mature fruit (Augspurger 1983), or they

only consider the relative number of open flowers or mature fruit of the individual and

ignore the importance of the relative numbers of open flowers or mature fruit on the other

individuals (Marquis 1988, Freitas and Bolmgren 2008).

(1)

In this index the synchronization level (SIi) of an individual i is a function of the ratio of

the total cones opening in the season that newly opened in the jth

interval between counts

(yi,j) and the mean of that value for every other individual in the population during the

same interval between counts (ӯi,j) with n representing the number of intervals between

counts. This results in an index for each tree that ranges between zero and one, with one

indicating complete synchrony with the other trees in the population and zero indicating

complete asynchrony.

Red squirrel seed predation and hoarding behavioral observations

We also concurrently monitored red squirrel cone clipping activities to determine

the extent and timing of pre-dispersal cone predation (2009; n = 21 adult males from Aug

33

20 to Oct 1, 2010; 9 adult males from Aug 25 to Sep 30). In both years this encompassed

the beginning, peak, and decline in cone clipping activities. We followed the

methodology presented in detail by Fletcher et al. (2010), using repeated daily seven-

minute focal observations of individuals three times weekly, facilitated by radio collars

and color-marked ear tags. During focal periods we recorded the number of cones

clipped from trees and we used the average clipping rate on a given day to estimate the

daily total number of cones clipped by each squirrel by multiplying this average by the

number of minutes between sunrise and sunset (61°N, 138°W; Herzberg Institute of

Astrophysics; National Research Council of Canada). We used the shape-preserving

piecewise cubic modeling technique (Fritsch and Carlson 1980) adopted by Fletcher et al.

(2010) to estimate clipping activities for each individual on un-observed days between

August 15th

and October 15th

. In order to assess the phenology of cone clipping we

plotted the average of both modeled and observed values for all squirrels on each day

throughout the season.

All animal use procedures were approved by McGill University Faculty of

Agricultural and Environmental Sciences Animal Care Committee.

Statistical analysis

To evaluate the strength and form of phenotypic selection on intra-annual

reproductive synchrony during these two episodes of selection we calculated selection

differentials and the selection gradients (Lande and Arnold 1983) in each year separately.

Standardized selection differentials (S’) were measured as the covariance between

relative fitness and the individual synchrony index. Relative fitness for each tree was

calculated as the absolute fitness of the tree (number of open cones visible during the last

count) divided by the mean fitness of all trees in the population. Standardized linear

selection gradients (β’) were estimated from the partial regression coefficients of a

multiple regression that included individual synchrony index and the number of cones

produced (number of cones visible during the first count) as predictors of relative fitness.

Individual synchrony index and the number of cones produced were each standardized to

a mean of zero and standard deviation of one prior to analysis (Lande and Arnold 1983).

Non-linear (i.e. stabilizing or disruptive) selection gradients (γ’) were estimated as twice

34

the regression coefficients from the quadratic terms in the model (Lande and Arnold

1983, Stinchcombe et al. 2008). Since relative fitness was calculated relative to the mean

fitness for each population there were no differences among study areas in relative fitness

so this factor was not included in our selection models. Due to non-normality of the

residuals from the models, we used jackknife re-sampling to generate standard errors and

test significance (Mitchell-Olds and Shaw 1987).

Repeatability is a measure of the proportion of total variance in a trait that is

accountable to differences among individuals (Falconer and Mackay 1996), and therefore

represents an upper limit to heritability (but see Dohm 2002). In balanced designs with

paired measures it is most simply estimated as a correlation, since it is the within-class

correlation of repeated measurements (Nakagawa and Schielzeth 2010). We assessed the

repeatability between years of intra-annual reproductive synchrony and the number of

cones produced in white spruce using Pearson‟s product moment correlation coefficient

(r). Thus, we measured whether the relative, not absolute, measurement of the trait of an

individual was correlated with its relative measurement the following year (Berkum et al.

1989, Chappell et al. 1995). Due to non-normal error distributions the number of cones

produced were ln-transformed (ln +1) prior to this analysis.

All statistical tests were conducted using the statistical software R (R

Development Core Team 2011) with an alpha level of 0.05. We report means ± standard

error throughout the paper.

Results

In the non-mast year (2009) 44.8 percent of all white spruce trees sampled

produced cones, whereas 83.2 percent produced cones in the mast year (2010). On

average, the number of cones produced on sampled trees increased 25-fold from the non-

mast to mast year. The number of closed cones decreased throughout the study in both

years due to the clipping activities of the red squirrels and cone opening (Figure 1).

About two weeks prior to cone opening, red squirrel cone clipping activities increased,

and then peaked as cones were beginning to open in both years.

35

In the non-mast year, 34.9 percent of all trees sampled had at least one cone

survive to opening, whereas 72.6 percent of all trees sampled had cones survive to

opening in the mast year. Among individual trees that produced cones, there was a

significant (p<0.001; Wilcoxon rank sum test) 9-fold increase in cone production from

the non-mast to mast year (non-mast; 55 ± 6 cones counted, mast; 480 ± 36 cones

counted), however the percentage of these cones that escaped pre-dispersal seed

predation and opened in the mast (43.9 ± 2.7 percent) and non-mast (45.0 ± 2.8 percent)

were not significantly different (p=0.09; Wilcoxon rank sum test).

Individual intra-annual reproductive synchrony indices were significantly higher

(p<0.001; Wilcoxon rank sum test) in the mast (0.56 ± 0.02; range 0.03 – 0.9) than in the

non-mast year (0.47 ± 0.01; range 0.01-0.89). There were significant positive selection

differentials for increased intra-annual reproductive synchrony in both years (non-mast;

S’= 0.48 ± 0.12, t1,211 = 4.13, p < 0.001, mast; S’ = 0.45 ± 0.09, t1,205 = 5.20, p < 0.001).

However, when accounting for the number of cones produced, there was significant

directional selection for increased intra-annual reproductive synchrony in the non-mast

year but no evidence of selection on synchrony in the mast year (Table 1). There was

significant directional selection for an increase in the number of cones produced in the

non-mast year. In the mast year there were significant linear and non-linear selection on

the number of cones produced, indicating that trees that produced more cones had higher

fitness but also that the strength of selection also increased with increasing cone

production (Table 1). Analysis of white spruce trees included in both years of the study

revealed that intra-annual reproductive synchrony was significantly repeatable between

years (r91 = 0.25, p = 0.015), as was the number of cones produced (r275 = 0.49, p <

0.001).

Discussion

Individual white spruce trees varied widely in the number of cones produced and

their reproductive synchrony. These two traits had important consequences for tree

fitness, but effects differed between a mast year and a non-mast year. It is unsurprising

that the number of cones present at the start of the hoarding period was a significant

36

determinant of the number of cones open at the end of the hoarding period, given the

tendency for most trees to have a substantial proportion of their cones escape squirrel

predation. However, our detection of strong selection (double the median value of

known selection gradients reported by Kingsolver et al. 2001) for intra-annual

reproductive synchrony is more surprising and novel. During the mast year, when trees

produced many cones and squirrels are satiated in their cone clipping and hoarding

(Fletcher et al. 2010), we found no evidence for selection on intra-annual reproductive

synchrony. However, in the non-mast year, when mean cone production was much

lower, we found evidence for strong directional selection favoring increased intra-annual

reproductive synchrony. Atlan et al. (2010) recognized two strategies for escape from

seed predation in a shrub (Ulex europaeus), escape in time or numbers, and they

suggested that the plants they studied exhibited polymorphic use of these strategies. Our

results also suggest that white spruce can enhance seed predation either through the

adjustment of seed number or the timing of seed maturation. The “too many cones”

strategy appears to be effective in years of high and low cone production, whereas the

“too little time” strategy seems to be most effective when cone abundance is limited.

To our knowledge, this is the first study to quantify the degree of intra-annual

reproductive synchrony exhibited by individuals in a masting species, and the first to

relate this to seed escape. It is also one of the few studies to find evidence of a positive

influence of intra-annual reproductive synchrony on seed escape in plants (for other

examples see, Augspurger 1981, Honek and Martinkova 2005). Most other studies have

found that fruiting or flowering off-peak is associated with reduced seed predation

(Pettersson 1994, Eriksson 1995, Pilson 2000, Freeman et al. 2003, Lacey et al. 2003).

When using the same synchrony index as this study, Mahoro (2002) found no correlation

between flowering synchrony and flower, fruit or seed predation for individuals of

Vaccinium hirtum. Gomez (1993) found neither consistency in the sign nor significance

of phenotypic selection on flowering synchrony of Hormathophyla spinosa across three

years. Differences between our findings and those of previous studies could be due to

our focus on a system where predation defines the plant-animal interaction, whereas most

previous studies of plant synchrony have focused on mutualistic plant-animal interactions

such as pollination, with seed escape being secondary. Synchrony patterns thought of as

37

detrimental for seed predators are often thought of as beneficial to pollinators and

dispersers (Benkman et al. 1984, Elzinga et al. 2007). Most of the studies of intra-annual

reproductive synchrony in plants also have animal pollinators present, and are interpreted

mainly in that context (Kolb et al. 2007). White spruce are wind pollinated and dispersed

(Nienstaedt and Zasada 1990), which eliminates any pollinator or disperser saturation that

could result in a cost of reproductive synchrony.

Other studies of conifer cone phenology have sought to determine population, and

occasionally within individual patterns of maturation or opening (Vander Wall and Balda

1977, Tomback and Kramer 1980, Benkman et al. 1984, Samano and Tomback 2003),

and have compared species traits that facilitate avian dispersal or deter red squirrel

predation (Benkman et al. 1984, Samano and Tomback 2003). But none of these studies

have investigated the implications of inter-individual variation in cone phenology within

a single species. By using a synchrony index we were able to quantify differences

between individuals. Our index accounted for both within and among individual

synchrony by accounting for the relative number of open cones not only on the individual

of interest, but also that of all the other individuals in the population (Mahoro 2002).

