UCLA

Many mammalian species display sex differences in the frequency of play behavior, particularly in the domain of social play. Yet the animal literature largely lacks longitudinal studies on developmental patterns of play over the lifespan, which are important for understanding the timing of sex differences and the evolutionary functions of this ubiquitous behavior. I analyzed sex-specific patterning of social play, solitary play, and grooming using an 18-year dataset on a cohort of 38 wild white-faced capuchin monkeys (Cebus capucinus) followed since infancy. Rates of each behavior were measured as the proportion of point samples taken during focal follows in which the individual was engaging in social play, solitary play, or grooming. To determine sex differences in the rates of these behaviors, I ran three Poisson generalized linear mixed models with a random effect for each individual. Results for social play and solitary play showed significant fixed effects for age, such that rates of both types of play decreased with age. Results for social play showed significant fixed effects for sex and the interaction between sex and age, such that males had higher overall rates of social play than females and had a slower decline in the rate of social play with age compared to females. Solitary play did not show a significant fixed effect for sex; however, rates of solitary play decreased more quickly over time for females than for males. Females had significantly higher overall rates of grooming than males, and the interaction between sex and age was significant such that rates of grooming increased more quickly over time for females than males; in fact, the proportion of time spent grooming increased over the lifespan for females but decreased over the lifespan for males. My results suggest that males allocate more time overall toward social play than females, particularly throughout the juvenile period, but that females may compensate for lower bonding opportunities in social play through increases in time spent grooming. Results were consistent with two functional hypotheses of play, the practice and bonding hypotheses. This longitudinal study demonstrates that play behavior may be critical for the development of sex-specific social strategies and emphasizes the importance of lifespan perspectives on behaviors such as play and grooming.