Factors Associated with Injury Rate and Pregnancy Success in Rhesus Macaques

SIMPLE SUMMARY: Rhesus macaques are bred around the world for use in biomedical research. They have a strict dominance hierarchy which they reinforce with aggression, and fight injuries are a major welfare concern in group-housed macaques. In this study we examined 10 years of injury records at a colony in which the monkeys were housed in small breeding groups with a single adult male. We found that breeding females were disproportionally likely to be injured. We examined how factors, such as group size, might affect both the injury rate and the probability of the females becoming pregnant. We found that introducing a new adult male to a group had the greatest effect on the injury rate but also significantly increased the probability of the females getting pregnant. ABSTRACT: Fight injuries are a major welfare concern in group-housed rhesus macaques. This is particularly a problem in breeding groups. We investigated which factors might affect the injury rate in group-housed macaques and also looked at how the same factors might affect productivity. We analysed 10 years of health records at a breeding colony in which monkeys were kept in small breeding groups consisting of a single adult male and 2–13 females and their offspring or single-sex juvenile groups. We found that females over the age of 2.5 years in breeding groups were the most likely to be injured. We focused on these females and used generalised mixed-effect models to examine which factors affected the injury rate and their productivity (probability of getting pregnant). The biggest risk factor for injury was the introduction of a new adult male to a breeding group. However, this also produced a large increase in the proportion of females that became pregnant, suggesting that there may be a trade-off between the risk of injury and the productivity. We also found that females in large groups with a young breeding male had a very high risk of injury. We recommend keeping young breeding males (<7 years) in smaller groups.