Cooperation and affiliation between males may be key to the evolution of large multimale-multifemale primate groups in some species. Cacajao and Chiropotes form multimale-multifemale groups larger than those of most other platyrrhines (Cacajao: over 150 and Chiropotes: up to 80 individuals), and groups exhibit a high degree of fission-fusion dynamics. In both genera, males engage in affiliative, sex-specific behaviors and form all-male parties. Males in both genera also have conspicuous genitalia but can demonstrate sexual crypsis, or mimicry, wherein testes are retracted, resembling labia. Observed egalitarian interactions among males suggest that there is scramble competition for access to females, and aggression between males is uncommon relative to other social primates. As of yet, there are no genetic data to clarify dispersal patterns, and while relatedness among males would in part explain their affiliative relationships, there is some limited evidence for dispersal by males in Cacajao. In this review of recent studies of male-male social interactions in Chiropotes and Cacajao, we posit that the ability to maintain large groups in these genera may be related to the affiliative and perhaps coalitionary relationships between males, who may or may not be related. Affiliative male-male relationships may allow for monopolization of groups of females and facilitate group cohesion by reducing intragroup aggression; however data on male-male interactions with identified individuals will be required to determine patterns of affiliation, while genetic studies may be the most practical way of determining dispersal patterns for these genera.
Keywords: affiliation; cooperation; dispersal; kinship; large groups.
© 2015 Wiley Periodicals, Inc.