However, this index was unable to separate the importance of within and among

individual synchrony, of which variation in maturation or opening pattern could be

important to seed predation (Benkman et al. 1984). Future research should focus on the

relative importance of within and among individual synchrony, which will require

monitoring individual cones on a given tree and following their progression throughout

the autumn.

We further demonstrate that cone production and intra-annual synchrony are both

repeatable traits. Although repeatabilities of these traits (0.25 for synchrony and 0.49 for

production) were lower than repeatabilities of morphological traits in other plants (0.64 to

0.99; Beavis et al. 1991, Jordano 1995, Shykoff and Kaltz 1998, Santos 1999, Di Renzo

et al. 2000), it establishes the potential for heritable variation, which has been shown for

phenological traits in other plants (Chao et al. 2003, Botto and Coluccio 2007, Johnson

2007, Atlan et al. 2010), including a conifer (Matziris 1994). However, differentiating

between the environmental and genetic contributions to the variation in cone production

and intra-annual synchrony of white spruce will be challenging. Nevertheless, a

38

comparison of the fitness of more and less synchronous individuals in a population

indicates an adaptive advantage of this strategy (Ims 1990a).

Red squirrels removed all of the available cones before any had opened on about

ten percent of sampled trees in both years. Our inability to estimate intra-annual

synchrony for these highly depredated trees (the „invisible fraction‟, sensu Grafen

(1988)), could have introduced bias into our selection gradient estimates (Grafen 1988,

Bennington and McGraw 1995, Hadfield 2008). However, since the number of trees in

this category is small, consistent across years, and there is still large variation in intra-

annual synchrony index values within the population, the bias is likely to be small.

Our use of the number of open cones as a measure of fitness assumes that the

number of open cones correlated with the number of seeds dispersing. Although quality

and number of seeds per cone can vary depending on cone size or year (Zasada 1970,

Waldron 1965), years with large cone crops tend to also have more seeds per cone

(Zasada and Viereck 1970), indicating our results may underestimate the importance of

these traits to tree fitness in the mast year and overestimate their importance in the non-

mast year. However, the difficulty of assessing seedfall of individual trees requires

additional indicies of fitness such as cone numbers to be used (Parchman et al. 2007).

This study provides additional evidence that red squirrels interact strongly with,

and may act as selective agents on, a variety of conifer species (Smith 1970, Elliott 1974,

Benkman et al. 1984, Benkman and Siepielski 2004). The two years over which we

measured selection on spruce cone number and synchrony represent a small proportion of

the lifetime of these trees, but provide unbiased estimates of the fitness consequences of

these traits during these two episodes of selection. Further estimates of selection on cone

production and synchrony would reveal how representative the two years that we studied

were of mast and non-mast years more generally. Overall selection on these traits could

also be affected by interactions with other potential pre-dispersal seed predators (Smith

and Balda 1979, Nienstaedt and Zasada 1990), but these appear to be less common pre-

dispersal seed predators of spruce in our area. The importance of cone production and

synchrony to post-dispersal seed predation is also unknown. The high proportion of

cones clipped by red squirrels, however, suggests that red squirrel seed predation has

important consequences for the fitness of spruce trees. As a result, the natural selection

39

on the timing and amount of cone production in white spruce resulting from squirrel cone

predation that we have documented here likely represents an important component of the

overall selection on these traits.

40

Literature Cited

Atlan, A., M. Barat, A. S. Legionnet, L. Parize, and M. Tarayre. 2010. Genetic variation

in flowering phenology and avoidance of seed predation in native populations of

Ulex europaeus. Journal of Evolutionary Biology 23:362-371.

Augspurger, C. K. 1981. Reproductive synchrony of a tropical shrub - experimental

studies on effects of pollinators and deed predators on Hybanthus-prunifolius

(Violaceae). Ecology 62:775-788.

Augspurger, C. K. 1983. Phenology, flowering synchrony, and fruit set of six neotropical

shrubs. Biotropica 15:257-267.

Beavis, W. D., D. Grant, M. Albertsen, and R. Fincher. 1991. Quantitative trait loci for

plant height in four maize populations and their associations with qualitative

genetic loci. Theoretical and Applied Genetics 83:141-145.

Benkman, C. W., R. P. Balda, and C. C. Smith. 1984. Adaptations for seed dispersal and

the compromises due to seed predation in limber pine. Ecology 65:632-642.

Benkman, C. W., T. L. Parchman, A. Favis, and A. M. Siepielski. 2003. Reciprocal

selection causes a coevolutionary arms race between crossbills and lodgepole

pine. American Naturalist 162:182-194.

Benkman, C. W. and A. M. Siepielski. 2004. A keystone selective agent? Pine squirrels

and the frequency of serotiny in lodgepole pine. Ecology 85:2082-2087.

Bennington, C. C. and J. B. McGraw. 1995. Phenotypic selection in an artificial

population of Impatiens pallida: The Importance of the invisible fraction.

Evolution 49:317-324.

Berkum, F. H. V., R. B. Huey, J. S. Tsuji, and T. Garland. 1989. Repeatability of

individual differences in locomotor performance and body size during early

ontogeny of the lizard Sceloporus occidentalis (Baird & Girard). Functional

Ecology 3:97-105.

Boonstra, R., S. Boutin, A. Byrom, T. Karels, A. Hubbs, K. Stuart-Smith, M. Blower, and

S. Antpoehler. 2001. The role of red squirrels and arctic ground squirrels. Pages

179-214 in C.J. Krebs, S. Boutin, and R. Boonstra, editors. Ecosystem dynamics

of the boreal forest: the Kluane project Oxford University Press, Oxford.

41

Botto, J. F. and M. P. Coluccio. 2007. Seasonal and plant-density dependency for

quantitative trait loci affecting flowering time in multiple populations of

Arabidopsis thaliana. Plant, Cell & Environment 30:1465-1479.

Chao, C. T., D. E. Parfitt, L. Ferguson, C. Kallsen, and J. Maranto. 2003. Genetic

analyses of phenological traits of pistachio (Pistacia vera, L.). Euphytica

129:345-349.

Chappell, M. A., G. C. Bachman, and J. P. Odell. 1995. Repeatability of maximal aerobic

performance in Belding's ground squirrels, Spermophilus beldingi. Functional

Ecology 9:498-504.

Cram, W. H. and H. A. Worden. 1957. Maturity of white spruce cones and seed. Forest

Science 3:263-269.

Darling, F. F. 1938. Bird flocks and the breeding cycle. Cambridge University Press,

Cambridge.

Di Renzo, M. A., M. A. Ibanex, N. C. Bonamico, and M. M. Poverene. 2000. Estimation

of repeatability and phenotypic correlations in Eragrostis curvula. The Journal of

Agricultural Science 134:207-212.

Dohm, M. R. 2002. Repeatability estimates do not always set an upper limit to

heritability. Functional Ecology 16:273-280.

Elliott, P. F. 1974. Evolutionary responses of plants to seed-eaters: Pine squirrel

predation on lodgepole pine. Evolution 28:221-231.

Elzinga, J. A., A. Atlan, A. Biere, L. Gigord, A. E. Weis, and G. Bernasconi. 2007. Time

after time: flowering phenology and biotic interactions. Trends in Ecology &

Evolution 22:432-439.

Eriksson, O. 1995. Asynchronous flowering reduces seed predation in the perennial forest

herb Actaea spicata. Acta oecologica 16:195.

Estes, R. D. 1976. The significance of breeding synchrony in the wildebeest. African

Journal of Ecology 14:135-152.

Falconer, D. S. and T. F. C. Mackay. 1996. Introduction to quantitative genetics.

Longman, Essex, England.

42

Findlay, C. S. and F. Cooke. 1982. Synchrony in the Lesser Snow Goose (Anser

caerulescens caerulescens). II the adaptive value of reproductive synchrony.

Evolution 36:786-799.

Fletcher, Q. E., S. Boutin, J. E. Lane, J. M. LaMontagne, A. G. McAdam, C. J. Krebs,

and M. M. Humphries. 2010. The functional response of a hoarding seed predator

to mast seeding. Ecology 91:2673-2683.

Freeman, R. S., A. K. Brody, and C. D. Neefus. 2003. Flowering phenology and

compensation for herbivory in Ipomopsis aggregata. Oecologia 136:394-401.

Freitas, L. and K. Bolmgren. 2008. Synchrony is more than overlap: measuring

phenological synchronization considering time length and intensity. Revista

Brasileira de Botânica 31:721-724.

Fritsch, F. N. and R. E. Carlson. 1980. Monotone piecewise cubic interpolation. SIAM

Journal on Numerical Analysis 17:238-246.

Gomez, J. M. 1993. Phenotypic selection on flowering synchrony in a high-mountain

plant, Hormathophylla spinosa (Cruciferae). Journal of Ecology 81:605-613.

Grafen, A. 1988. On the uses of data on lifetime reproductive success. Pages 454 – 471 in

T. H. Clutton-Brock, editor. Reproductive success. Univeristy of Chicago Press,

Chicago, IL.

Hadfield, J. D. 2008. Estimating evolutionary parameters when viability selection is

operating. Proceedings of the Royal Society B: Biological Sciences 275:723-734.

Honek, A. and Z. Martinkova. 2005. Pre-dispersal predation of Taraxacum officinale

(dandelion) seed. Journal of Ecology 93:335-344.

Hughes, D. A. and J. D. Richard. 1974. The nesting of the Pacific ridley turtle

Lepidochelys olivacea on Playa Nancite, Costa Rica. Marine Biology 24:97-107.

Ims, R. A. 1990a. The ecology and evolution of reproductive synchrony. Trends in

Ecology & Evolution 5:135-140.

Ims, R. A. 1990b. On the adaptive value of reproductive synchrony as a predator-

swamping strategy. American Naturalist 136:485-498.

Janzen, D. H. 1971. Seed predation by animals. Annual Review of Ecology and

Systematics 2:465-492.

43

Johnson, M. T. J. 2007. Genotype-by-environment interactions leads to variable selection

on life-history strategy in Common Evening Primrose (Oenothera biennis).

