The roles of phylogeny and sociality in the evolution of social play in muroid rodents

A composite index incorporating the frequency and structure (target, type of defence, etc.) of play fighting was used to compare the complexity of such play in 13 species of muroid rodents whose behaviour has been previously described. A phylogenetic comparison of the distribution of the complexity of play fighting revealed that relatedness did not predict complexity. The most likely pattern for the ancestral rodent was moderate levels of complexity, from which increases or decreases in complexity then appeared to have evolved independently, at the level of subfamily and genus. Given that phylogeny did not predict the distribution of the pattern of play fighting, an alternative hypothesis was tested. That is, that instead, the distribution was produced by species differences in sociality, as reflected by the degrees of male-female association amongst adults. The analysis revealed that play complexity was unrelated to species differences in sociality, with both highly social and relatively asocial species being equally likely to have high or low levels of play complexity. The implications of these results for the evolution of mammalian play are considered. (C) 1999 The Association for the Study of Animal Behaviour.