Primate Mating Systems PDF

# Primate Mating Systems ## Chapter Objectives By the end of this chapter you should be able to: - Explain why reproduction is the central act of all living things. - Describe how mammalian reproductive biology influences the reproductive strategies of primate females. - Discuss the factors that...

# Primate Mating Systems ## Chapter Objectives By the end of this chapter you should be able to: - Explain why reproduction is the central act of all living things. - Describe how mammalian reproductive biology influences the reproductive strategies of primate females. - Discuss the factors that influence female reproductive success. - Describe the process of sexual selection and explain why it favors traits that would not be favored by normal natural selection. - Describe how competition among males over access to females influences male reproductive strategies. - Explain why infanticide is an adaptive strategy for male primates in some circumstances. ## Introduction - Reproduction is the central act in the life of every living thing. - All primate behaviors have evolved to enhance reproduction. - Complex adaptations exist because they evolved through natural selection. - Mating systems (the way animals find mates and care for offspring) play a crucial role in understanding primate societies. - Understanding the diverse reproductive strategies of nonhuman primates illuminates human evolution because we share many elements of our reproductive physiology with other species of primates. ## The Evolution of Primate Mating Systems ### The Language of Adaptive Explanations - **Strategy** refers to behavioral mechanisms that lead to particular courses of behavior in particular functional contexts. - Biologists often use the term **strategy** to describe the behavior of animals. - **Cost and benefit** refer to how particular behavioral strategies affect reproductive success. - **Beneficial** behaviors increase the genetic fitness of individuals. - **Costly** behaviors reduce the genetic fitness of individuals. ### The Evolution of Reproductive Strategies - Primate females always provide lots of care for their young, but males do so in only a few species. - The amount of parental care varies greatly within the animal kingdom. - Primates provide much more than just resources included in gametes. - **Parental care** has profound consequences for the evolution of social behavior and morphology. - Males do not care for offspring (a) when they can easily use their resources to acquire many additional matings or (b) when caring for their offspring would not appreciably increase the offspring's fitness. ### Reproductive Strategies of Females - Female primates invest heavily in each of their offspring. - Pregnancy and lactation are time-consuming and energetically expensive activities. - Larger animals tend to have longer pregnancies than smaller animals. - The extended duration of pregnancy in primates is related to the fact that brain tissue develops very slowly. - A female's reproductive success depends on her ability to obtain enough resources to support herself and her offspring. - There is considerable evidence that female reproductive success is limited by the availability of resources within the local habitat. - **Sources of Variation in Female Reproductive Performance:** - Very young and very old females do not reproduce as successfully as middle-aged females. - Females reproduce at lower rates than do middle-aged females because they are not yet fully grown and may lack experience in handling newborn infants. - In contrast to humans, most primate females continue to reproduce throughout their lives. - **Longevity** is a major source of variation in female fitness. - **Dominance rank** influences access to valuable resources and access to resources influences female reproductive success. ### A Closer Look: Dominance Hierarchies - Dominance relationships occur when dominance interactions between two individuals have predictable outcomes. - A **dominance relationship** is established when one individual regularly defeats the other (based on size, weight, experience, or aggressiveness). - Dominance rankings can be assigned to individuals when dominance relationships have predictable outcomes. - Dominance relationships are said to be **transitive** when females can defeat all the females ranked below them and none of the females ranked above them. - A hierarchy is **linear** when the relationships within all sets of three individuals (trios) are transitive. - Dominance rank may vary greatly within animal societies and may not always perfectly predict the outcome of encounters between individuals, but dominance relationships can have a strong influence on the reproductive performance of both male and female primates. ### Reproductive Trade-offs - Females must make a trade-off between the number of offspring they produce and the quality of care they provide. - Natural selection favors individuals that can convert effort into offspring most efficiently. - Mothers cannot maximize both the quantity and the quality of their offspring. - Maternal behavior reflects the trade-off between the requirements of a growing infant and the energy costs to the mother of catering to her infant's needs. ## Sexual Selection and Male Mating Strategies - Sexual selection leads to adaptations that allow males to compete more effectively with other males for access to females. - **Sexual selection** is a special category of natural selection that favors traits that increase success in competition for mates. - There are two types of sexual selection: - **Intrasexual selection:** Results from competition among males. - **Intersexual selection:** Results from female choice. - **Intrasexual competition** favors traits that enhance success in male-male competition, such as large body size, horns, tusks, antlers, and large canine teeth. - **Sexual dimorphism** refers to consistent differences in the size or appearance of the two sexes. Intrasexual competition favors larger body size, larger teeth, and other traits that enhance fighting ability. - The fact that sexual dimorphism is greater in primate species forming one-male, multifemale groups than in pair-bonded species indicates that **intrasexual selection** is the likely cause of **sexual dimorphism** in primates. - In multimale, multifemale groups, in which females mate with several males during a given estrous period, sexual selection favors **increased sperm production.** ### Male Reproductive Tactics - **Investing males** are commonly seen in pair-bonded species. - In pair-bonded species, males' reproductive success depends mainly on their ability to establish territories, find mates, and rear surviving offspring. - **Mate guarding** and **offspring care** are important components of males' reproductive tactics in pair-bonded species. - **Cooperatively breeding species** typically consist of one dominant pair and helpers of both sexes. - The presence of multiple male helpers seems to enhance female fertility. ### Male-Male Competition in Groups without Pair Bonds - In non-pair-bonded groups, the reproductive success of males depends on their ability to gain access to groups of unrelated females and to obtain matings with receptive females. - Male-male competition is intense in species that form one-male groups. - Residence in one-male groups does not always ensure exclusive access to females. - In species that form both one-male and multimale groups, males adopt different strategies based on the circumstances. - For males in multimale groups, conflict arises over group membership and access to receptive females. ### Infanticide - **Infanticide** is a sexually selected male reproductive strategy. - Infanticide may enhance male reproductive success because the death of nursing infants hastens the resumption of maternal receptivity. - If infanticide is a male reproductive strategy, then we should expect that (1) infanticide would be associated with changes in male residence or status; (2) males should kill infants whose deaths hasten their mothers' resumption of cycling; (3) males should kill other males' infants, not their own; and (4) infanticidal males should achieve reproductive benefits. - **Infanticide** is sometimes a substantial source of mortality for infants. ### Females Have Evolved a Battery of Infanticide Counterstrategies - Females may try to prevent males from harming their infants. - Females may try to confuse males about paternity by obscuring information about their reproductive state, by mating with multiple males when they are sexually receptive, and by mating with males at times when they are not likely to conceive. - Gelada females have adopted a different strategy to reduce the impact of infanticide. Gelada females terminate pregnancies in the days that follow takeovers. - The threat of infanticide seems to influence the nature of male-female relationships in baboons. Females form close relationships with adult males (male "friends"). ## Key Terms - Strategy - Primiparous - Multiparous - Dominance - Dominance matrix - Transitive - Sexual selection - Intrasexual selection - Intersexual selection - Bachelor males - Estrus - Sexual selection infanticide hypothesis ## Further Reading