Primate Mating Systems - Anthropology 201 Fall 2024 PDF

11. Primate Mating Systems and Sexual Selection Anthropology 201 Fall 2024 Primate Mating Systems (A Closer Look - 5.2) Solitary or “noyau” Orangutans, lorises Pair-bonded (monogamy)...

11. Primate Mating Systems and Sexual Selection Anthropology 201 Fall 2024 Primate Mating Systems (A Closer Look - 5.2) Solitary or “noyau” Orangutans, lorises Pair-bonded (monogamy) Gibbons, owl monkeys Uni-female/multi-male (polyandry) Tamarins [cooperative breeding in text] 2 Primate Mating Systems Uni-male/multi-female (polygyny) Langurs, mountain gorillas Non-resident males may form “bachelor groups” Multi-male/multi-female (polygynandry) Vervets, macaques Special case: fission-fusion (spider monkeys, chimpanzees) 3  Primate Mating Systems How do these mating systems (or groups) influence the reproductive strategies of males and females? 4  Objectives Define a strategy in evolutionary terms Distinguish reproductive strategies of female and male primates – Competition over different resources Define and describe sexual selection – How it differs from natural selection Reading: Chapter 6; Ch 5 p. 133-136 5  The language of adaptation Strategy = set of behaviours occurring in specific functional context – e.g. folivory is a foraging/feeding strategy – e.g. polygyny is a mating strategy à Strategies are products of selection, not conscious plans 6  The language of adaptation Different behaviours have different impacts on fitness* – Beneficial = increases fitness – Costly = decreases fitness What currency should we use to measure costs and benefits? – Best: reproductive success (RS): number of offspring surviving to reproductive age – When RS difficult to measure: use proxies (e.g. foraging efficiency) *Fitness = an individual’s genetic contribution to the next generation 7 The language of adaptation Sets of behaviours (i.e. strategies) that tend to increase fitness will evolve through selection Strategies may be different for males and females, and in different group compositions 8  An observation Primate females always provide lots of care for their young Primate males do so in very few species WHY? 9  Why females invest more Why is it female primates that invest more, and not males? – In fish and bird species the males sometimes provide more/most parental care Mammalian reproductive system constrains female strategies – Must carry offspring to term – Must nurse them until they can forage independently – Infant’s survival (hence mother’s fitness) in mammals depends on these factors, more than in other animals 10  Why females invest more Primates: extended pregnancy (large brain), altriciality Extra time and energy per infant Relatively small number of surviving infants produced over lifetime Each infant = greater portion of lifetime fitness 11  Why males invest less Males typically less involved in care of offspring – Male care often less important for survival of offspring – Time, energy and resources are limited – Paternity uncertain – Often better strategy to use their resources to access additional females 12  When males invest less Males will be less involved: – 1) when attracting additional mates is relatively easy (low cost) – 2) when fitness of offspring raised by one parent is high (benefit of additional care is minimal) When benefits of investment outweigh costs of seeking new matings, males will provide more care (tamarins, owl monkeys – females can’t provide enough care?) 13  Reproductive Strategies Sexes invest unequally in offspring – Female investment typically higher Causes of variation in RS are likely to be different: – Females: securing resources (food) – Males: securing mates 14 Female Reproductive Strategies Female RS depends on ability to obtain enough resources (esp. food) for her and her offspring Abundance leads to high birth rates, shorter interbirth intervals (e.g., provisioning by humans) Resource crashes lead to low rates, long intervals (e.g., cyclones) (provisioned years in open squares, 1964-1971) 15 Female Reproductive Strategies Access to food is important, so females often compete for this access Dominance hierarchies often form – High-ranking females gain access to more/better quality food 16 Female Reproductive Strategies Rank is positively correlated with RS 1. Macaques: rank 2. Hanuman langurs: and group size rank and age influence RS influence RS 17 Female Reproductive Strategies Rank is positively correlated with RS High rank Mid rank Low rank 3. Chimpanzees: Mother’s rank influences survival to weaning in offspring (hence RS) 18 Female Reproductive Strategies Socializing can positively influence RS May insulate some