Journal of Evolutionary Biology 20:190-200.

Jordano, P. 1995. Frugivore-mediated selection on fruit and seed size: Birds and St.

Lucie's Cherry, Prunus mahaleb. Ecology 76:2627-2639.

Kelly, D. 1994. The evolutionary ecology of mast seeding. Trends in Ecology &

Evolution 9:465-470.

Kelly, D. and V. L. Sork. 2002. Mast seeding in perennial plants: Why, how, where?

Annual Review of Ecology and Systematics 33:427-447.

Kingsolver, J. G., H. E. Hoekstra, J. M. Hoekstra, D. Berrigan, S. N. Vignieri, C. E. Hill,

A. Hoang, P. Gibert, and P. Beerli. 2001. The strength of phenotypic selection in

natural populations. The American Naturalist 157:245-261.

Kolb, A., J. Ehrlén, and O. Eriksson. 2007. Ecological and evolutionary consequences of

spatial and temporal variation in pre-dispersal seed predation. Perspectives in

Plant Ecology, Evolution and Systematics 9:79-100.

Krebs, C., S. Boutin, and R. Boonstra, editors. 2001. Ecosystem dynamics of the boreal

forest: the Kluane project Oxford University Press, Oxford.

Lacey, E. P., D. A. Roach, D. Herr, S. Kincaid, and R. Perrott. 2003. Multigenerational

effects of flowering and fruiting phenology in Plantago lanceolata. Ecology

84:2462-2475.

LaMontagne, J. M. and S. Boutin. 2007. Local-scale synchrony and variability in mast

seed production patterns of Picea glauca. Journal of Ecology 95:991-1000.

LaMontagne, J. M., S. Peters, and S. Boutin. 2005. A visual index for estimating cone

production for individual white spruce trees. Canadian Journal of Forest Research

35:3020-3026.

Lande, R. and S. J. Arnold. 1983. The measurement of selection on correlated characters.

Evolution 37:1210-1226.

Mahoro, S. 2002. Individual flowering schedule, fruit set, and flower and seed predation

in Vaccinium hirtum Thunb. (Ericaceae). Canadian Journal of Botany-Revue

Canadienne De Botanique 80:82-92.

44

Marquis, R. J. 1988. Phenological variation in the neotropical understory shrub Piper

arielanum: Causes and consequences. Ecology 69:1552-1565.

Matziris, D. I. 1994. Genetic variation in the phenology of flowering in black pine. Silvae

Genetica 43:321-328.

Mitchell-Olds, T. and R. G. Shaw. 1987. Regression analysis of natural selection:

Statistical inference and biological interpretation. Evolution 41:1149-1161.

Nakagawa, S. and H. Schielzeth. 2010. Repeatability for gaussian and non-gaussian data:

a practical guide for biologists. Biological Reviews 85:935-956.

Nienstaedt, H. and J. C. Zasada. 1990. Picea glauca (Moench) Voss white spruce.in R.

M. B. a. B. H. and Honkala, editors. Silvics of North America. Volume 1.

Conifers. USDA Agricultural Handbook. U.S. Department of Agriculture,

Washington, D.C.

Odonoghue, M. and S. Boutin. 1995. Does reproductive synchrony affect juvenile

survival rates of northern mammals. Oikos 74:115-121.

Parachnowitsch, A. L. and C. M. Caruso. 2008. Predispersal seed herbivores, not

pollinators, exert selection on floral traits via female fitness. Ecology 89:1802-

1810.

Parchman, T.L., C.W. Benkman, and E.T. Mezquida. 2007. Coevolution between

Hispaniolan Crossbills and pine: Does more time allow for greater phenotypic

escalation at lower latitude? Evolution 61: 2142-2153.

Peters, S., S. Boutin, and E. Macdonald. 2003. Pre-dispersal seed predation of white

spruce cones in logged boreal mixedwood forest. Canadian Journal of Forest

Research-Revue Canadienne De Recherche Forestiere 33:33-40.

Pettersson, M. W. 1994. Large plant size counteracts early seed predation during the

extended flowering season of a Silene uniflora (Caryophyllaceae) population.

Ecography 17:264-271.

Pilson, D. 2000. Herbivory and natural selection on flowering phenology in wild

sunflower, Helianthus annuus. Oecologia 122:72-82.

R Development Core Team. 2011. R: A language and environment forstatistical

computing in R Foundation for Statistical Computing, editor., Vienna, Austria.

45

Rathcke, B. and E. P. Lacey. 1985. Phenological patterns of terrestrial plants. Annual

Review of Ecology and Systematics 16:179-214.

Robertson, R. J. 1973. Optimal niche space of the Redwinged Blackbird: Spatial and

temporal patterns of nesting activity and success. Ecology 54:1085-1093.

Rutberg, A. T. 1987. Adaptive hypotheses of birth synchrony in ruminants: An

interspecific test. The American Naturalist 130:692-710.

Samano, S. and D. F. Tomback. 2003. Cone opening phenology, seed dispersal, and seed

predation in southwestern white pine (Pinus strobiformis) in southern Colorado.

Ecoscience 10:319-326.

Santos, C. A. F. 1999. In situ evaluation of fruit yield and estimation of repeatability

coefficient for major fruit traits of umbu tree [Spondias tuberosa (Anacardiaceae)]

in the semi-arid region of Brazil. Genetic Resources and Crop Evolution 46:455-

460.

Shykoff, J. A. and O. Kaltz. 1998. Phenotypic changes in host plants diseased by

Microbotryum violaceum: Parasite manipulation, side effects, and trade-offs.

International Journal of Plant Sciences 159:236-243.

Silvertown, J. W. 1980. The evolutionary ecology of mast seeding in trees. Biological

Journal of the Linnean Society 14:235-250.

Sinsch, U. 1988. Temporal spacing of breeding activity in the natterjack toad, Bufo

calamita. Oecologia 76:399-407.

Smith, C. C. 1970. The coevolution of Pine squirrels (Tamiasciurus) and conifers.

Ecological Monographs 40:349-371.

Smith, C. C. and R. P. Balda. 1979. Competition among insects, birds and mammals for

conifer seeds. American Zoologist 19:1065-1083.

Smith, J. N. M. and N. F. G. Folkard. 2001. Other herbivores and small predators:

arthropods, birds, and mammals. Pages 261-272 in C.J. Krebs, S. Boutin, and R.

Boonstra, editors. Ecosystem dynamics of the boreal forest: the Kluane project. .

Oxford Univsersity Press, Oxford.

Steele, M. A. 1998. Tamiasciurus hudsonicus. Mammalian Species 586:1-9.

46

Stinchcombe, J. R., A. F. Agrawal, P. A. Hohenlohe, S. J. Arnold, and M. W. Blows.

2008. Estimating nonlinear selection gradients using quadratic regression

coefficients: Double or nothing? Evolution 62:2435-2440.

Tomback, D. F. and K. A. Kramer. 1980. Limber pine seed harvest by Clark's Nutcracker

in the Sierra Nevada: Timing and foraging behavior. The Condor 82:467-468.

Vander Wall, S. B. and R. P. Balda. 1977. Coadaptations of the Clark's Nutcracker and

the Pinon pine for efficient seed harvest and dispersal. Ecological Monographs

47:89-111.

Waldron, R.M. 1965. Cone production and seedfall in a mature white spruce stand. The

Forestry Chronicle 41:314-321.

Zasada, J.C. and L.A. Viereck. 1970. White spruce cone and seed production in interior

Alaska, 1957-1968. U.S. Department of Agriculture, Forest Service, Pacific

Northwest Forest and Range Experiment Station Res. Note PNW-129 Portland,

Oregon, USA.

47

Tables

Table 1. Standardized directional (β‟) and stabilizing/disruptive (γ’) selection gradients

calculated for intra-annual reproductive synchrony and the number of cones produced by

white spruce during a non-mast (2009; n=212) and mast (2010; n=206) year. Separate

models were used to assess linear and non-linear selection gradients within each year.

Bold font indicates significance estimated with jackknife tests (Mitchell-Olds and Shaw

1987). Stabilizing/disruptive selection gradients have been doubled to accurately

represent their strength (Stinchcombe et al. 2008).

Year Selection

Gradient Model

Parameter

Estimate

SE

P value

Non-mast β' Synchrony index 0.32 0.12 0.007

Non-mast β' Cones produced 1.01 0.45 0.026

Non-mast γ' Synchrony index -0.18 0.10 0.388

Non-mast γ' Cones produced -0.12 0.38 0.864

Mast β' Synchrony index 0.06 0.05 0.216

Mast β' Cones produced 1.30 0.16 <0.001

Mast γ' Synchrony index -0.10 0.08 0.520

Mast γ' Cones produced 0.25 0.08 0.002

48

Figures

Figure 1. Seasonal decline in the number of closed cones on white spruce trees resulting

from red squirrel cone clipping activities as well as cone opening. Average number of

closed cones (closed triangles) and average number of open cones (open circles) per tree

in the study (non-mast; n=607, mast; n=292) plotted with the average observed and

modeled red squirrel cone clipping (dashed line) rate per day (non-mast; n=21, mast;

n=9) throughout the autumn of a non-mast (2009) and (2010) mast year. Cone count

rounds occurred over more than one day but are plotted on median date of each round.

Cone number values represent means ± SE and clipping activity values are means.

49

Connecting Statement

In this thesis I explore how reproductive timing influences red squirrel hoarding of white

spruce cones, from the perspective of both the trees and the red squirrels. In the first

chapter I showed that the degree of intra-annual reproductive synchrony exhibited by

individual white spruce trees was positively correlated with seed escape from red

squirrels, and was under positive directional selection in a low cone year. This furthers

the evidence of strong interactions between white spruce and their dominant pre-dispersal

seed predator, the red squirrel, and highlights the importance of the brief period of cone

availability to hoarding red squirrels. In the second chapter I explore the implications of

the varying separation of reproduction in red squirrels from this brief period of time that

they have available to hoard resources for overwinter survival and future reproduction.

