To play or not to play? Effects of social isolation length and stimulus rat familiarity on social play engagement in three laboratory rat strains

Social play is displayed by juveniles of many mammalian species and is vital to the development of social skills as shown in humans and rats. Here, we determined how two external conditions, social isolation length and social stimulus familiarity, modulate social play levels (by the experimental animals) and social play defense tactics (by the stimulus animals) in juveniles of three common laboratory rat strains: Long-Evans, Sprague-Dawley, and Wistar. Experimental rats were socially isolated for either 2 h or 48 h and were then exposed in their home cage to a familiar (cage mate) or novel sex-, age-, and strain-matched stimulus rat. We found that Long-Evans rats played more with familiar than with novel rats, irrespective of time isolated, Sprague-Dawley rats played the most in the 48h-Familiar condition, and Wistar rats played the least in the 2h-Familiar condition. Moreover, Wistar rats played more with novel rats than Long-Evans and Sprague-Dawley rats. Among the defense tactics analyzed, Long-Evans and Sprague-Dawley stimulus rats showed more counter nape attacks with familiar than with novel rats, Sprague-Dawley stimulus rats also showed more counter nape attacks after a 2 h versus 48 h isolation while Wistar rats showed more counter nape attacks after a 48 h versus 2 h isolation. The effects of the external conditions on social play levels and defense tactics were similar across sex for the three strains. Overall, more social play and more counter nape attacks were observed with a familiar playmate in Long-Evans rats, with a familiar playmate and 48 h isolation (social play) or 2 h isolation (counter nape attacks) in Sprague-Dawley rats, and with a familiar or novel playmate after 48 h isolation in Wistar rats. Future research could determine whether strain differences in neuronal mechanisms underlie these condition-induced variations in social play engagement.