1. Sexual selection and Gibbons Readings: Chapters 16 and 26

4. Gibbon

5. Body weight in gibbons M = F, or M 10% heavier Sexual selection Body mass & social structure

6. Sexual “monomorphism” in monogamous species...... Is not necessarily associated with lack of aggression. Sexual selection Canines

7. Sexual selection Testis size (scramble competition)

9. Testes larger in multi-male species: Scramble competition

10. Testes larger in multi-male species

11. Testes larger (than expected) in multi-male species

12. Adult male chimpanzee, Kanyawara

13. Integrating 3 effects of sexual selection M=F

14. MonogamyPolygyny Polyandry Multi MM-FF G Gorilla Gibbon Chimp Tamarin

17. 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, 1999. Anim Behav.

18. 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.

19. Dispersed mating systems MM-FF mating system

20. Penile morphologies are LESS divergent in polygynous mating systems than MM-FF or dispersed mating systems. Why?

21. Male genital morphology: human penes fit multi-male = size and form of external penis

22. 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)

23. Gibbons (Hylobatidae)

24. Gibbons or “lesser apes” Hylobatidae

25. Gibbons. 8-10 species, SE Asia 5-8 kg 10-15 kg

26. Gibbons Mostly allopatric

27. Weird species colors: F/M same F/M different (sexual dichromatism) Variable within sex (gold/dark)

28. Terminal branch specialists Frugivory predominates (+ leaf) F normally leads foraging activities MM respond first to predators High survival within groups MOVIE

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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!

35. 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

36. 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

37. “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”

38. Is Territorial Defense a form of Mate Guarding?

39. 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

40. 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)

41. 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)

42. 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?

44. See Chapter 17 (orangutans)

45. e.g. sexual swellings e.g. Good gene theory e.g. M looks strong

46. Sexual selection of specific characters ?

47. What about humans ?

48. Fuentes (2000) Gibbon ‘community’ concept “Neighborhood” ?? -- maybe “Community” -- No --- no joint action.