Cage aggression in group-housed laboratory male mice: an international data crowdsourcing project

Abstract

Aggression in group-housed laboratory mice is a serious animal welfare concern. Further understanding of the causes of mouse aggression could have a significant impact on a large number of laboratory animals. The NC3Rs led a crowdsourcing project to collect data on the prevalence and potential triggers of aggression in laboratory mice. The crowdsourcing approach collected data from multiple institutions and is the first time such an approach has been applied to a laboratory animal welfare problem. Technicians observed group-housed, male mice during daily routine cage checks and recorded all incidents of aggression-related injuries. In total, 44 facilities participated in the study and data was collected by 143 animal technicians. A total of 788 incidents of aggression-related injuries were reported across a sample population of 137,580 mice. The mean facility-level prevalence of aggression-related incidents reported across facilities was equivalent to 15 in 1,000 mice. Key factors influencing the prevalence of aggression included strain; number of mice per cage; how mice were selected into a cage; cage cleaning protocols; and transfer of nesting material. Practical recommendations have been provided to minimise aggressive behaviour in group-housed, male mice based upon the results of the study and taking into consideration the current published literature.

Group and Single Housing of Male Mice: Collected Experiences from Research Facilities in Sweden

“…In line with what has previously been reported [10][11][12]16,17], this study confirms in both workshops and the survey, that aggression is a common problem across animal facilities also in Sweden. This is also clear considering that Swedish authorities have received many requests for permission to deviate from present legislation and keep male mice single-housed.…”

“…This confirms results from another study were survey respondents considered C57BL/6 to be one of the strains too aggressive to group house [10]. However, in a recent study where data was systematically collected, C57BL/6 was among the strains showing low levels of aggression [12]. C57BL/6 is one of the most commonly used strains, which might skew the perception since this strain is encountered more often and this could explain the high number of comments related to this strain.…”

“…They were also reported to fight after being temporarily removed from their homecage and after transport. Effects on aggression related to different husbandry routines has also been reported in the literature [12,15,22]. This was further confirmed by the responses to the survey where fighting was reported to be common after cage changing and at sexual maturity.…”

“…Several studies have identified factors associated with aggression in mice [12][13][14][15] and some general husbandry recommendations have been suggested to decrease levels of aggression. Some examples are to keep mice in small groups (three animals have been suggested), to group animals at an age of 3-4 weeks, to keep siblings or familiar mice together, and to transfer nesting material at cage cleaning [13,16,17].…”

The occurrence of tarsal injuries in male mice of C57BL/6N substrains in multiple international mouse facilities

“…As these injuries were observed in three different C57BL/6N substrains it is possible that this genetic background is predisposed to tarsal injuries. Male C57BL/6 mice are widely reported to display aggressive behaviours towards cage-mates (Lidster, Owen, Browne, & Prescott, 2019). Both threat (thrust and mounting) and aggressive behaviours (boxing, parrying, fighting) are associated with establishing and maintaining dominance hierarchies in group house male mice.…”