Why are most primates polygynous ? Polygyny (1M : xF) 2 Preconditions for the evolution of polygyny 1. Must be feasible for males to defend females (costs vs. benefits) 2. Males must be able to capitalise on potential for polygyny • e.g. offspring will survive without care 3 1 Why most primates are polygynous • In most species, females congregate spatially in small, stable groups – can be defended • Long interbirth intervals – only a few reproductively active females per sexually active male • Intense male-male competition for limited number of fertilisable females • Result : polygyny 4 When does multi-male polygyny occur? When males can’t monopolise access to females 1. Females are dispersed 2. Large groups 5 Dispersed females 6 2 Too many females Males either • can’t exclude other males • or cooperate to defend territories 7 Polygyny threshold # males generally correlated with # females 8 Evolution of monogamy 3 Monogamy (1M : 1F) 10 Monogamy is not common • Birds – 90% • Mammals – under 5% • Primates – 37/200 = ~18% • Traditional human societies – 20% 11 Characteristics of monogamous primates • Limited mating opportunities • Male investment in offspring HIGH • Male paternity confidence HIGH • Little sexual dimorphism • Territoriality & sex-specific aggression 12 4 Monogamy evolves when … … males can only defend one female due to 1. ecological factors or 2. constraints imposed by parental care Monogamous birds : parental care most important 13 Gibbons : ecology • Females evenly distributed in environment – mutual aggression • Playback experiments: female of a pair reacts aggressively to female sounds, male doesn’t • Females reduce males' options – no choice but to be monogamous 14 New world monkeys : care • Body size – small adults, but large offspring relative to adult size • Very hard for a lone female to raise offspring • Males that abandon may mate more often, but they won’t end up passing along more genes if offspring die 15 5 Monogamy =/= fidelity • Siamangs – 5 EPCs during a 2.5 yr study of 1 group • Gibbons – EPCs observed on ~9% of days 16 Monogamy =/= fidelity 2 • Fat-tailed dwarf lemurs – 7 / 16 social fathers not the genetically determined sire • Fork-marked lemurs – 4 / 7 genotyped offspring not sired by social father 17 Why do females mate with multiple males? 6 Multiple mating in females • A female primate needs to mate only once, at optimum stage of her cycle, to conceive • But, polyandrous mating is widespread – e.g. a female chimpanzee copulates >>100 times for each conception – females often mate with all (unrelated) males in group 19 Sexual advertising • Females of many species advertise impending ovulation to multiple males – prominent behavioural signals (e.g. proceptive displays), – visual signals (e.g. sexual swellings) – acoustic signals (copulation calls) – olfactory signals 20 Visual signals 21 7 Copulation calls • During or after copulation 22 Olfactory signals 23 Mating with multiple males is costly • Time & energy • Risks of harassment or predation • Risk of disease transmission • Potentially negative effects of male seminal fluid 24 8 Why should females seek multiple mates? • Paternity confusion • Indirect mate choice • Promotes sperm competition & cryptic female choice • Exchange for immediate resources • Gaining parental investment from multiple males • Fertilisation insurance • Recruitment of males for group defense 25 Why should females seek multiple mates? cont. • Avoidance of the costs of sexual coercion • Pure “fun” • Spite (sperm depletion) • Social reward 26 Male dominance & mating success 9 Males compete 1.0 Proportion remaining .8 .6 .4 .2 0.0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Age (yr) 28 MM-MF groups • Males compete for high rank • High-ranking males have “priority of access” to females • Behaviour: High rank -> high mating success • DNA studies: High rank -> high reproductive success 29 Priority of access model • 1 female : male #1 monopolises her • 2 females males #1 and 2 monopolise them • 3 females males #1, 2 & 3 monopolise them • Etc. 30 10 Example Male #1 sires Males 1, 2, 3 & 4 sire 31 Priority of access in male mandrills 60 Offspring sired 50 40 30 20 10 0 1 2 3 4 5 6 7 8 9 10 Male rank 32 Rank & reproductive success • Relationship weaker where – females overlap – subordinates sneak – many rivals – males form coalitions – ranks unstable – female choice 33 11 Variation in male RS over lifespan • Rank (therefore RS) changes with age • Tends to even out variation in RS • Need a lifetime view 4 – long-term study # offspring 3 2 1 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Age (yr) 34 12