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).