Guidelines on Mouse and Rat Breeding and Housing Management

Unit for Laboratory Animal Medicine
Sep 9, 2021 12:00 am

To address the health and well-being of mice and rats by ensuring safe breeding practices and population densities. Limiting breeding cage occupancy reduces the incidence of morbidity and mortality due to overcrowding.  

In all cases, cages should be regularly monitored to ensure the well-being of the neonates (e.g., size, age, and activity level of litter), as well as characteristics of the cage environment and colony breeding performance. Exceptions to this guideline require prior IACUC approval.

  • Responsibility

    1. Husbandry Personnel
    2. Veterinary Personnel
    3. Investigative Personnel
  • Breeding & Housing Management of Mice

    1. Breeding Density
      1. When breeding, no more than 2 adults and 1 litter, regardless of the size of the litter, will be allowed in a 75 square inch "standard" #1 mouse cage.
      2. Mice can be bred under the following conditions:
        1. Recommended Strategy:
          1. Monogamous pairs in a "standard" #1 mouse cage.
        2. Alternative Strategies:
          1. Breeding trios (one male, two females) or harem breeding (1 male, 3-4 females) in a "standard" #1 mouse cage.
            1. Separate pregnant females prior to parturition to ensure only 1 litter of pups with up to 2 adults remain in the cage after pups are born if using this breeding strategy.
          2. Breeding trios (one male, two females) in a 140 sq. in.  #3 "standard" rat cage.
        3. Wean the first litter of pups by 22 days if post-partum estrus is used in "standard" #1 mouse cages to prevent the presence of two litters in a cage, i.e., no extended weaning is allowed if post-partum estrus is used.
      3. The IACUC acknowledges that some specialized genetically modified lines may require more than 2 adults and 1 litter in the breeding cage to facilitate adequate production. Breeding records demonstrating poor breeding must first be reviewed with a faculty veterinarian and scientific justification submitted and approved of by the IACUC. See Guidelines for Documenting Scientific Justification for Exceeding Cage Densities for details on required data.
  • Management of Fighting Mice

    1. Fighting among adult male mice is a well-documented behavior and can result in severe wounding and death. To minimize fighting, the following group-housing practices are followed for mice:
      1. Do not combine adult male mice (older than 5 weeks). This does not apply to:
        1. Littermates that have been cage mates since birth.
        2. New arrival mice. Follow procedures outlined in Small Animal Receiving.
      2. Do not recombine adult male mice (older than 5 weeks) that have been separated.
      3. Group-house weaned males with littermates.
        1. Males from different litters can be combined within seven days of weaning, only if age difference is 7 days apart or less.
          1. Use a clean cage when combining males from different litters to prevent territorial behavior.
      4. Some strains are more aggressive than others and fight regardless of the age at combination.
        1. House males individually in these circumstances, or with females if intending to breed.
      5. Mice observed with fight wounds:
        1. Mild to moderate wounds (those involving minimal damage to the epidermis) necessitates consultation with ULAM veterinary personnel for determination of appropriate therapy.
        2. Severe wounds (e.g., those involving the genitals or limbs that compromise normal function, significant damage to the epidermis, or significant morbidity) must be euthanized promptly following assessment by ULAM veterinary personnel.
        3. See Mouse and Rat Husbandry SOP for policy on separation of animals with fight wounds.
      6. Justification for housing situations that fall outside of the normal must be approved in the IACUC animal care and use protocol.
  • Breeding & Housing Management of Rats

    1. Rats can be bred in the following conditions:
      1. Monogamous Pairs (on male, one female):
        1. Separate male before parturition if caging is <212 square inches, e.g., if housed in "standard" #3 rat caging.
      2. Breeding Trios (one male, two females):
        1. Follow housing densities for the type of caging used.
        2. See Appendix A for maximum housing densities for rat cages.
    2. Wean pups by 22 days of age in either breeding scheme.
      1. Delayed weaning (weaning at >22 days) requires prior IACUC approval. Additional floor space may be required in order to ensure animal well-being.
  • Appendix A: Maximum Housing Densities for Rat and Mouse Cages

Species: Mice Rats
Topic: Husbandry

Questions or concerns about the content of this document should be directed to the Unit for Laboratory Animal Medicine (ULAM) at (734) 764-0277 or