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Lecture 3: Physiological ecology of
animals: heat balance
1. Example of one environmental challenge tophysiology: Heat balance/thermal ecology ofanimals
a.
Modes of heat gain and loss; homeostasis
b.
Size, shape, insulation, evaporative cooling
c. Tradeoff principle and adaptive compromises(example of weasel body shape)
Copyright James D Thomson
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Radiationheat transfer by electromagneticradiation
Conductiondirect contact with substrate (e.g.,
feet lose heat to ground)
Convectionheat transfer mediated by movingfluid (usually air or water)
Evaporationefficient cooling from wet surfaces
Redistributioncirculatory system redistributesheat among body parts, esp. core toappendages
One selected limiting factor:
heat budgets
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Size matters to heat balance (and other
balances of gains and losses)
Homeostasis and surface area:volume ratio
Surface area determines equilibration rate
Volumeprovides the inertia
Radius: 1 2 3
SA/V: 3 1.5 1
Equilibrates most slowlyEquilibratesquickly
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Sizematters
Bergmanns rule: Homeotherms tend to
be larger at higher latitudes (colder)
Helarctos malayanus65 kg, short fur
Ursus americanus275 kg, medium fur
Ursus maritimus650 kg, long fur
All WikipediaCopyright James D Thomson
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Fossil record: horses shrank during two past
episodes of climate warming, grew during ice ages
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An exception to Bergmann
s Rule?
National Geographic
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Shapematters:
Allens rule: extremities reduced in cold
climates
Sphere has least SA:V, so why shouldnt
homeotherms be spheres in cold climates?
Sometimes SA is needed for function
Sometimes particular shapes are neededfor function
Tradeoffs and adaptive compromises
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Maximum SA:V ratio?
Chrysopelea gliding snake, Borneo;
restricted to warm tropics
National Geographic, Tim Laman
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Minimum SA:V ratio?
Pika, Ochotona princeps: alpine tundra rabbit;
restricted to cold habitats; note spherical shape,
reduced ears (for a rabbit)
National GeographicCopyright James D Thomson
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Variation in appendages: arctic and
desert hares
Lepus arcticus (Art Wolfe) Lepus californicus (www.pestproducts.com)
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What else matters? Insulation more
important than size/shape
www.dinosaur-museum.org
mercury.bio.uaf.edu
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Thick insulation: Half-sheared sheep
National GeographicCopyright James D Thomson
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What else matters? Shaving! Can aquatic
mammals use fur for insulation? Drag is a drag.
When shaving is not enough:Michael Phelps, 16 22 Olympic medals
HGH?
Anti-drag
suit
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Thick insulation: Cross-section of seal
Blubber
Guts
Musculature42%
58%of cross-sectional
area
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f
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Hair and feathers provide
adjustableinsulation
Video clip, Clarks Nutcracker on a cold
morning in Colorado
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What else matters? Convective
cooling enhanced by vascularization
Lepus californicus(National Geographic) L. americanus (JDT)Copyright James D Thomson
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Another way to provide well-
vascularized surface area for cooling
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What else matters? Countercurrent
circulation to limbs conserves heat
Arteries and veins should be appressed in appendagesto conserve heat; separated in appendages designed to
shed heat (Compare Ricklefs Fig. 3.24)
Countercurrent flow maintains gradient, so heat isalways flowing from outgoing blood to incoming blood
Outside (0!"
Body (35!" 30 20
5 10
15
Body (35!"30 15
25 10
0
Vessels not appressed,returning blood is chilled Vessels appressed,returning blood is warmerCopyright James D Thomson
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What else matters?
Convection enhanced by evaporation
/www.bluegina.com
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?
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What else matters?
Convection enhanced by evaporation
/www.bluegina.com
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Extra slide if time: Range dependence on other organisms:saguaro, gila woodpecker, cactus wren, elf owl
www.southwestbirders.comCopyright James D Thomson
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Reconciling an apparent paradox:
weasels are small predators, short-furred,very long and thin
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Weasel in winter = ermine: active all year;
camouflaged for snowy environment
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24Henry VIII: Low SA / V Mustela erminea: High SA / V
Holbein
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25Henry VIII: Better? Mustela erminea: Worse?Copyright James D Thomson
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Ecology53(5): 939-943 (1972)
Weasel WoodratWarm
((Cold))Curls into flat disk Curls into ball
Copyright James D Thomson
So the weasel body plan is well suited
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So, the weasel body plan is well-suitedfor warm climates!
M.erminea
!making their actual distribution paradoxical!
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28Mustela frenataand Thomomys talpoides, Rocky Mountain Biological Lab, D. Inouye photosCopyright James D Thomson
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Typical weasel prey:
Pocket gopher,Thomomys talpoides
Seldom leaves
underground burrows;mostly eats roots, tubers
So, the paradoxinvolves
the requirements of theweasels predatory
lifestyle
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Example of a tradeoff:
Being long and thin makes weasels subject to
thermal stresses (costly)!
!but allows them to be better predators
(beneficial).
Because they are long and thin, we infer that the
fitness gainsof being a good hunter offset the
fitness costsof an expensive metabolism Phenotypes of all organisms are riddled with
compromises dictated by tradeoffs
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Two reasons why natural selection
produces deeply imperfect organisms
Tradeoffs
Being good atxmay necessarily imply being
bad at y
Constraints
Selection builds on what is already there,especially existing developmental programs
Tinkering, yes; fundamentally fresh redesign,
no
31Copyright James D Thomson