Sexual selection and Gibbons Readings: Chapters 16 and 26

Gibbon

Body weight in gibbons M = 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

MonogamyPolygyny Polyandry Multi MM-FF G Gorilla Gibbon 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-MM Mangabeys: 12 days Macaques:5-20 d. Baboons: 6-10 d. Chimps:6-9 d. Monogamous-1M Gibbons:6 days Gorillas:1 day ! Exaggerated Swellings Serve 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, 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 species, SE Asia 5-8 kg kg

Gibbons Mostly 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. concolor25% groups H. hoolock12% 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 = 2 nd 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.