Monomorphism, male-male competition, and mechanisms of sexual dimorphism.

Reconstructing sociosexual variables such as mating system and social organization from fragmentary fossils and other sources of data remains a major challenge in understanding the evolution of behavioral and morphological diversity within the primate order. Often, a particular mating system can lead to the evolution of novel behavior (e.g., ‘‘competitive’’ paternal care in tamarins; Garber, 1997) or contribute to the evolution of a particular morphology (e.g., relatively larger sagittal crests in male gorillas with large harems; Caillaud et al., 2008). Indeed, within paleoanthropology, numerous hypotheses concerning the evolution of major adaptive traits in humansdbipedalism, language, increased reliance on stone tool-usedoften rely on a particular inferred mating system. For example, Lovejoy (1981) proposed that monogamy was the ancestral mating system in Australopithecus afarensis and from this went on to craft a creative hypothesis for the evolution of bipedalism. Similarly, Deacon (1997) suggested that male-female pairbonding helped foster the evolution of language by allowing males and females to vocalize their monogamous commitment to other group members. Increased reliance on stone tools has also been linked to a particular mating system and social organization, with a concomitant reduction in canine size (e.g., Wolpoff, 1976). These examples, and others like them, suggest particular sociosexual behaviors and mating systems were a potential driver of the evolution of unique human characteristics within the order Primates. It is not surprising, then, that any hypothesized mating system inferred from fragmentary fossil data is likely to draw scrutiny; this has particularly been the case for hypotheses