50

Chapter 2: Reproductive and resource constraints on food hoarding in male

and female red squirrels.

Authors:

Devan W. Archibald (email: devan.archibald@mail.mcgill.ca)

Natural Resource Sciences, Macdonald campus, McGill University

21 111 Lakeshore Drive, Ste-Anne-de-Bellevue, QC, H9X 3V9, Canada

Andrew G. McAdam (email: macadam@uoguelph.ca)

Department of Integrative Biology, University of Guelph,

50 Stone Road East, Guelph, ON, N1G 2W1, Canada

Stan Boutin (email: stan.boutin@ualberta.ca)

Department of Biological Sciences, University of Alberta

Z1109 Biological Sciences Building, Edmonton, AB, T6G 2R3, Canada

Quinn Fletcher (email: quinn.fletcher2@mail.mcgill.ca)

Department of Biological Sciences, University of Alberta

Z1109 Biological Sciences Building, Edmonton, AB, T6G 2R3, Canada

Murray M. Humphries (email: murray.humphries@mcgill.ca)

Natural Resource Sciences, Macdonald campus, McGill University

21 111 Lakeshore Drive, Ste-Anne-de-Bellevue, QC, H9X 3V9, Canada

51

Abstract

When investment in accumulating energy stores is not adequately separated from

reproduction, costs associated with current reproduction could trade-off with the

acquisition of resources for future reproduction and survival. We quantified conifer cone

clipping and hoarding by male and female red squirrels (Tamiasciurus hudsonicus) in

four years with varying resource levels and seasonal separation of reproduction from

hoarding. There was a 50-fold increase in the availability of cones between the lowest

and highest cone years, with higher cone years associated with a later end to the breeding

season that extended into the cone-hoarding season. The total numbers of cones clipped

and hoarded were more related to cone production than reproductive timing. However,

females, who experienced peak reproductive demands approximately three months after

males, were more likely than males to pursue the low cost and low reward strategy of

clipping more cones than they hoarded and scatter-hoarding more cones than they larder-

hoarded. These differences between males and females were most pronounced in the

highest cone year, when females were lactating while hoarding and their time spent

hoarding increased from parturition to post-weaning. Nevertheless, even in this year,

females allocated as much time to hoarding activities as males while successfully

reproducing and accumulated sufficient cone reserves to survive winter. Our findings

demonstrate interacting effects of resource availability and reproductive timing on

hoarding behavior in red squirrels, and show how high resource availability reduces the

temporal separation and trade-offs between reproduction and capital accumulation.

52

Introduction

Trade-offs are at the core of ecological and evolutionary theory (Stearns 1989,

Lima and Dill 1990, Werner and Anholt 1993, Roff 2002, Kneitel and Chase 2004,

Ridley 2004). The basis of trade-offs are often presumed to be energy ; energy invested

in one activity cannot simultaneously be invested in another (eg. Brown et al. 2004). But,

trade-offs can also be generated by time allocation, because limited time allocated to one

activity comes at the expense of time allocated to another (Enright 1970, Halle and

Stenseth 2000). One way animals minimize trade-offs in time and energy allocation is to

organize activities seasonally such that investment in one activity does not overlap with

investment in another. For example, many animals accumulate energy reserves at one

time of the year and use these reserves as capital to support reproductive demands at

another time of year (Drent and Daan 1980, Stearns 1992, Jönsson 1997).

Opportunities for capital accumulation are often restricted to one time of the year,

due to annual cycles in resource availability (Jönsson 1997). Year-round residents of

high latitude regions may have particular difficulty in separating reproduction and capital

accumulation if both activities need to be accomplished during short growing seasons.

Furthermore, inadequate separation of the two events in time should generate a trade-off

between current reproduction and future survival and reproductive success.

Alternatively, if animals engaged in reproduction remain capable of capital accumulation,

this trade-off could be avoided or minimized. Most research on capital breeders has

focused on animals using endogenous lipid and protein stores, but many species use

exogenous stores of hoarded food (Jönsson 1997). Food hoarding is important to over-

winter survival in many species, and occasionally is used to fuel reproductive activity in

the following spring (Vander Wall 1990).

Female mammals experience their peak energetic demands during lactation,

whereas males experience their peak energetic demands during mating (Gittleman and

Thompson 1988). As a result, the generally higher energetic costs associated with

reproduction and its later completion in female mammals relative to male mammals

potentially allows females less time and energy for capital accumulation following

reproduction than males. North American red squirrels (Tamiasciurus hudsonicus) have

53

a highly promiscuous, scramble competition mating system with energy demands of

reproduction highest in males when most females are sexually receptive (Lane et al.

2010) and highest in females during late lactation (Humphries and Boutin 2000). Given a

33 day gestation period in this species (Steele 1998), and a lactation period lasting about

50 to 70 days (Layne 1954, Humphries and Boutin 1996), females experience peak

energy demands three or more months later than males. In addition to these gender

differences, there is extensive inter-annual variation in the timing of reproduction, with

Boutin et al. (2006) reporting average parturition dates ranging from late March to early

June.

In northern areas of their range red squirrels experience extreme resource-pulses,

where their main food resource, white spruce seed (Picea glauca), exhibits yearly

variation spanning three orders of magnitude (Boutin et al. 2006, LaMontagne and Boutin

2007). Red squirrels hoard up to 15,000 white spruce cones, which are produced in late

summer and autumn, by clipping cones from the tree tops, and then burying these clipped

cones as either scatter-hoards spread throughout their territory or as larder-hoards

concentrated in a central midden (Hurly and Lourie 1997, Fletcher et al. 2010). Burying

cones below the ground surface keeps cones moist, ensuring they remain closed and

retain their seeds (Smith 1968). In contrast, clipped cones that are left on the surface

usually open and lose their seeds, either in autumn before snow accumulation or in spring

after snow melt. Scatter-hoarding is generally presumed to require less in energy and

time than larder-hoarding (Vander Wall 1990, Clarke and Kramer 1994); but see, (Hurly

and Lourie 1997), whereas larder-hoarded cones are most defensible from pilferage

(Donald 2010) and provide the most concentrated (Hurly and Lourie 1997), easily-

accessed and safely consumed energy source during winter and reproduction (Smith

1968, Steele 1998, Boutin et al. 2006, McAdam et al. 2007). Overall, cone hoarding in

autumn has been shown to be as energetically demanding for red squirrels as mating in

males and lactation in females (Lane et al. 2010, Fletcher 2011), suggesting that it might

be incompatible with other energy and time demanding activities.

Here we test the hypothesis that reduced temporal separation between

reproduction and capital accumulation in a food-hoarding rodent compromises hoarding

performance. This hypothesis predicts that in years when hoarding and reproduction are

54

least separated, red squirrels will spend less time hoarding, total hoard accumulation will

be reduced, and/or squirrels will pursue low cost, low benefit strategies such as clipping

cones that are not immediately hoarded, and/or scatter-hoarding rather than larder-

hoarding. The alternative hypothesis is that reproduction and hoarding only overlap

under high resource circumstances when they can be simultaneously accomplished

without demonstrable costs. Because females experience peak reproductive demands

much later in the season than males, the alternative hypothesis predicts that females

would spend less time hoarding, accumulate smaller hoard sizes, and be more likely to

clip but not hoard or scatter-hoard than males. To test these predictions, we quantified

the total number of cones clipped, hoarded, and the propensity to larder-hoard in male

and female red squirrels in four years with varying resource levels and seasonal

separation of reproduction from hoarding.

Methods

Study area

The study was conducted on two study areas (approximately 40ha each) located

near Kluane National Park in southwestern Yukon, Canada (61°N, 138°W). The study

areas were located in a glacial valley composed of boreal forest dominated by white

spruce with a willow (Salix spp.) understory. Red squirrels are present at an average

density of 1.5-2.8 squirrels/ha (Boonstra et al. 2001). For a more detailed description of

the general ecology of the study site see Krebs, Boutin, and Boonstra (2001).

Red squirrel populations and white spruce cone production have been monitored

continuously since 1988 (Boutin et al. 2006, LaMontagne and Boutin 2007, McAdam et

al. 2007). Red squirrel population monitoring covered the entire reproductive season,

typically starting in early March and ending in late August, facilitated by approximately

bi-weekly live-trapping and handling of individuals (McAdam et al. 2007). Every

summer the number of cones produced by white spruce trees of cone bearing age was

counted in a consistent manner before red squirrel cone hoarding behavior started

(LaMontagne et al. 2005). Study areas were staked every 30 meters and lines flagged to

55

create a grid allowing the recording of spatial locations of field observations with a 3m

resolution (Boutin et al. 2006, McAdam et al. 2007).

Hoarding behavioral observations

Behavioral data from autumn 2010 were combined with data from 2002, 2003,

and 2005, which were included in the analysis of the functional response of red squirrels

to white spruce cone levels presented by Fletcher et al. (2010). The dates of observations

and sample sizes varied among years (autumn 2002, 20 adult females and 8 adult males

monitored Sep 1-Oct 8; autumn 2003, 15 adult females and 17 adult males monitored

Aug 20-Oct 14; autumn 2005, 12 adult females and 9 adult males monitored Aug 28-Sep

24; autumn 2010, 13 adult females and 10 adult males monitored Aug 22-Sep 30).

Red squirrels were live trapped in Tomahawk traps baited with peanut butter and

each animal was marked with ear tags (Monel #1). Colored wires or pipe cleaners were

threaded through the ear tags in unique combinations to allow individual identification in

the field. Animals were fitted with radio collars (Model PD-2C, 4g, Holohil Systems

Limited, Carp, Ontario, Canada) to allow individuals to be reliably located. Each

individual received three focal observation periods per day three days per week. Red

squirrels are diurnal, so observation periods were spaced evenly throughout the day.

Individuals were monitored along the most efficient visitation circuit that minimized

distances between territories, but with starting locations and directions varying randomly

between days. During a focal observation period squirrels were located with radio

telemetry and their behavior was monitored for seven minutes. During the focal

observation period we recorded the number and location of all cones clipped from trees,

and all items hoarded.

