BACKGROUND

The Guide for the Care and Use of Laboratory Animals (the Guide, NRC 2011) states: "Not all members of a social species are necessarily socially compatible. Social housing of incompatible animals can induce chronic stress, injury, and even death. In some species, social incompatibility may be sex biased; for example, male mice are generally more prone to aggression (p. 64) and "Social stability should be carefully monitored; in cases of severe or prolonged aggression, incompatible individuals need to be separated." (p. 64)

IACUC Policy

Fighting Prevention:

To minimize fighting, the following group-housing practices must be followed for mice:

1. Adult male mice (older than 5 weeks) may be co-housed if they have been co-housed continuously since weaning and/or arrival at Wayne State University

2. Adult male mice (older than 5 weeks) that have previously been housed separately or used for breeding must NEVER be combined.

1. This is especially true if the males have been used as breeders or if males have been separated for more than 24 hours for experimental purposes (i.e., metabolic caging, volunteer wheel running).
2. Should a cage of male siblings be flagged as a health concern for fighting, the male aggressor/s will be removed from the cage and housed individually. This cage (or cages if multiple aggressors are found) will be labeled as an “aggressor” and is no longer permitted to be housed with its siblings or with any other male mice.
3. Unfamiliar adult male mice will almost certainly fight.

 3. Weaned males should be group-housed only with their littermates.

1. Males grouped from different litters should be combined within one week of weaning, and only with males that are one week apart in age or less.
2. When combining males from different litters, a clean cage should be used to prevent territorial behavior.

Fight Wound Treatment:

Some strains are more aggressive than others and will fight regardless of the age at which they are combined. When fight wounds are seen, the veterinary staff should be contacted, and the following steps may be taken:

1. Mice with mild to moderate wounds (those involving minimal damage to the epidermis, i.e., small dry scabs) should be treated with topical or systemic therapy as prescribed by veterinary staff.
2. Mice with severe wounds (e.g., those involving the genitals or extremities that compromise normal function, significant damage to the epidermis, or significant morbidity) must be euthanized in a timely manner. DLAR will euthanize the mice if they are unable to reach the PI or laboratory staff, or if the laboratory cannot attend to the animal in a timely manner.
3. The aggressor (typically the mouse without wounds) must be placed in a separate cage immediately, and NEVER recombined with other males.
4. If there are multiple aggressors (multiple mice without wounds, or all mice have wounds), mice should be separated into groups of 2 or 3 per cage and closely monitored. If aggression continues, the aggressive mouse (or mice) should be housed individually, clearly labeled as aggressive, and NEVER recombined with other males.
5. Occasionally males may demonstrate aggression towards females or vice versa. Also, intra-group fighting among females can occur. These should be dealt with on a case-by-case basis and may result in the separation of some mice. Every effort will be made to keep mice group housed.

Approved: December 2012

Revision Approved: October 2017, June 2023, July 2023, October 2023

Reviewed: October 2019