The role of the cortex in play fighting by rats: developmental and evolutionary implications

Play is a distinctive behavior of young mammals, especially mammals with a well-developed forebrain. For this reason it is thought that there may be a relation between forebrain evolution and highly elaborated play behavior. This study investigated the contribution of the cortex to play behavior by comparing play in control and neonatally decorticated rats (Rattus norvegicus). Play fighting in rats involves the combination of attack by one rat and defense by the recipient, with pinning arising when specific patterns of defense are used. Whether paired with another decorticate or with an intact pairmate, decorticates attacked pairmates as frequently as did intacts, and they were just as likely to defend against playful attacks as were intacts. Where decorticates differed from intacts was on a measure of pinning, in which one rat stands over a supine partner, decorticate rats displayed a reduction of 50% relative to control rats during the juvenile stage in which play is most pronounced (days 25 to 40). Juvenile decorticate rats adopted types of defensive responses which were less likely to result in the pinning configuration. Thus, a reduced pinning frequency reflects an altered pattern of defense, not a reduced level of play fighting. Rather, the decorticate patterns of defense were typical of those defensive responses displayed by adult rats. That is, decorticate juveniles exhibit a precociously mature pattern of playful defense. As intact controls mature, they come to resemble the decorticates in their defensive responses, and hence the difference in pinning frequency between decorticate and intact pairs diminishes. This suggests that the cortex may inhibit the escalation of defense in juveniles and thus promote prolonged ventral-ventral contact during play fighting. The results further suggest that the cortex is involved in the development of adult behavioral skills by facilitating juvenile play.