In 2010, females were concurrently reproductively active while hoarding,

allowing a direct investigation of the impacts of seasonal overlap of reproduction with

time allocated to hoarding activities. In this year we altered our methodology slightly to

obtain more detailed observations. In addition to the methodology described above

(Fletcher et al. 2010), we also employed an instantaneous sampling approach (Dantzer et

al. 2011) recording the behaviors that squirrels were engaged in every 30 seconds.

Specifically, these behaviors included whether the focal squirrel was in a nest, travelling,

56

foraging, feeding, resting, vigilant, grooming, or out of site (Stuart Smith and Boutin

1995). In 2010 we also increased the number of focal observation periods per day from

three to four.

Estimation of total number of cones clipped, hoarded, and the propensity to larder-

hoard

As in Fletcher et al.(2010), we estimated the total number of cones clipped and

hoarded by each squirrel on a given day based on its average clipping or hoarding rate per

minute of focal observation, multiplied by the number of minutes between sunrise and

sunset (61°N, 138°W; Herzberg Institute of Astrophysics; National Research Council of

Canada). In this analysis, hoarding included both scatter- and larder-hoarding. In order

to obtain seasonal estimates of total cones hoarded and total cones clipped, we used the

shape-preserving piecewise cubic modeling technique (Fritsch and Carlson 1980)

employed by Fletcher et al. (2010) to estimate hoarding activities on un-observed days

between August 15th

and October 15th

. The seasonal totals were equal to the sum of the

observed and modeled number of cones hoarded or clipped per day, but we limited our

analysis to only red squirrels monitored throughout the whole sampling period.

Larder-hoarding propensity was quantified as the proportion of cone hoarding

events observed in a season that were brought to the central larder (Jenkins and Breck

1998). Hurly and Lourie (1997) found a strong correspondence between the estimated

proportion of cones scatter or larder-hoarded by red squirrels recorded by behavioral and

direct-sampling methods (quadrats placed throughout territories), indicating behavioral

estimates provide a reliable index of hoarding behavior. We limited this analysis to red

squirrels with at least 10 observed caching events (n= 67 squirrels, median number of

events observed = 65).

Reproductive timing

The seasonal timing of reproduction relative to the food hoarding period was

assessed based on the date each individual was last observed reproductively active in

each year. For females that successfully weaned a litter the date last reproductively

active was defined as 70 days postpartum, the time at which red squirrels wean their

57

young (Layne 1954, Humphries and Boutin 1996). Parturition dates were estimated from

the weight of the pups when nests were entered shortly after the female gave birth to her

litter (McAdam et al. 2007), and successful completion of reproduction was confirmed

with post-weaning trapping or observation of juveniles. When biweekly trapping

confirmed the mother had lost her litter before weaning, we estimated the date last

reproductively active as the median date between when a female was last trapped

lactating and first trapped non-lactating. For males, the date last reproductively active

was defined as the date of testes ascension, estimated as the median date between when a

male was last trapped with testes scored as scrotal and the first day trapped with testes

scored as abdominal (empty scrotum).

All animal use procedures were approved by McGill University Faculty of

Agricultural and Environmental Sciences Animal Care Committee.

Statistical analysis

The degree of seasonal separation between reproduction and food hoarding is

likely to be affected by both current and past white spruce cone levels. Female red

squirrels anticipate upcoming large cone crops in the current year by producing a second

litter just prior to new cone availability (Boutin et al. 2006), contributing to overlap in

reproduction and hoarding. Large cone crops in the previous year are correlated with

earlier parturition dates the following spring (Boutin et al. 2006), contributing to reduced

overlap with the hoarding period. Because an autumn with a large cone crop is invariably

followed by an autumn with a small cone crop (LaMontagne and Boutin 2007), high

overlap of reproduction and hoarding typically occurs in years of high cone production,

whereas low overlap between reproduction and hoarding typically occurs in the

subsequent low-cone year. Because of this close correspondence between resource levels

and the amount of separation between reproduction and hoarding, we did not attempt to

separate the unique effects of cone production and reproductive timing on hoarding

behavior. Instead, we focused on differences among years, combined with qualitative

assessments of each year‟s cone levels (lowest, low, medium, high) and date of

reproductive completion (early, middle, late).

58

To determine if there were differences in (1) total cones clipped, (2) total cones

hoarded, and (3) the proportion of cones larder-hoarded among years and between

genders we used three separate generalized linear models (GLM). Total cones clipped

and hoarded were each evaluated with quasi-Poisson errors to account for over-dispersion

(Bolker 2008, O‟Hara and Kotze 2010). The proportion of cones larder-hoarded was

evaluated with quasi-binomial errors to account for over-dispersion (Bolker 2008). Each

of these models included the effect of year, sex, and their interaction. In all of our GLMs

we tested for the significance of year, sex, and their interaction using an using an F-test

based on the ratio of deviances (Venables and Ripley 2002, Bolker 2008). When

significant effects of year were found we investigated which years differed by conducting

post-hoc Tukey tests with Bonferroni corrected p-values (Bretz et al. 2011) using the

“glht” function in the “multcomp” package in the statistical software R (Hothorn et al.

2008).

To examine how directly overlapping reproduction and hoarding affected the time

budget of females in autumn of 2010, we modeled: (1) the proportions of time feeding,

(2) in the nest, and (3) devoted to cone hoarding-related activities (clipping cones,

hoarding cones, travelling with cones) in each focal observation period as a function of

days postpartum using three separate generalized linear mixed effects models (GLMM).

Because red squirrel hoarding activity rates increase to a peak and then decline (Fletcher

et al. 2010), we first determined the appropriate effect of the date on which the behavior

was sampled (days since January 1st; hereafter Julian date) without other factors included

in the model. We tested for non-linearity using a quadratic term for Julian date

(hereafter, Julian date2). If there was a significant effect of Julian date, it was included in

the model. We fit the models with quasi-binomial errors (logit link, models fit with

Penalized Quasi-Likelihood) due to over-dispersion (Bolker et al. 2009). Since we

expected maternal behavior to vary non-linearly throughout the lactation period (see

Dantzer et al. 2011), we also tested a quadratic term for days postpartum (hereafter, days

postpartum2) in our models. We then investigated differences between female and male

red squirrels in the proportion of time devoted to cone hoarding-related activities. In this

case, hoarding activity was predicted by sex and Julian date using a fourth GLMM fit as

above, but using all male and female focal data combined. To control for repeated

59

observations on the same individuals we included a random intercept term for squirrel

identity in all of our models using the “glmmPQL” function in the “MASS” package

(Venables and Ripley 2002) in the statistical software R (R Development Core Team

2011). The choice to use penalized quasi-likelihood (PQL) was made following Bolker

et al. (2009), since for each focal observation bout our data had an expected number of

successes and failures greater than five (fifteen 30-second intervals). We tested the

significance of our fixed effects using Wald t tests (Bolker et al. 2009). However, since

we used PQL which is not true maximum likelihood, we could not use likelihood ratio

tests to test the significance of our random factors (Bolker et al. 2009) and therefore do

not report these results.

All statistical analysis were conducted using the statistical software R (R

Development Core Team 2011) with an alpha level of 0.05, and means ± standard error

are reported.

Results

Reproductive completion and resource levels

The estimated dates of reproductive completion varied considerably among years,

as did cone availability (Table 1). 2002 and 2003 had low cone levels and well separated

reproduction and hoarding, while 2005 had limited separation with moderate cone levels.

In contrast, 2010 had high cone levels and direct overlap in reproductive activity and

hoarding among females, but little to no overlap among males.

Total number of cones clipped and hoarded

The median and maximum rate of cone clipping (median 2.3 cones/min; max 50

cones/min, n=430 focals in which clipping was observed) far exceeded the median and

maximum rate of cone hoarding (median 0.3 cones/min; max 4.3 cones/min, n=1411

focals in which hoarding was observed).

The total number of cones clipped by red squirrels differed significantly among

years (Figure 1A, F3,93 = 37.5, p<0.001), ranging from 1201 ± 466 cones clipped in 2003

(n=32 squirrels) to 38,046 ± 4914 cones clipped in 2010 (n=21 squirrels). More cones

60

were clipped in the small mast (2005) and large mast (2010) years compared to the lower

cone years with early reproductive completion, despite a lower separation of reproduction

and hoarding in the mast years (Figure 1A). On average, female red squirrels clipped 2.2

times as many cones as males (Figure 1A; F1,92 = 9.20, p = 0.003) and this gender

difference was consistent across years (sex x year interaction: F3,89 = 1.84, p=0.15).

The total number of cones hoarded (scatter plus larder-hoarded) by red squirrels

differed significantly among years (Figure 1B, F3, 93 = 24.7, p<0.001), ranging from 860 ±

194 cones hoarded in 2003 (n=32 squirrels) to 8050 ± 977 cones hoarded in 2002 (n=27

squirrels). Red squirrels hoarded significantly fewer cones in the lowest cone year (2003)

than all other years, which did not differ significantly (Figure 1B). On average, male red

squirrels hoarded 1.5 times as many cones as females (Figure 1B, F1,92 = 7.53, p=0.007),

this gender difference was consistent across years (sex x year interaction: F3,89 = 0.58,

p=0.63).

Propensity to larder-hoard

The proportion of cones that we observed being hoarded that were taken to the

central-larder differed significantly among years (Figure 1C, F3, 63 = 17.4, p<0.001),

varying from 25% larder-hoarded in 2010 (i.e. 75% scatter-hoarded) to 75% larder-

hoarded in 2002. Red squirrels larder-hoarded the lowest proportion of cones when cone

levels were highest and reproduction was latest (2010), but also larder-hoarded

proportionately small amounts when cone levels were lowest and reproduction earliest

(2003; 37%). On average, the proportion of cones larder-hoarded by males was 1.5 times

higher than females (Figure 1C, F1,62 = 4.25, p=0.044), and this gender difference was

consistent across years (sex x year interaction: F3,59 = 1.21, p=0.31).

