Introduction

Sexual selection theory suggests that divergent reproductive interests of the sexes impede the evolution of enduring social bonds between males and females. Males are more likely to increase reproductive success by acquiring multiple mates whereas females enhance fitness more by discriminative choice of individual mate(s) (Darwin 1871; Trivers 1972). Consequently, insofar as postcopulatory bonds limit a male's sexual access to additional fertile females and are irrelevant to antecedent female mate choice, such bonds should be relatively rare. The mostly polygynous mammals, for example, fulfill this expectation: male mating effort generally exceeds paternal effort, and affiliative interactions between the sexes center on the period of copulation or female fertility (Clutton-Brock 1989b).

And yet, males and non-fertile or anestrous females maintain stable relationships with one another in some species. The same theoretical framework that predicts the rarity of persistent heterosexual bonds also highlights a primary context for their evolution: when a male restricts his mating to a single female, a postcopulatory relationship with her is not only less costly to him, but may also offer fitness advantages to both parties. The proposed benefit to the female is the extensive parental care she receives from a male that is now certain of paternity. Thus durable male–female relationships were originally viewed as part of a coevolved suite of behaviors including monogamy, biparental care, and, in gregarious animals, “nuclear families” of parents and offspring (Morris 1967; Wittenberger & Tilson 1980; Gubernick 1994). The primarily monogamous, biparental birds have long served as vivid examples of this system (Lack 1968).

Introduction

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.