Fig. 1

Males play more than females. Male and female pups were assessed for play behavior on PN27–PN38 with a same- and an opposite-sex partner. Play partners were the same every day, except for PN33, on which novel play partners were introduced. a Combining frequency of play from PN27–PN38 showed sex differences in both same- and mixed-sex pairs (Student’s t tests with Welch’s correction: ***p < 0.001). However, sex of the play partner did not change total frequency of play (paired t tests: ns = not significant). b Separation of play behavior into individual days did not indicate an interaction between the pair type and age (two-way repeated measures ANOVA). c Comparison of play on PN32 (familiar partner) and PN33 (novel partner) revealed a reduction in play for females paired with novel female partners (two-way repeated measures ANOVA with Bonferroni post hoc: *p < 0.05)