Abstract

The establishment of bachelor groups of western lowland gorillas (Gorilla gorilla gorilla) in captivity was considered to be a necessary husbandry step in response to growing concern within the European and North American co-operative breeding programmes that breeding situations are limited [Harcourt, A.H., 1988. Bachelor groups of gorillas in captivity: the situation in the wild. Dodo 25, 54–61; Johnstone-Scott, R.A., 1988. The potential for establishing bachelor groups of western lowland gorillas (Gorilla gorilla gorilla). Dodo 25, 61–66]. Comparisons of behaviour between bachelor groups and breeding groups, targeting situations of social conflict, give an insight into the functions of male/male interactions within bachelor groups.

Observations were undertaken at two zoos in the United Kingdom, one housing a bachelor group and one housing a breeding group of G. g. gorilla. Data were collected on the frequencies of escalated and non-escalated aggression (aggression), display, play-fighting and displacement (avoidance of aggression), and food-sharing and resource-sharing behaviours (tolerance) between males in three age-classes: silverback, blackback and sub-adult.

Significant differences in behaviour were found between the three age-classes. In both the bachelor (Kruskal–Wallis: K[2] = 13.58, p < 0.001, post hoc critical value = 9.43) and breeding group (K[2] = 14.50, p < 0.001, post hoc critical value = 9.87) the silverback exhibited higher levels of escalated aggression than the sub-adults and higher levels of non-escalated aggression than either the blackbacks or the sub-adults (K[2] = 14.90, p < 0.001, post hoc critical value = 9.43; K[2] = 21.16, p < 0.001, post hoc critical value = 9.87 for bachelor and breeding groups, respectively). The bachelor group sub-adult initiated significantly lower numbers of displacements (approach–retreat behaviour) than either the silverback or the blackback (K[2] = 9.72, p < 0.01, post hoc critical value = 9.43), whereas in the breeding group the sub-adult only initiated significantly lower numbers of displacements than the silverback (K[2] = 11.52, p < 0.01, post hoc critical value = 9.87). Significant differences in play-fighting between the age-classes were only seen in the breeding group, the blackback initiating more play-fights than the silverback (K[2] = 8.63, p < 0.01, post hoc critical value = 9.87). Significant differences in behaviour were also found between the groups. In all age-classes the breeding group individuals exhibited higher levels of chest beat display than the bachelor group (Mann–Whitney: U[10,10] = 9.00, p < 0.001; U[10,10] = 26.0, p < 0.05; U[10,10] = 10.5, p = 0.001 for silverbacks, blackbacks and sub-adults, respectively). The breeding group silverback also showed higher levels of non-escalated aggression than the bachelor group silverback (U[10,10] = 20.0, p < 0.05). The blackbacks in the bachelor group exhibited higher levels of displacements than the breeding group blackback (U[10,10] = 13.00, p < 0.01). These results indicate possible differences between bachelor and breeding groups of captive gorillas in their methods of managing social interaction. However, these results are based on a small sample size and represent the initial findings of a larger study. They should be treated with caution until the full study is completed.