Attack and defense during play fighting appear to be motivationally independent behaviors in muroid rodents

Play fighting is composed of two subcomponent behaviors-attack and defense. If these two components are controlled by the same motivational system, it is expected that they will co-vary. Therefore, it is expected that (1) when species are compared, those that exhibit high levels of playful attack should also exhibit high levels of playful defense; (2) when individuals are compared within a species, those that exhibit high levels of playful attack should also exhibit high levels of playful defense; and (3) developmentally, as play fighting wanes, both playful attack and playful defense should wane simultaneously. A comparison of playful attack and playful defense among six species of muroid rodents reveals that they are not correlated; high rates of defense can occur with low rates of attack, and vice versa. A comparison of individuals from one species (i.e., rats) reveals that those exhibiting the highest levels of attack are not necessarily those exhibiting the highest levels of defense. Finally, a developmental analysis of rats and other species of muroid rodents shows that attack wanes with age whereas defense does not. These data indicate that playful attack and playful defense are motivationally distinct behaviors in these species.