Breeding Policy for Mice and Rats

IACUC Policy: 007-08, Effective Date: 11/18/2005, Last Revision: 03/18/2026

Overview/Purpose

The purpose of this policy is to ensure compliance with the Guide regarding space requirements for mice and rats utilized for breeding activities under an approved Institutional Animal Care and Use Committee (IACUC) protocol.

Definitions

• Monogamous/pair breeding: one male and one female in breeding cage.
• Trio breeding: one male and two females in breeding cage.
• Harem breeding: one male and three or four females in breeding cage.
• Weaning extension: delayed weaning of underdeveloped or small pups beyond 21 days of age. See the requirements below.

Requirements

• Breeding must be listed as an animal source in the IACUC protocol.
• Only one breeding male may be housed with female(s) in the cage.

The following are acceptable breeding schemes:

Mice

• Monogamous/pair
• Trio
  • The total number of offspring in the cage should be less than 12 pups.
  • Pups should be weaned at 21 days of age and prior to the arrival of a third litter.
  • In the event of large litters (>12 pups present in the cage) or disparate ages of the litters, females should be separated with their litters to prevent overcrowding.
  • This scheme is not appropriate for animals needing a weaning extension or for strains with large litters/pups.
• Harem
  • Pregnant females must be moved into another cage before parturition with no more than two pregnant females per cage.

Rats

• Monogamous/pair
• Alternative breeding schemes (trio and harem) are not appropriate based on cage size limitations.

Colony Management

Colony management including setting up breeding cages, documenting date of birth for litters on breeding cards, and weaning is the responsibility of the Principal Investigator/research team. Animals must be weaned and separated from the cage at appropriate times to prevent overcrowding that could impact performance and animal welfare.

• Normally developing pups (mice and rats) should be weaned at 21 days of age when able to eat and drink on their own.
• Pups that are present in the breeding cage 3 days past the standard weaning age, and large enough to eat and drink on their own, will be weaned by ULAR staff. The research team will not be contacted or reminded of weaning dates.
• Cages with litters of significantly different ages (i.e. newborn pups and weanlings) including animals that are at least 21 days of age and of adequate size to eat and drink on their own must be separated immediately. ULAR will separate females with their litters if there are immediate welfare concerns or performance issues.
• If pup development is delayed, a weaning extension (e.g. 28 days) must be noted on the breeding card. Individual pups must be assessed regularly starting at 21 days of age and weaned promptly once they are able to eat and drink on their own. Normal-sized pups within the litter should be weaned at 21 days of age. Pups present in the breeding cage beyond the extension period that are large enough to eat and drink on their own will be weaned by ULAR.
• Weaning and colony management provided by ULAR to ensure animal welfare and compliance with this policy may incur cage separation and/or technical time charges.

Additional Information/Guidance

The Guide recommends 51 in² for a female mouse plus litter with a comment that "other breeding configurations will depend upon considerations." Current IVC (individually ventilated) mouse caging provides 77.5 in² of floor space.

The IACUC recognizes that trio breeding for mice may be an exception based on The Guide recommendations for floor space. Performance criteria including ammonia levels, pre- and post-weaning mortality, soiling of bedding, the difference in age of pups of different litters, and growth rate have been evaluated and the use of IVC cages for trio breeding of mice is acceptable when pups are weaned at approximately 3 weeks of age.

The Guide recommends 124 in² of floor space for a female rat plus litter. Current IVC (individually ventilated) rat caging provides 142 in² of floor space. Paired breeding cages of rats with a litter may exceed the Guide recommendations for floor space. Breeders will remain paired to allow for continual social housing of the male based upon performance criteria including pup survival and lack of fighting.