Characterization of juvenile play in rats: importance of sex of self and sex of partner

Background: Juvenile social play is observed in many mammalian species, and its disruption in several neuropsychiatric disorders has greatly increased interest in understanding the origins and sources of variability in this behavior. Methods: We quantified social play behavior in juvenile rats and investigated the impact of sex and familiarity of the play partner. Sex differences in play behavior were investigated by comparing males and females from either same-or mixed-sex pairs with data pooled over 12 days of analysis. Whether play was altered based on the sex of the play partner was assessed using a paired analysis to compare play with a same-or opposite-sex play partner for both males and females. Additionally, a repeated measures design was utilized to determine whether play changed with increasing age. On postnatal day 33, a novel play partner was introduced. We used a repeated measures analysis to compare postnatal day 33 with the previous day. These approaches were used to assess the effects of age, sex, sex of partner, and familiarity of partner on total social play behavior as well as how play was broken down into components, such as pouncing, pinning, chasing, and boxing. Results: There were sex differences in total frequency of play, and specific parameters of play behavior, such as chasing, pouncing, pinning, and boxing. Additionally, males significantly altered their play behavior in response to the sex of their play partner, whereas females were more sensitive to the familiarity of the play partner. Conclusions: This study provides critical groundwork for uncovering factors that regulate social play behavior and can be used to guide future mechanistic based work.