Chapter seven:primate mating systems

Chapter Review

Reproductive Strategies and Parental Investment

Studying the varied ways that primates organize themselves, reproduce, and care for their young is key for the reconstruction of human (hominid) evolution. Being able to answer what aspects of primate mating systems we share and do not share with our primate brethren can illuminate the selective pressures our ancestors had to face. Therefore, as a starting point for understanding reproductive strategies among primates—as well as in other mammals—we should examine the differences between male and female roles in reproduction. At the cellular level, the female gamete (egg) is metabolically expensive to produce compared to sperm, which are little more than a bundle of chromosomes. As a result, eggs are produced one at a time, whereas sperm are produced in much larger quantities (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 concomitant energy expenditure required to carry the fetus while moving), and increased nutrition for adequate support of both the fetus and the mother. In addition, all mammalian infants require a period of nursing (Fig. 7.1). This period is called lactation; here again the mother will incur metabolic costs to provide adequate nutrition for the developing newborn. Contrary to this complicated scenario, however, males do not generally experience same enforced biological costs of reproduction that females must endure.

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Figure 7.1 Baboon mother and infant.
Image credit: Joan Silk.

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 preferentially adopt that activity—rather than investing in their current or soon-to-be offspring—to further their Darwinian fitness. Furthermore, 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, larger than the males, and generally more “male-like” in their behavior. Female reproductive strategies—when the female is the primary or often sole caretaker—include, then, investing a good deal of time and energy in her offspring, the timely weaning of older juveniles (so she does not over-invest in them), the careful choosing of mates, and coalitioning to protect their offspring from infanticidal males (see below for an explanation of infanticide). 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.

Sexual Selection, Strategies, and Competition

In primates, sexual selection manifests itself in two scenarios:

Intrasexual competition, 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.

The second case of sexual selection—intersexual mate choice—must also result in higher reproductive success for an individual, but why certain traits are selected revolves around three points:

Traits that increase the fitness of females.
Traits that indicate good genes, which thus improve the fitness of offspring.
Traits that have little adaptive significance but have been preferentially selected by females over many generations

The benefits conferred on a female by a male who defends the young, provides food, or defends territories better than other males are readily apparent. However, traits that indicate good genetic makeup act in an indirect manner. While good genes are not easily discerned, physical signs of good genes may be easier to spot—perhaps brightly colored peacock tails or loud mating calls bear a relationship with genetic fitness. That peacocks select mates on the conscious basis of good genes is not very likely, for example. Yet, the fact that brightly colored males do tend to have more fit and vigorous offspring could affect mate selection, even if the genetic basis of this decision remains unknown to the actors.

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 monogamous 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, then, 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 rather macabre phenomenon in which a male will kill the offspring of a female 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 become sexually receptive to him. Researchers have demonstrated that infanticide that enhances a male’s reproductive success, while certainly a macabre practice, is not a pathological reaction to overcrowding conditions, but a logical competitive response in certain environments and with given group compositions.

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