International Journal of Comparative Psychology

Play is rare in the Animal Kingdom, but relatively common in the larger brained vertebrate taxa. Comparisons at the level of classes, orders, and, in some cases, families, suggest that larger brained taxa are more likely to contain playful species. However, at the species level, such relationships generally disappear. In some well documented mammalian taxa, such as Rodentia, it is clear that there are species which do not play at all, some where the play is quite complex and some showing all grades in between. Comparative methods are used here to supplement proximal analyses of the content of one particular form of play, play fighting, so as to identify the neurobehavioral mechanisms that are needed in rodents to evolve complex play from simpler antecedents. At least five independent neural mechanisms are shown to be necessary to produce the most complex example of play fighting in rodents. The identification of such levels of control provides a new method for systematizing the diversity of play present in mammals. Furthermore, this approach sets the stage for re-evaluating the relationship between brain size and play. That is, the issue can be reconceptualized in terms of whether species with larger brains are more likely to have a greater number of control mechanisms. It is not that larger brained species are more likely to play, but rather, that when they do play, the content of their play is more flexible. Suitable comparative data sets are needed to test these possibilities.