Hoarding time allocation during concurrent reproductive activity

In 2010, the year of extensive overlap of reproduction and hoarding in females

(see Table 1), female time allocated to feeding increased linearly throughout the autumn

(Julian date = 0.02 ± 0.006, Wald t1,648 = 3.62, p<0.001) and was not a quadratic function

of Julian date (Julian date2 = -0.0009 ± 0.0006, Wald t1,647 = -1.41, p=0.16, Julian date =

0.47 ± 0.32, Wald t1,647 = 1.48, p=0.14). Time spent feeding was initially low close to

61

parturition, increased, and then declined again to lower levels (Figure 2A ), as indicated

by a significant negative quadratic function of days postpartum (Days postpartum2 = -

0.0005 ± 0.0002, Wald t1,646 = -2.74, p=0.006, Days postpartum = 0.04 ± 0.02, Wald t1,646

= 2.20, p=0.03) with the seasonal effect of Julian date accounted for (Julian date = 0.02 ±

0.01, Wald t1,646 = 1.73, p=0.08).

Female time spent in the nest decreased linearly with throughout the autumn

(Julian date = -0.06 ± 0.01, Wald t1,648 = 0.01, p<0.001) and was not a quadratic function

of Julian date (Julian date2 = 0.001 ± 0.001, Wald t1,647 = 1.02, p=0.31, Julian date = -0.66

± 0.59, Wald t1,647 = -1.12, p=0.26). Time spent in the nest was initially high close to

parturition, decreased, and then increased slightly again (Figure 2B), as indicated by a

significant positive quadratic function of days postpartum (Days postpartum2 = 0.0009 ±

0.0003, Wald t1,646 = 3.24, p=0.001, Days postpartum = -0.10 ± 0.03, Wald t1,646 = -4.12,

p<0.001) with the seasonal effect of Julian date accounted for (Julian date = -0.02 ± 0.02,

Wald t1,646 = -1.07, p=0.29).

Female time allocated to hoarding-related activities increased, peaked, and then

declined throughout the autumn as evidenced by a significant negative quadratic function

of Julian date (Julian date2 = -0.006 ± 0.001, Wald t1,647 = -5.47, p<0.001, Julian date =

3.18 ± 0.58, Wald t1,647 = 5.45, p<0.001). Time allocated to hoarding-related activities

increased further from parturition (Figure 2C), as indicated by a significant linear term

for days postpartum (Days postpartum = 0.02 ± 0.007, Wald t1,646 = 2.48, p=0.013) with

the seasonal effect of Julian date2 accounted for (Julian date

2 = -0.006 ± 0.001, Wald t1,646

= -5.39, p<0.001, Julian date = 3.13 ± 0.59, Wald t1,646 = 5.34, p<0.001), and was not a

quadratic function of days postpartum (Days postpartum2 = 0.0003 ± 0.0003, Wald t1,645

= 0.95, p=0.34, Days postpartum = -0.006 ± 0.026, Wald t1,645 = -0.22, p=0.82) with the

seasonal effect of Julian date2 accounted for (Julian date

2 = -0.007 ± 0.001, Wald t1,645 = -

5.42, p<0.001, Julian date = 3.29 ± 0.61, Wald t1,645 = 5.36, p<0.001).

During this year of extensive overlap of reproduction and hoarding in females but

little to no overlap in males (2010; see Table 1), with both genders included in the model

time allocated to hoarding-related activities was still a negative quadratic function of

Julian date (Figure 3, Julian date2 = -0.007 ± 0.0008, Wald t1,1194 = -8.00, p<0.001, Julian

date = 3.30 ± 0.42, Wald t1,1194 = 7.96, p<0.001). The genders did not differ significantly

62

in the proportion of time allocated to cone hoarding-related activities (sex = -0.12 ± 0.21,

Wald t1,1193 = -0.57, p=0.57), with the effect of Julian date2 accounted for (Julian date

2 = -

0.007 ± 0.0008, Wald t1,1193 = -8.01, p<0.001, Julian date = 3.31 ± 0.41, Wald t1,1193 =

7.97, p<0.001).

Discussion

In this study, we examined the effects of varying separation of reproduction and

hoarding, two energetically demanding activities for red squirrels (Lane et al. 2010,

Fletcher 2011), across four years of varying resource availability. The total number of

cones clipped was more affected by cone production than reproductive timing, with more

cones clipped when more cones were available regardless of reproductive timing. The

total number of cones hoarded was less directly related to cone production, with some

evidence of an interacting influence of reproductive timing. The number of cones

hoarded was lowest in the year with the lowest cone availability, but did not differ

significantly among the three remaining years despite substantial variation in cone

production (Figure 1). In these years, cones hoarded tended to vary according to

reproductive timing, with fewer cones hoarded when reproduction occurred later.

Gender differences in hoarding behavior were consistent with the predicted

consequences of females experiencing peak reproductive demands much later in the year

than males (Humphries and Boutin 2000, Lane et al. 2010). In particular, females tended

to pursue lower cost-lower benefit hoarding strategies than males. Females consistently

clipped more cones than males, whereas males consistently hoarded more cones than

females. Furthermore, the ratio of larder-hoards to scatter-hoards was higher in males

than females. This combination of females hoarding less and scatter-hoarding more will

lead to much smaller larder-hoards in females than males, which has been confirmed by

two studies that counted and compared the central-larder food stores of male and female

red squirrels (Gerhardt 2005, Donald 2010). The gender differences in hoarding behavior

that we observed persisted across all years, as indicated by non-significant year by gender

interactions, but the magnitude of the difference in the lower cost-lower benefit hoarding

63

strategies tended to increase when reproduction and hoarding over-lapped more (Figure

1).

An additional way in which differences in reproductive timing could promote

gender-differences in hoarding behavior involves the timing of hoard recovery. Recovery

of scatter-hoarded and clipped but not hoarded cones will become easier and less

energetically costly when the snow melts in spring. For example, in spring 2011

following the large mast of 2010, females were observed consuming and hoarding cones

clipped but un-hoarded the previous autumn, which were exposed by melting snow (S.

Boutin, A.G. McAdam, M.M. Humphries, E. Studd, unpublished data). Given that males

experience peak reproductive demands in late winter and early spring when snow cover

remains extensive, whereas females typically experience peak energy demands in late

spring and early summer when snow cover is reduced or gone, clipped and scatter-

hoarded cones are likely to have higher energetic value for reproducing females than

reproducing males. Nevertheless, cones hoarded within the central-larder will be of

much higher energetic value for both genders at all times of the year, given their

centrality, defensibility, and low perishability. The number of cones accumulated in the

central-larder is an important determinant of over-winter survival (Larivée et al. 2010)

and is potentially very important to subsequent reproductive success. Thus, the observed

gender and year differences in larder-hoard accumulation are likely to have important life

history and demographic consequences.

Two other studies of gender-specific hoarding behavior also attributed hoarding

variation to differing reproductive roles (Clarke and Kramer 1994, Jenkins In press).

Clarke and Kramer (1994) argued that female eastern chipmunks (Tamias striatus) were

more likely to scatter-hoard to avoid hoard depletion from young present in their burrow.

Juvenile red squirrels remain near their natal territory until they establish their own

territory (Larsen and Boutin 1994), so it is possible that female red squirrels may scatter-

hoard more to avoid depletion of their central-larder by their young. Bequeathal of part

or all of a territory also occurs in red squirrels (Price 1992, Boutin et al. 1993, Price and

Boutin 1993, Boutin et al. 2000), so having cones scattered throughout the territory, a

portion of which may eventually be acquired by offspring, could be a form of parental

investment. However, juvenile dispersal and territory settlement usually occurs prior to

64

the start of the hoarding season (Boutin et al. 1993). Therefore, assuming that the

majority of a female‟s hoard is consumed over the course a single year, these offspring

avoidance-investment arguments only apply to one year of this study (2010) when

hoarding preceded juvenile settlement. Thus, the occurrence of gender differences in

hoarding behavior that persisted across all study years is inconsistent with these

alternative offspring avoidance or parental investment explanations.

Despite finding general support for the hypothesis that low separation of

reproduction from hoarding affects the total number of cones hoarded, our results also

confirms that reproduction and hoarding can be accomplished simultaneously, at least

under high resource conditions. In 2010, when cone production was high and females

were lactating throughout most of the hoarding season, females spent as much time

hoarding as males and differences in the number of cones clipped and hoarded by males

and females were not notably larger than in other years. Although lactating females were

characterized by reduced time spent hoarding shortly after their litters were born, time

allocated to hoarding increased as their litters aged, allowing them to make-up for lost

time later in the hoarding season. All of the females for which we sampled behavior

survived the winter (S. Boutin, A.G. McAdam, M.M. Humphries, unpublished data),

indicating they accumulated the cone hoards necessary for over-winter survival.

Furthermore, their reproductive performance did not appear to be compromised; growth

rates of pups raised during the hoarding season (1.85 ± 0.08 g/day) of the sampled

females, quantified using methodology described in McAdam et al. (2007), were similar

to other years (1.46-2.05 g/day; Humphries and Boutin 2000), and each female had at

least one of their offspring recruit into the population in spring 2011 (S. Boutin, A.G.

McAdam, M.M. Humphries, unpublished data).

The very high cone production in 2010 was likely a key contributor to the ability

of females to reconcile the competing demands of reproduction and hoarding. This

capacity to sustain overlapping demands allows red squirrels flexibility in the separation

of reproduction and hoarding in high cone years, and facilitates their ability to produce

second litters in late summer when they anticipate large cone crops in advance of their

availability (Boutin et al. 2006). Resource availability affects the phenological timing of

energy demanding activities in many animals. For example, the timing of reproduction is

65

among the most responsive of traits to experimental food supplementation in vertebrates

(Boutin 1990) and some birds respond to food supplementation by molting sooner after

reproduction (Siikamäki 1998). Together with previous literature, our results highlight

the extent of gender variation in hoarding behavior and how resource-availability can

mediate seasonal separation and trade-offs between reproduction and capital

accumulation (van Noordwijk and de Jong 1986, Dunham et al. 1989, Beilharz and Nitter

1998, Reznick et al. 2000, Boggs 2009).

