Effects of age and group type on social behaviour of male western gorillas (Gorilla gorilla gorilla) in North American zoos

The long-term management of male gorillas in zoos is a significant challenge. The demographics of the population – specifically a 50/50 sex ratio and the desire to form breeding groups that contain a single male and multiple females – necessitates housing a majority of adult males outside of mixed-sex groups. The primary approach for socially housing these individuals within the North American population has been the formation of all-male groups. Our previous research had found that captive all-male groups were cohesive and showed behavioural patterns similar to what had been observed in the wild. To gain a more complete understanding of male social dynamics as a function of group type, we examined social interactions among male gorillas living in either bachelor or mixed-sex groups. Subjects included 48 individuals ranging in age from 6 to 44 years. Overall, we found age to be the primary predictor of social behaviour. Rates of affiliative behaviour were highest in immaturity with a gradual shift to more dominance related behaviours (displacements, non-contact aggression) in young adulthood. Non-contact aggression increased as a function of number of adult males in the group, which likely accounts for higher rates of non-contact aggression in silverbacks living in all-male as compared to mixed-sex groups. The results further reiterate that all-male groups are a feasible, long-term housing strategy for male gorillas in zoos and highlight important considerations for male management including: introduction of males in immaturity, delayed formation of breeding groups until males reach social as well as physical maturity, and the role of dominance-related behaviours, as opposed to escalated aggression, in adult male interactions.