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.