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I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see the whole problem is so intricate that it is safer to leave its solution for the future.
Charles Darwin, 1871
Sexual selection is an important evolutionary force in mammalian species because of one simple fact – males are a glut on the market. From the females' point of view, there are more males than needed to meet their reproductive requirements. And from the males' point of view, there are not nearly enough females to go around to satisfy their reproductive potential. The relative abundance of males generates strong intrasexual competition among them.
The goals of this chapter are to explore the selective factors that influence the evolution of birth sex ratios, and to weigh the empirical evidence that primate females facultatively manipulate birth sex ratios to enhance their own fitness. We will begin by briefly enumerating some of the ways in which adult sex ratios influence the evolution of male and female life histories, morphology and reproductive strategies in primate groups. Then, we will explain how natural selection shapes the evolution of birth sex ratios, and consider the empirical evidence for adaptive manipulation of birth sex ratios in primate groups.
Sexually selected infanticide is relatively widespread among primates, but has been documented primarily in one-male–multifemale reproductive units, e.g., in guenons (Cercopithecus spp.) (Butynski 1982; Fairgrieve 1995), langurs (Presbytis spp.) (Hrdy 1974; Newton 1988; Sommer 1994), howler monkeys (Alouatta spp.) (Crockett & Sekulic 1984), and mountain gorillas (Gorilla gorilla berengei) (Fossey 1984; Watts 1989). Although male infanticide has been invoked as a selective force in multimale–multifemale groups, such as in macaques (Macaca fascicularis) (van Noordwijk 1985), capuchin monkeys (Cebus olivaceus) (O'Brien 1991), and chimpanzees (Pan troglodytes) (Smuts & Smuts 1993), it has rarely been observed in these species (e.g., Valderrama et al. 1990; Camperio Ciani 1984) or follows patterns partly inconsistent with Hrdy's (1974) sexual selection hypothesis (Hiraiwa-Hasegawa & Hasegawa 1994). Thus current data suggest that the presence of multiple males in a primate group discourages infant-killing by other males.
Relative to one-male groups, the presence of additional, reproductively active males may both dilute the benefits of infanticide and increase its costs. Exploitation of the reproductive opportunity created by infanticide depends upon the perpetrator's ability to monopolize subsequent fertilizations, which is a function of social variables such as the number of males in a group, the intensity of male–male mating competition, and the potential for effective mate guarding.
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