Guidelines on Mouse and Rat Breeding and Housing Management

Unit for Laboratory Animal Medicine
Feb 23, 2024 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, are 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. 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 (i.e., “Trio Birthing”.) Justification must be approved by the IACUC in order to use this breeding strategy. See Guidelines for Documenting Justification for Exceeding Cage Densities for additional details.
  • 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 (animal >28 days old). This does not apply to:
        1. Adult male mice that are within seven days of weaning
        2. Littermates that have been cage mates since birth.
        3. New arrival mice. Follow procedures outlined in Small Animal Receiving.
      2. Do not recombine adult male mice 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 seven days apart or less.
          1. Use a clean cage when combining males from different litters to prevent territorial behavior.
      4. Examples of when combining male mice is acceptable or not
        1. Combining Male Mice Examples Allowed to combine? (yes or no)
          Male mice are weaned at 21 days old and another litter of male mice, in a different cage, is weaned at 28 days old. Yes
          Male mice are 35 days old and another cage of male mice are 38 days old. No
          Male mice are 50 days old and another cage of male mice are 60 days old. No
          Adult male mouse (over 35 days old) and a male mouse at 21 days old, and just weaned. No


    2. 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.
    3. Mice observed with fight wounds:
      1. Report fight wounds to ULAM veterinary personnel for determination of appropriate therapy. See Animal Care Identification and Communication Procedures for information on reporting clinical issues to veterinary personnel.
      2. See Mouse and Rat Husbandry SOP for policy on separation of animals with fight wounds.
    4. 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: Rats Mice
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