For many mammals, play fighting is the predominant social behavior of juveniles. However, the neural circuits regulating this behavior have remained elusive and, in females, have been completely ignored. In rats, play fighting is often reported to be sexually dimorphic, with males exhibiting higher levels than females. Many studies, however, fail to detect this sex difference, likely due to differences in experimental conditions. In the present study, we first asked whether the social environment influences play fighting in male and female juvenile rats. After a 24-hour isolation period, 35 - 36 day-old male and female Wistar rats were paired with a same-sex playmate for 30 min in 1 of 3 social contexts: 1) in their home cage with an age-matched intruder (Resident-Intruder), 2) in their home cage with a weanling-aged intruder (Resident-Weanling; age of weanlings = 18 - 21 days), 3) in a novel cage with an age-matched playmate (Neutral Cage). Play sessions were recorded, and play behavior was scored as the number of pounces and pins occurring in the 30 min session. Solitary controls were treated identically to experimental animals except that they were recorded alone in either their home cage or a novel cage during the “play session.” To probe the neural circuitry of play, rats were perfused with 4% paraformaldehyde 30 - 45 min after the end of the play session, and the brains were processed for cFOS immunohistochemistry. Preliminary results indicate that there were no sex differences in the number of pounces or pins in the Resident-Intruder context, nor was there a difference in these behaviors between the resident and the intruder. In Resident-Weanling pairs, levels of play fighting were decreased in both males and females to a similar degree. The younger the play partner, the lower the amount of play; play fighting was virtually absent in the majority of juveniles paired with 18 day-old weanlings. A sex difference in play was only seen in the Neutral Cage context with females displaying decreased pounces and pins. These experiments demonstrate that the social context can influence play: a younger playmate incites less play fighting in both males and females, whereas a neutral environment incites less play fighting in females than in males. Forthcoming cFOS immunohistochemical analyses will identify putative male and female play circuits.
Paul, MJ., Zhu, VZ., Terranova, JI., Murray, E., Ismail, N., & De Vries, GJ. (2011). The social environment impacts sex differences in play fighting. In Unknown Host Publication Society for Neuroscience.