66

Literature Cited

Beilharz, R. G. and G. Nitter. 1998. The missing E: the role of the environment in

evolution and animal breeding. Journal of Animal Breeding and Genetics

115:439-453.

Boggs, C. L. 2009. Understanding insect life histories and senescence through a resource

allocation lens. Functional Ecology 23:27-37.

Bolker, B. M. 2008. Ecological models and data in R. Princeton University Press,

Princeton, N.J.

Bolker, B. M., M. E. Brooks, C. J. Clark, S. W. Geange, J. R. Poulsen, M. H. H. Stevens,

and J.-S. S. White. 2009. Generalized linear mixed models: a practical guide for

ecology and evolution. Trends in Ecology & Evolution 24:127-135.

Boonstra, R., S. Boutin, A. Byrom, T. Karels, A. Hubbs, K. Stuart-Smith, M. Blower, and

S. Antpoehler. 2001. The role of red squirrels and arctic ground squirrels. Pages

179-214 in C.J. Krebs, S. Boutin, and R. Boonstra, editors. Ecosystem dynamics

of the boreal forest: the Kluane project Oxford University Press, Oxford.

Boutin, S. 1990. Food supplementation experiments with terrestrial vertebrates: patterns,

problems, and the future. Canadian Journal of Zoology 68:203-220.

Boutin, S., K. W. Larsen, and D. Berteaux. 2000. Anticipatory parental care: acquiring

resources for offspring prior to conception. Proceedings of the Royal Society of

London Series B-Biological Sciences 267:2081-2085.

Boutin, s., Z. Tooze, and K. Price. 1993. Post-breeding dispersal by female red squirrels

(Tamiasciurus hudsonicus): the effect of local vacancies. Behavioral Ecology

4:151-155.

Boutin, S., L. A. Wauters, A. G. McAdam, M. M. Humphries, G. Tosi, and A. A.

Dhondt. 2006. Anticipatory reproduction and population growth in seed predators.

Science 314:1928-1930.

Bretz, F., T. Hothorn, and P. Westfall. 2011. Multiple Comparisons Using R. Chapman &

Hall/CRC Taylor & Francis Group, Boca Raton, FL, USA.

Brown, J. H., J. F. Gillooly, A. P. Allen, V. M. Savage, and G. B. West. 2004. Toward a

metabolic theory of ecology. Ecology 85:1771-1789.

67

Clarke, M. F. and D. L. Kramer. 1994. Scatter-hoarding by a larder-hoarding rodent:

intraspecific variation in the hoarding behaviour of the eastern chipmunk, Tamias

striatus. Animal Behaviour 48:299-308.

Dantzer, B., A. G. McAdam, R. Palme, M. M. Humphries, S. Boutin, and R. Boonstra.

2011. Maternal androgens and behaviour in free-ranging North American red

squirrels. Animal Behaviour In press.

Donald, J. L. 2010. Intraspecific cache pilferage by larder-hoarding red squirrels

(Tamiasciurus hudsonicus) in Kluane, Yukon. M.Sc. Thesis. University of

Alberta, Edmonton.

Drent, R. H. and S. Daan. 1980. The prudent parent: Energetic adjustments in avian

breeding. Ardea 68:225-252.

Dunham, A. E., B. W. Grant, and K. L. Overall. 1989. Interfaces between biophysical

and physiological ecology and the population ecology of terrestrial vertebrate

ectotherms. Physiological Zoology 62:335-355.

Enright, J. T. 1970. Ecological aspects of endogenous rhythmicity. Annual Review of

Ecology and Systematics 1:221-238.

Fletcher, Q. E., S. Boutin, J. E. Lane, J. M. LaMontagne, A. G. McAdam, C. J. Krebs,

and M. M. Humphries. 2010. The functional response of a hoarding seed predator

to mast seeding. Ecology 91:2673-2683.

Fletcher, Q. F. 2011. The role of energy expenditure in resource acquisition and energy

allocation in free-ranging North American red squirrels (Tamiasciurus

hudsonicus). Ph.D. Thesis. McGill University, Montreal.

Fritsch, F. N. and R. E. Carlson. 1980. Monotone piecewise cubic interpolation. SIAM

Journal on Numerical Analysis 17:238-246.

Gerhardt, F. 2005. Food pilfering in larder-hoarding red squirrels (Tamiasciurus

hudsonicus). Journal of Mammalogy 86:108-114.

Gittleman, J. L. and S. D. Thompson. 1988. Energy allocation in mammalian

reproduction. American Zoologist 28:863-875.

Halle, S. and N. C. Stenseth. 2000. Activity patterns in small mammals: An ecological

approach. Springer, Berlin; New York.

68

Hothorn, T., F. Bretz, and P. Westfall. 2008. Simultaneous inference in general

parametric models. Biometrical Journal 50:346-363.

Humphries, M. M. and S. Boutin. 1996. Reproductive demands and mass gains: A

paradox in female red squirrels (Tamiasciurus hudsonicus). Journal of Animal

Ecology 65:332-338.

Humphries, M. M. and S. Boutin. 2000. The determinants of optimal litter size in free-

ranging red squirrels. Ecology 81:2867-2877.

Hurly, T. A. and S. A. Lourie. 1997. Scatterhoarding and larderhoarding by red squirrels:

Size, dispersion, and allocation of hoards. Journal of Mammalogy 78:529-537.

Jenkins, S. H. In press. Sex differences in repeatability of food-hoarding behaviour of

kangaroo rats. Animal Behaviour In Press, Corrected Proof.

Jenkins, S. H. and S. W. Breck. 1998. Differences in food hoarding among six species of

Heteromyid rodents. Journal of Mammalogy 79:1221-1233.

Jönsson, K. I. 1997. Capital and income breeding as alternative tactics of resource use in

reproduction. Oikos 78:57-66.

Kneitel, J. M. and J. M. Chase. 2004. Trade-offs in community ecology: linking spatial

scales and species coexistence. Ecology Letters 7:69-80.

Krebs, C., S. Boutin, and R. Boonstra, editors. 2001. Ecosystem dynamics of the boreal

forest: the Kluane project Oxford University Press, Oxford.

LaMontagne, J. M. and S. Boutin. 2007. Local-scale synchrony and variability in mast

seed production patterns of Picea glauca. Journal of Ecology 95:991-1000.

LaMontagne, J. M., S. Peters, and S. Boutin. 2005. A visual index for estimating cone

production for individual white spruce trees. Canadian Journal of Forest Research

35:3020-3026.

Lane, J. E., S. Boutin, J. R. Speakman, and M. M. Humphries. 2010. Energetic costs of

male reproduction in a scramble competition mating system. Journal of Animal

Ecology 79:27-34.

Larivée, M. L., S. Boutin, J. R. Speakman, A. G. McAdam, and M. M. Humphries. 2010.

Associations between over-winter survival and resting metabolic rate in juvenile

North American red squirrels. Functional Ecology 24:597-607.

69

Larsen, K. W. and S. Boutin. 1994. Movements, survival, and settlement of red squirrel

(Tamiasciurus hudsonicus) offspring. Ecology 75:214-223.

Layne, J. N. 1954. The biology of the red squirrel, Tamiasciurus hudsonicus-Loquax

(Bangs), in central New York. Ecological Monographs 24:227-267.

Lima, S. L. and L. M. Dill. 1990. Behavioral decisions made under the risk of predation:

a review and prospectus. Canadian Journal of Zoology 68:619-640.

McAdam, A. G., S. Boutin, A. K. Sykes, and M. M. Humphries. 2007. Life histories of

female red squirrels and their contributions to population growth and lifetime

fitness. Ecoscience 14:362-369.

O‟Hara, R. B. and D. J. Kotze. 2010. Do not log-transform count data. Methods in

Ecology and Evolution 1:118-122.

Price, K. 1992. Territorial bequeathal by red squirrel mothers: A dynamic model. Bulletin

of Mathematical Biology 54:335-354.

Price, K. and S. Boutin. 1993. Territorial bequeathal by red squirrel mothers. Behavioral

Ecology 4:144-150.

R Development Core Team. 2011. R: A language and environment forstatistical

computing in R Foundation for Statistical Computing, Vienna, Austria.

Reznick, D., L. Nunney, and A. Tessier. 2000. Big houses, big cars, superfleas and the

costs of reproduction. Trends in Ecology & Evolution 15:421-425.

Ridley, M. 2004. Evolution. Oxford University Press, Oxford; New York.

Roff, D. A. 2002. Life history evolution. Sinauer Associates, Sunderland, Mass.

Siikamäki, P. 1998. Limitation of reproductive success by food availability and breeding

time in Pied Flycatchers. Ecology 79:1789-1796.

Smith, M. C. 1968. Red squirrel responses to Spruce cone failure in interior Alaska. The

Journal of Wildlife Management 32:305-317.

Stearns, S. C. 1989. Trade-Offs in life-history evolution. Functional Ecology 3:259-268.

Stearns, S. C. 1992. The evolution of life histories. Oxford Univ. Press, Oxford.

Steele, M. A. 1998. Tamiasciurus hudsonicus. Mammalian Species 586:1-9.

Stuart-Smith, A. K. and S. Boutin. 1995. Behavioural differences between surviving and

depredated juvenille red squirrels. Ecoscience 2: 34-40.

70

van Noordwijk, A. J. and G. de Jong. 1986. Acquisition and allocation of resources: Their

influence on variation in life history tactics. The American Naturalist 128:137-

142.

Vander Wall, S. B. 1990. Food hoarding in animals. University of Chicago Press,

Chicago.

Venables, W. N. and B. D. Ripley. 2002. Modern applied statistics with S. Springer, New

York.

Werner, E. E. and B. R. Anholt. 1993. Ecological consequences of the trade-off between

growth and mortality rates mediated by foraging activity. The American

Naturalist 142:242-272.

