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

Abstract

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.

Patterns of wounding in hamadryas baboons (Papio hamadryas) in North American zoos

“…This finding may initially put caution towards housing hamadryas in this way, but the observed wounding rates are similar, if not lower, than what has been reported in zoo‐housed apes. Hamadryas averaged a single wound every 4 months, compared to every 1.3 months in chimpanzees (Ross et al, ) and every 2.5 months in gorillas (Leeds et al, ). Hamadryas in multi‐male groups received a wound approximately every 2.5 months, similar to the frequencies observed in chimpanzees and gorillas.…”

“…Methods for this study were modeled after multi‐institution assessments of wounding in chimpanzees ( Pan troglodytes ; Ross et al, ) and gorillas ( Gorilla gorilla gorilla ; Leeds et al, ). Instructions and training materials were sent to participating institutions.…”

“…(2) group size is not associated with wounding in other baboon species (Archie, Altmann & Alberts, 2014), thus these larger multi-male troops should not have increased rates of wounding compared to small singlemale and bachelor troops; (3) the housing of other primate species in natural social groupings, even when including multiple males, has not increased rates of wounding (Leeds et al, 2015;Ross et al, 2009), and (4) males use extreme coercion to maintain cohesion with females (Pines, Chowdhury, Saunders & Swedell, 2015) and generally respect the male-female bond within other OMU's (Kummer, 1968), thus housing multiple OMU's together in multi-male troops should not increase wounding.…”

“…Methods for this study were modeled after multi-institution assessments of wounding in chimpanzees (Pan troglodytes; Ross et al, 2009) and gorillas (Gorilla gorilla gorilla; Leeds et al, 2015).…”

“…The purpose of this study was to assess wounding rates across three group types managed in the North American Hamadryas Baboon SSP® population: multi‐male/multi‐female (hereafter multi‐male), single male/multi‐female (hereafter single male), and bachelor troops. We hypothesized that there would be no difference in wounding rates across troop types because (1) hamadryas have evolved to live in multi‐male groupings (Kummer, ; Swedell, ), thus this represents a natural social grouping that should not elicit major changes in behavior; (2) group size is not associated with wounding in other baboon species (Archie, Altmann & Alberts, ), thus these larger multi‐male troops should not have increased rates of wounding compared to small single‐male and bachelor troops; (3) the housing of other primate species in natural social groupings, even when including multiple males, has not increased rates of wounding (Leeds et al, ; Ross et al, ), and (4) males use extreme coercion to maintain cohesion with females (Pines, Chowdhury, Saunders & Swedell, ) and generally respect the male‐female bond within other OMU's (Kummer, ), thus housing multiple OMU's together in multi‐male troops should not increase wounding.…”