4 bachelor gorillas pose at the Bronx Zoo. Studies show that oxytocin levels are higher in gorillas in bachelor group housing than those in single management. Photograph by JULIE LARSEN MAHER for the World Conservation Society.

Understanding how primates navigate their social environment in human care is critical to the advancement of welfare-forward animal management and conservation efforts. Researchers typically find mixed-sex groups with a single adult silverback male in nature. However, in North American zoos, Western lowland gorillas (WLG, Gorilla gorilla gorilla) have often been housed successfully in bachelor (all male) groups. Recent research published in the journal Primates looked at the neuroendocrine hormone oxytocin in urine to evaluate the affiliative nature of various social groupings of WLG living in North American zoos. The collaborative team of primatologists from Case Western Reserve University, Cleveland Metroparks Zoo, Disney’s Animal Kingdom, Ohio State University, and the Dian Fossey Gorilla Fund also looked at cortisol concentration as a measure of social stress to achieve a more comprehensive view of the gorilla’s social environment.  

In the largest endocrine study of male WLG to date, both oxytocin and cortisol hormones were measured in the urine of 71 male Western lowland gorillas living at 23 accredited zoos throughout the United States. The male gorillas were managed in one of four group strategies: mixed-sex (n = 29), bachelor groups (n = 36), single management (n = 3), or fission-fusion (n = 5) – an alternation strategy of single management and mixed-sex. Singly managed WLG were kept in their own enclosure to prevent physical altercations but with visual, auditory, and olfactory access to other gorillas. Hormone levels from urine were measured using enzyme immunoassays, including Arbor Assays Oxytocin ELISA Kit (K048-H) with a protocol validated previously by the research group.  

The Results

The study found that oxytocin levels were significantly higher (P ≤ 0.05) in both bachelor and fission-fusion group males than in mixed-sex and single-management housing. The cortisol levels did not significantly differ between bachelor and mixed-sex groups, but both were lower than singly housed males (P ≤ 0.05). These results suggest males living in social grouping benefit from decreased stressors and that male and female gorillas may not form as strong of an affiliation bond as previously assumed, given the predominance of mixed-sex groups in nature.  

These findings highlight the importance of maintaining social housing of WLG and encourage the continued adoption of bachelor grouping as a positive management strategy when housing resources are scarce.  

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