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Tables

Table 1. Range and median date of reproductive completion for adult red squirrels included in the study in 2002, 2003, 2005, and 2010

with white spruce average cone index (ln transformed cone count; mean ± SE) calculated from annual cone counts conducted prior to

red squirrel harvesting (n = 167-171 trees per year). Days until hoarding season was calculated by subtracting the median date last

reproductively active from August 16th

, the approximate date when hoarding activity initiates (Fletcher et al. 2010).

Females Males

Year

Cone

Index

Reproductive

Completion

Qualitative

Classification

Days

Until

Hoarding

Season

Date of

Reproductive Completion

Days

Until

Hoarding

Season

Date of

Reproductive Completion

Median

Range

Median

Range

2002 1.9 ± 0.1 Early 37 July 12 May 13 – Aug 15 52 June 29 May 30 – July 30

2003 0.9 ± 0.1 Early 34 July 13 May 6 – Sep 18 30 July 17 June 23 – Aug 8

2005 3.4 ± 0.2 Middle 13 Aug 4 June 30 – Sep 21 11 Aug 5 July 27 – Sep 6

2010 4.9 ± 0.1 Late -63 Oct 18 Sep 13 – Nov 3 -4 Aug 20 July 31 – Sep 3

72

Figures

Figure 1. Total number of cones clipped (A), hoarded (B) and the proportion of hoarded

cones that were larder-hoarded (C) by adult male and female red squirrels over four study

years with varying cone availability and separation of reproduction and hoarding. All

models contained significant sex and year effects with different letters indicating

significant differences found between years using post hoc testing. None of the models

contained significant interactions between year and sex. Values are means ± SE.

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Figure 2. The proportion of time adult female red squirrels spent feeding (A) and in the

nest (B) varied as a quadratic function of days postpartum, but time spent conducting

cone hoarding-related activities (C) varied linearly with days postpartum during autumn

2010, after accounting for the appropriate fit of Julian date (time spent feeding; linear,

time spent in the nest; linear, time spent hoarding; quadratic). Parturition dates ranged

from Jul 12 to Aug 24 (median; Aug 9). Data were analyzed using generalized linear

mixed models with squirrel identity as a random factor, but plotted values are raw data

representing means ± SE for each day postpartum.

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Figure 3. The proportion of time adult female (closed circles) and male (open squares)

red squirrels spent conducting cone hoarding-related activities throughout autumn 2010

varied non-linearly with Julian date and was not significantly different between the sexes.

The dashed grey line indicates the proportion of study females that were yet to pass the

mid-point of lactation (proportion less than 45 days postpartum). Data were analyzed

using a generalized linear mixed model with squirrel identity as a random factor, but

plotted values are raw data representing means ± SE for each day.

75

General Conclusions

This thesis evaluated how the seasonal timing of reproduction influenced red

squirrel hoarding of white spruce cones, from the perspective of both the trees (Chapter

1) and the squirrels (Chapter 2).

In the trees we found evidence that the degree of intra-annual reproductive

synchrony exhibited by individual white spruce trees relative to others in the population

enhanced seed escape from red squirrels in both a mast and non-mast year, and that this

trait was under strong selection in the lower cone year. This indicates masting species

can employ a mixture of strategies to avoid seed predation, both escape in numbers and

time, but that the relative importance of these strategies varies with levels of seed

production. This second aspect of reproductive synchrony, intra-annual synchrony, has

rarely been addressed in masting species (Rathcke and Lacey 1985). Selection on intra-

annual reproductive synchrony is likely imposed by a combination of many members of

the seed predator community. Post-dispersal seed predation can also be severe (Janzen

1971), and if intra-annual reproductive synchrony is under selection from seed predators

they will likely have an influence as well. Other studies should examine the importance

of intra-annual reproductive synchrony in other masting plants, how it varies with

variation in seed production, and the community of seed predators.

In red squirrels, our findings suggest that cone hoarding behaviour is more

affected by annual cone levels than reproductive timing, and in years with high cone

production both activities can successfully be combined despite their associated high

energetic costs (Lane et al. 2010, Fletcher 2011). Since both reproduction and hoarding

can be accomplished by red squirrels, their ability to anticipate large cone crops in

advance of their availability by producing second litters in late summer (Boutin et al.

2006) does not appear to compromise their ability to simultaneously establish a food

hoard necessary for future survival and reproduction. The compatibility of reproduction

and hoarding under high resource conditions provides red squirrels considerable

flexibility in reproductive timing relative to the autumn hoarding period, ranging from

multi-month separation in a year of low cone production preceded by high cone

production to almost complete overlap in a year of high cone production preceded by low

76

cone production. However, we also found that males and females used different hoarding

strategies that were consistent with differences in the timing of reproductive completion,

indicating that although overall hoarding behaviour was driven by resource levels, the

timing of reproduction may be a factor in gender differences. These findings highlight

the importance of considering the timing of environmental and physiological events

throughout the year in understanding animal behaviour (McNamara and Houston 2008).

The timing of reproduction is important in predator-prey dynamics, but often only

the reproduction of the prey is considered (Ims 1990b, a). Combined, the findings of this

thesis illustrate the importance of considering the timing of reproduction of both the prey,

and the predator, which will lead to a more complete understanding of how phenological

patterns in reproduction influence predator-prey dynamics. Future research should

investigate the relationship of hormone levels of both the predator and the prey and how

their variation throughout the year influences predator-prey dynamics. Hormones are

important components in mediating animal behaviour (von Holst 1998, Lonstein and De

Vries 2000) and plant growth and timing of reproduction are also under hormonal control

(Hooley 1994). Red squirrel maternal behavioural patterns are correlated with androgen

levels suggesting hormones may play a role in mediating red squirrel behaviour during

reproduction (Dantzer et al. 2011). In other rodents removal of the testis or ovaries has

influenced hoarding (Nyby et al. 1973, Borker and Gogate 1984) as have metabolic

hormones (Keen-Rhinehart et al. 2010), indicating that hoarding is under hormonal

control. Additionally, variation in hoarding strategies are often explained in terms of

varying levels of territoriality (Vander Wall 1990), and aggression is often linked with

testosterone levels (Soma 2006). Hormone levels of both predator and prey could be

linked via ingestion of the prey by the predator. Red squirrel anticipation of large cone

crops in advance of their availability (Boutin et al. 2006) may be influenced by white

spruce hormone levels in the differentiating cone buds. The plant hormone gibberellins is

associated with increased cone production in white spruce (Pharis et al. 1986), and in the

low cone year prior to the mast of 2010 we noted red squirrels were eating many buds

during the time that cone bud differentiation occurs. Understanding the hormonal

changes throughout the year of both the predator and the prey, in addition to the timing of

reproduction, may lead to a greater understanding of predator-prey dynamics.

77

Literature Cited

Borker, A. S. and M. G. Gogate. 1984. Role of ovarian hormones in hoarding pattern.

Indian Journal of Physiology and Pharmacology 28:115-120.

Boutin, S., L. A. Wauters, A. G. McAdam, M. M. Humphries, G. Tosi, and A. A.

Dhondt. 2006. Anticipatory reproduction and population growth in seed predators.

Science 314:1928-1930.

Dantzer, B., A. G. McAdam, R. Palme, M. M. Humphries, S. Boutin, and R. Boonstra.

2011. Maternal androgens and behaviour in free-ranging North American red

squirrels. Animal Behaviour In press.

Fletcher, Q. F. 2011. The role of energy expenditure in resource acquisition and energy

allocation in free-ranging North American red squirrels (Tamiasciurus

hudsonicus). PhD Thesis. McGill University, Montreal.

Hooley, R. 1994. Gibberellins: perception, transduction and responses. Plant Molecular

Biology 26:1529-1555.

Ims, R. A. 1990a. The ecology and evolution of reproductive synchrony. Trends in

Ecology & Evolution 5:135-140.

Ims, R. A. 1990b. On the adaptive value of reproductive synchrony as a predator-

swamping strategy. American Naturalist 136:485-498.

Janzen, D. H. 1971. Seed predation by animals. Annual Review of Ecology and

Systematics 2:465-492.

Keen-Rhinehart, E., M. J. Dailey, and T. Bartness. 2010. Physiological mechanisms for

food-hoarding motivation in animals. Philosophical Transactions of the Royal

Society B: Biological Sciences 365:961-975.

Lane, J. E., S. Boutin, J. R. Speakman, and M. M. Humphries. 2010. Energetic costs of

male reproduction in a scramble competition mating system. Journal of Animal

Ecology 79:27-34.

Lonstein, J. S. and G. J. De Vries. 2000. Sex differences in the parental behavior of

rodents. Neuroscience & Biobehavioral Reviews 24:669-686.

78

McNamara, J. M. and A. I. Houston. 2008. Optimal annual routines: behaviour in the

context of physiology and ecology. Philosophical Transactions of the Royal

Society B: Biological Sciences 363:301-319.

Nyby, J., P. Wallace, K. Owen, and D. D. Thiessen. 1973. An influence of hormones on

hoarding behavior in the Mongolian gerbil (Meriones unguiculatus). Hormones

and Behavior 4:283-288.

Pharis, R. P., D. Tomchuk, F. D. Beall, R. M. Rauter, and G. Kiss. 1986. Promotion of

flowering in white spruce (Picea glauca) by gibberellin A4/7, auxin

(naphthaleneacetic acid), and the adjunct cultural treatments of girdling and

Ca(NO3)2 fertilization. Canadian Journal of Forest Research 16:340-345.

Rathcke, B. and E. P. Lacey. 1985. Phenological patterns of terrestrial plants. Annual

Review of Ecology and Systematics 16:179-214.

Soma, K. K. 2006. Testosterone and aggression: Berthold, birds and beyond. Journal of

Neuroendocrinology 18:543-551.

Vander Wall, S. B. 1990. Food hoarding in animals. University of Chicago Press,

Chicago.

von Holst, D. 1998. The concept of stress and its relevance for animal behavior. Pages 1-

131 in M. M. Anders Pape Møller and J. B. S. Peter, editors. Advances in the

study of behavior. Academic Press.