Urinary oxytocin and cortisol concentrations vary by group type in male western lowland gorillas (Gorilla gorilla gorilla) in North American zoos.

Evaluating how primates in human care function within their social environment is important for understanding and optimizing their management and welfare. The neuroendocrine hormone oxytocin is associated with affiliation and bonding, suggesting it can be used to evaluate the affiliative nature of social groupings. When paired with cortisol concentrations, social stressors can simultaneously be assessed, providing a more complete picture of primate social environments than if measuring either hormone independently. Here, we measured both oxytocin and cortisol in urine within a large subset of male western lowland gorillas (Gorilla gorilla gorilla; n = 71) living in North American zoos. Both endocrine measures were compared between social group types, with an emphasis on comparing bachelor and mixed-sex groupings to understand how these broad management practices affect male gorillas in zoos. Oxytocin concentrations were greater in bachelor group males than mixed-sex group males and singly housed males, providing physiological evidence that males in bachelor groups form comparatively stronger affiliative relationships than males in other group types. Cortisol concentrations did not differ between bachelor and mixed-sex group males and males in both group types had lower cortisol concentrations than singly housed males. These results indicate that males are similarly capable of coping with group-specific social stressors, and single management may expose males to additional stressors for which further study is needed. These data contribute to a larger body of research highlighting the value of bachelor groups from both a population management and individual welfare perspective.