females from cost of low rank – Benefits from association with high ranking females? J B Silk et al. Science 2003;302:1231-1234 19  Reproductive Strategies Females Females produce few offspring, invest heavily in each, and can raise them (largely) without males Limiting factor that causes competition and influences the RS of females = access to food Males Males less constrained by food, seldom invest in offspring, hence can potentially mate with many females Limiting factor that causes competition and influences the RS of males = access to mates 20  Male Reproductive Strategies All about access to mates Traits that increase success in competition for mates evolve through a special form of selection: Sexual Selection 21  Sexual Selection Originally proposed by Darwin Special category of natural selection Accounts for features with no obvious survival function (secondary sexual characteristics) Helps explain traits that seem maladaptive in terms of natural selection (e.g. costly) 22  Natural vs. Sexual Selection Natural Selection Sexual Selection Favours phenotypes related to Favours phenotypes that survival/reproduction, e.g. increase success in competition predator avoidance, resource for mates acquisition... Affects one sex more strongly, Affects both sexes the one whose access to mates is limited 23  Components of Sexual Selection Intrasexual Selection – Competition within the sex(es) – Usually stronger among males – Common in primates Intersexual Selection – Mate choice – Usually stronger in females – Common in birds, ?primates 24 Intersexual Selection: What do females want? Females have high parental investment Also reproduce slowly Low reproductive potential Should mate with “quality” mates – Do not want to waste rare reproductive opportunities on unhealthy males or those that might produce less competitive offspring 25  Intersexual Selection Females can choose males that: – Will increase their fitness Defend resources, infanticide protection – Show good “genetic quality” Will produce more fit offspring Measured by condition, and costly traits Text suggests that intersexual selection not important in primates (female choice limited) But may occur via matings with multiple males and possibly cryptic female choice (female sperm storage) 26  Intrasexual selection in males Most basic form of male-male competition is to fight and drive other males away Winners have advantage in mating opportunities = higher RS Intrasexual selection favours traits that enable males to be effective fighters 27  Intrasexual selection in males Increased male body size and canine size are common targets of intrasexual selection Often leads to sexual dimorphism (SD) 28  Intrasexual selection in males SD should be most pronounced in groups where males compete most = single-male/multi-female 29  Reproductive Strategies: Single- male groups A male tries to establish residence in an unrelated group of females, and then restricts access to other males Other males constantly try to take over e.g. baboons, geladas, Hanuman langurs 30 Reproductive Strategies: Multi- male groups In most primates, females have estrus In multi-male/multi female groups, estrous females can mate with several males in group Less direct competition over access to females Here, intrasexual selection favours increased sperm production – “sperm competition” 31 Repro Strategies: Multi-male groups In multi-male/multi-female groups, dominance hierarchies may also form among males – Determine access to estrous females Dominance hierarchy reflects competitive ability (threats, etc.) Younger males have higher ranks – better physical condition? RS related to age Rank and RS in baboons 32  Reproductive Strategies: Pair- bonded males In pair-bonded species (e.g. gibbons), males don’t compete for access to females RS depends more on finding mates, defending territory, and rearing surviving offspring Mate guarding is a common rep. strategy Grooming in white-handed gibbons 33  Reproductive Strategies: Infanticide Adaptive strategy for males After group takeovers, males kill dependent offspring Females return to estrus allowing infanticidal male to reproduce more quickly (before they are ousted) Infanticide is costly for females – So expect counter-strategies such as false estrus, female coalitions, spontaneous abortions, male “friendships”, etc. Observations have been made in a large number of primate species, in the wild and in provisioned populations. 34 Infanticide Counterstrategies Concealed ovulation Regularly sexually receptive – with multiple males Terminate pregnancy after takeover (spontaneous abortion) Social supports/friendships to try to avoid infanticide 35