Chapter Review
Studying the varied ways that primates organize themselves, reproduce, and care for their young shows us what aspects of primate mating systems we share and do not share with our primate brethren, which can illuminate the selective pressures our ancestors had to face.
First, there are differences between male and female roles in mammalian reproduction. At the cellular level, the female gamete (egg) is metabolically expensive to produce compared to sperm, so eggs are produced one at a time, whereas sperm are produced in the millions. At the level of the organism, all female placental mammals carry and support the developing fetus within their wombs. Females need increased metabolic intake to offset the increasing weight and energy expenditure, and increased nutrition for adequate support of both the fetus and the mother. In addition, all mammalian infants require a period of nursing or lactation, and the mother will incur additional metabolic costs to provide adequate milk for the developing newborn. Males, on the other hand, do not generally experience the same enforced biological costs of reproduction as females.
Because of this disparity between the sexes in terms of required parental investment, males and females employ different strategies to further their reproductive success over the course of their lives. For example, since sperm is so plentiful and cheap to produce, male reproductive fitness is limited primarily by access to females. Therefore, if acquiring additional mates is relatively easy, males should focus on acquiring additional mates rather than investing time and resources in their current or soon-to-be offspring. Unless male parental contributions provide a substantial benefit to offspring, male investment remains low.
This disparity highlights a general point: the investing sex tends to be the choosier sex, whereas the noninvesting sex tends to be more flamboyant, aggressive, and competitive in the realm of mate access. In the case of mammals, nursing prevents males from taking over duties as primary caretakers. This is not the case with many species of birds (for example, sandpipers) or sea horses; in these situations and others, females deposit eggs in the males' care and provide little or no additional assistance. As predicted from disparities in parental investment, the females in these species engage in much higher degrees of competition with each other for access to mates, are aggressive, and are larger than the males. When the female is the primary or sole caretaker, female reproductive strategies include investing a good deal of time and energy in offspring, weaning of older juveniles (so she does not overinvest in them), the careful choosing of mates, and entering coalitions to protect offspring from infanticidal males. Additionally, in many primate groups there seems to be a correlation between high-ranking females and reproductive success; although it is difficult to shift one's place in a typical dominance hierarchy, such upward mobility is not completely impossible.
In primates, sexual selection manifests itself in two scenarios:
- Intrasexual selection, where competition is among males for access to females
- Intersexual selection, where selection results from mate choice by females
In male–male competition, selection generally favors individuals with larger body size, physical weaponry (such as the large canine teeth of baboons), and other characteristics that expedite the male's control over access to female(s). For this very reason, a high degree of sexual dimorphism (difference in size between the sexes) has been strongly correlated with male–male combat over access to females. In addition, this relationship holds true when we examine degrees of sexual dimorphism within different mating systems (for example, single-male, multifemale groups versus monogamous groups), particularly those in which the social environment engenders a high degree of male competition.
Different levels of conflict competition among males occur in varied types of primate groups. For example, in multimale groups, conflict arises frequently over access to sexually receptive females. Even in one-male groups, the resident male cannot always prevent females from mating with outsiders, a source of frequent and often violent conflict. In pair-bonded species, though, such as gibbons, males invest more in infant care and territory defense but do not compete directly over females. As we would expect then, without male–male competition, sexual dimorphism is lessened or negligible in pair-bonded species.
Males have their own strategies for maximizing fitness and reproductive success, which include competition among each other. Additionally, males may engage in behavior that benefits themselves while harming the general reproductive success of females. One practice along these lines is infanticide, a phenomenon in which a male will kill a female's offspring that are not his. Often, infanticide is perpetrated by a male actively attempting to take over a group of females from another male; after he has killed all the infants, the females will cease lactating and begin ovulating, becoming sexually receptive and able to reproduce again. Researchers have demonstrated that infanticide that enhances a male's reproductive success is not a pathological reaction to overcrowding conditions but rather a logical competitive response in certain environments and with given group compositions.