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Sexual selection
and
Gibbons
Readings: Chapters 16 and 26
Gibbon
Body weight in gibbonsM = F, or M 10% heavier
Sexual selection Body mass & social structure
Sexual “monomorphism” in monogamous species...... Is not necessarily associated with lack of aggression.
Sexual selection Canines
Sexual selection
Testis size
(scramble competition)
Testes larger in multi-male species: Scramble competition
Testes larger in multi-male species
Testes larger (than expected) in multi-male species
Adult male chimpanzee, Kanyawara
Integrating 3 effects of sexual selection
M=F
Monogamy Polygyny
Polyandry
Multi MM-FF
GGorillaGibbon
Chimp
Tamarin
Exaggerated Swellings mostly in Multi-Male Societies
Multi-Male species with Exaggerated Swellings have more than twice as many MM per breeding units(4.3 versus 1.8).
Ovulation at peak of swellings but …
…There is a high variation as to when exactly ovulation occurs.
Multi-MMMangabeys: 12 daysMacaques:5-20 d.Baboons: 6-10 d.Chimps: 6-9 d.
Monogamous-1MGibbons: 6 daysGorillas: 1 day !
Exaggerated SwellingsServe contradictory purposes for FF:
1- Increase paternity certainty (because dominant MM guard the FF around maximum swelling).
2- Confuse paternity (because of high variation of ovulation around maximum swelling)
Nunn, 1999. Anim Behav.
Male penile sexual functions
Display
A. To female: chimpanzee
B. To male: baboon
Sperm plug removal (?)
Chimpanzee, bonobo,
muriqui
Spines: “stimulate females”
Lorisoid
Twig (Adult M), Kibale N.P.
Dispersed mating systems
MM-FF mating system
Penile morphologies are LESS divergent in polygynous mating systems than MM-FF or dispersed mating systems. Why?
Male genital morphology: human penes fit multi-male
= size and form of external
penis
Monogamy and sexual selection
Unexpected: MM “should” compete for several FF (Female dispersal cf. food?)
Rare in mammals (3%?)
Primates: 15% (1970s-80s) 3-8% (1990s)
Distinct types
Callitrichids: include helping, reproductive suppression
Gibbons: territorial
Nocturnal primates = separate day range, yet exclusivity.
Definition of Monogamy:
“A prolonged association and essentially exclusive mating relationship
between one male and one female.” (Wittenberger & Tilson 1980)
Gibbons (Hylobatidae)
Gibbons or “lesser apes”
Hylobatidae
Gibbons.
8-10 species, SE Asia
5-8 kg
10-15 kg
GibbonsMostly allopatric
Weird species colors:
F/M same
F/M different (sexual dichromatism)
Variable within sex (gold/dark)
Terminal branch specialists
Frugivory predominates (+ leaf)
F normally leads foraging activities
MM respond first to predators
High survival within groups
MOVIE
Territorial defense
47% over food trees (Bartlett 1999)
Males do most of the fighting
Females mutually repulsive to each other
Benefit of food territoriality
Defense of specific food patches
Highly efficient (25% territory seen every day!)
Gibbons know their fruit patches very well
Trivers-derived model of monogamy.
Females defend territories
Males defend females.
Pro:
Explains “monogamy without male PI”
Fits “MM investing in FF” > “FF in MM”
Con:
Females don’t in fact establish territories
Activity patterns
20-50% resting
Little social activity
(mode = 1% of time grooming! cf. chimps 10-15%)
N.B.
Grooming model: competition to get favors
Evidence: groom up hierarchy (for aid down)
groomers are rewarded later
(chimps w/ food; vervets w/ support)
Reward is mainly the bond
Gibbon monogamy
Isolated pairs (unlike humans)
Long-term pairs (e.g. 3 pairs stable for 7 years)
(but: high divorce rate / adultery)
No helpers
Mating only for conception
(unless 2nd male)
Co-dominance or F > M
MM groom FF > FF groom MM (~80-90%)
(FF more when first paired!)
Strong territoriality
Call: species-specific, variable patterns
8 spp
F calls ---> M calls (+ overlap)
? = “This is my land” ---> “This is my female”
[Widowed F: F can mimic M presence]
5 spp
M long solos MOVIE
Dispersal and family formation
No sex bias in dispersal
Habitat saturated
- Empty territories quickly occupied
Solitaries disappear, die?
Ecological Constraints Hypothesis for Incest
Young adults should stay as long as possible
Kin less hostile than non-kin
Young may assist in range defense
Young may take over parental slot incest!
Why be monogamous?
*Female needs male help? NO!
Male does no caring for offspring
(except in one species: siamang, carries juvenile)
*Sexual selection: YES?
Male strategies depend on female distribution
F defends territory (but this does not work in gibbons...)
M defends F
*M aids in predator defense?
Looks trivial
Benefit to female of being consorted by male
*Range defense
Wins battles over food (e.g. 45% interactions over food)
M fights while F eats
*Infanticide defense? (van Schaik & Dunbar 1990)
Possible because there is a paternity certainty
Puzzle: why does M stay, if he could defend 2 ranges?
No attempts seen.
Lone mothers don’t call
*Mate-guard (Palombit 1999)
F accepts M because hard to remove him
M benefits by ensuring paternity
“Extra-pair copulations could be a serious threat to a male’s paternity; the group A female in the Khao Yai Park study area has been seen copulating with at least 4 different neighboring adult and subadult males over the past 15 years. These extra-pair copulations all occurred while the resident A-male was not accompanying her.”
(Brockelman 2004)
“A male often has to leave his mate unprotected to defend the territorial border” (Brockelman)
What if: conflict between mate-guarding and range-defense?
Range-defense wins...
“Mate guarding and territorial defense are demanding and often mutually exclusive activities”
Is Territorial Defense a form of
Mate Guarding?
Problems with view of monogamy
> 2 adults
H. concolor 25% groups
H. hoolock 12%
H. lar 10-18%
H. agilis 0%
Palombit,Fuentes (2000)
*Polygyny (esp. northern, more leaf-eating species, e.g. concolor)
F joins F-M pair if allowed to do so
*Movement
F leaves M1, joins widowed M2, mates with M2-M3-M4, rejoins M1
Exceptions to territoriality
Affiliative encounters
Khao Yai: 25% of one group’s encounters
Note: Khao Yai = 2nd largest NP in Thailand
e.g. M plays with juveniles of other group
(uncle!)
Bartlett (2000)
SUMMARY:
Reality:
Males defend territories
Females defend if necessary
Females are not effectively mate-guarded
Little infanticide pressure indicated
Hypothesis:
Females need a territory, defended as well as possible
Males constrained to defend territory for female
White-handed gibbon(Hylobates lar)
2- Male maintains a bond with a single female to increase paternity certainty (infanticide) (van Schaik and Dunbar, 1990)
3- Male seeks to maintain future mating opportunities (Palombit, 2000)
Three hypotheses to explain the male’s commitment to one female
Brockelman in press
1- Males attach themselves to females and defend their access to mating opportunities (Wrangham, 1979)
Are these hypotheses mutually exclusive?
See Chapter 17 (orangutans)
e.g. sexual swellings
e.g. Good gene theory
e.g. M looks strong
Sexual selection of
specific characters ?
What about
humans ?
Fuentes (2000)
Gibbon ‘community’ concept
“Neighborhood” ?? -- maybe
“Community” -- No --- no joint action.