Castration of adult male C57BL/6JRj mice allows for resocialization and social housing of previously single-housed males: a harm-benefit analysis

Veterinary ethics play a critical role in animal research since veterinarians practicing in laboratory animal facilities face ethical dilemmas every day, such as the animal welfare issues that come along with standard laboratory housing. It is a common code of practice to keep mice in single-sex groups, although housing males together can result in stress or fatal injuries due to aggressive territorial and resource-related behaviour. However, if they are separated, social isolation also reduces their welfare. To eliminate aggression, 19 adult male C57BL/6JRj mice, which had been individually housed for up to 32 weeks, were castrated and resocialized in groups in this case report. Surgical intervention increased Mouse Grimace Scale scores for up to 150 min post-castration and concentrations of faecal corticosterone metabolites on day 2 post-castration when compared to baseline but did not affect burrowing and nesting, suggesting short-term stress and pain. Mice gained body weight within the following two weeks. After mice were resocialized in groups, a transient loss in body weight followed by weight gain reflected the process of habituation. The lack of fight-associated lesions and the high nest building performance provided evidence that the new groups were stable. Our investigation demonstrates that harmonious group-housing of castrated adult male C57BL/6JRj mice is possible. In terms of a lifetime harm-benefit analysis for the individual animal in this case report, the short-term post-surgical stress and pain may outweigh the long-lasting negative effects of single housing. Scientific data on the well-being of the mice provided the basis of the harm-benefit analysis.