Adult-adult play in primates: Comparative analyses of its origin, distribution and evolution

Comparative analyses were conducted on a data set derived from the literature so as to test several hypotheses which were developed to explain the distribution of adult-adult play fighting within the order primates. Ratings for play occurring in sexual and non-sexual contexts were developed. Three hypotheses were evaluated: (i) that play occurring in non-sexual social contexts is a byproduct of its use in sex; (ii) that the occurrence of play is related to its use for social assessment and manipulation, and so is more likely to be present in species with reduced familiarity between individuals; and (iii) that phylogenetic affiliation influences the likelihood that species within clades engage in play. We used independent contrasts to test the first two hypotheses, and both were significant, with the presence of play in sexual contexts accounting for 14-16% of the variance of play in nonsexual contexts, and reduced social familiarity accounting for 30-40% of the variance in the occurrence of play in non-sexual contexts. To test the third hypothesis, we mapped the occurrence of both types of play onto known phylogenies. The overlap was not congruent, indicating that phylogenetic relationships did not account for the distribution of play. Given that play in both sexual and non-sexual contexts was more likely to occur in species with a social organization involving reduced frequency of contact between the sexes and other social group members, we suggest that the likely adaptive value of play fighting is as a tool for social assessment and manipulation. The possible factors that mitigate the use of play fighting for these purposes, such as the availability of other forms of communication that could serve similar functions, are discussed.