The effects of group type and young silverbacks on wounding rates in western lowland gorilla (Gorilla gorilla gorilla) groups in North American zoos

In North American zoos, male gorillas are often housed in all-male (bachelor) groups to provide socialization for males not managed in breeding groups. These groups exhibit long-term cohesion and stability and males in bachelor groups are no more aggressive than males in mixed-sex groups. Previous studies have shown that aggression in male gorillas is more directly related to age rather than group type, with young silverbacks (YSB; males 14-20 years of age) having higher rates of aggressive behavior than males of other age classes. Despite this, anecdotal reports have persisted that bachelor groups have higher wounding rates than mixed-sex groups. To assess wounding in zoo-housed gorillas, all instances of wounding across 28 zoos (180 gorillas, 45 social groups) were recorded over a 26 months period via a standardized data sheet. Similar to previous reports, we found age to be an important determinant in wounding. Bachelor groups that contained YSB's had significantly more wounds than bachelor groups without YSB's (U = 14.0, z = -2.193, P = 0.029). There was no difference in wounding rates between mixed-sex and bachelor groups without YSB's (U = 69.5, z = -0.411, P = 0.689). These data further demonstrate the importance of behavioral management of YSB's in zoos and the viability of bachelor groups as a long-term housing solution for male gorillas.