Sex-Based Aggression in Wild Chimpanzees (Pan troglodytes) at Gombe Stream National Park, Tanzania
Open Access DepositedInnate behavioral differences due to genetic sex alone can not be observed in human children due to sex differences in early socialization. Thus, researchers often conduct comparative studies in other model systems. As one of our closest living relatives, chimpanzees (Pan troglodytes) provide insight into the development of sex-based behaviors with evidence from several studies including variations in aggression displays, skill development, and play styles between the sexes. The classification of sex-based behavior in chimpanzees has traditionally been determined using broad behavioral averages, which may oversimplify interesting variations within the data collected and skew our understanding of complex behaviors, such as aggression, and their correlates. This study aims to rectify this issue by using a 20-year subset (1991 - 2012) from the Gombe Stream Research Center database (108 individuals, 60 females and 48 males). The data was summarized as aggression rates, initiation rates and retaliation rates. Individuals were assigned age groups (Infant, Juvenile, Subadult, and Adult) to control for age-based differences in aggression rates. Results from a multivariate ANOVA indicate that sex and age group are significant for overall aggression rates with males and adults being more aggressive than others. Sex is significant for retaliation contact rates with males retaliating more. Sex, age group, and their interaction are not significant for the rate of contact initiated by an individual. This may be due to the frequency of displays in males to assert dominance that does not include contact aggression. This could also be caused by differences in alpha male styles, more affiliative or assertive. A repeat analysis excluding displays may also resolve this. Importantly, while we found differences in the average rates of aggression, ‘outlier’ individuals existed in the dataset
some females were as or more aggressive than males and some males exhibited aggression rates comparable to females. Future analyses should determine the hormonal and experiential correlates for ‘outlier’ individuals.
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