Office of Research Integrity

Rodent Breeding Colonies

General Information | Breeding Schemes | Colony Management

Recent changes in NIH expectations require updating of institutional policies to remain in compliance with all applicable regulations. The entire text of the Guide for the Care and Use of Laboratory Animals (the Guide) is available for review online or may be downloaded as a PDF file.

General information

  • Optimal reproductive age span: 2 – 10 months1; some males can be productive longer
  • Estrus cycle: 4-5 days1
  • Postpartum estrus: A period within 24 hours after parturition when females are fertile and can conceive. After this period, they are not fertile until the pups’ weaning age (usually ~21 days)
  • Gestation period: 19-21 days1
  • Weaning age: ≥ 19 days old
  • Requests for exemptions from any of the policies related to breeding will be considered on a case-by-case by the IACUC, but must be specific and justified in the protocol.

Breeding Schemes

All schemes require only one male per cage.

  • Monogamous (Pair-breeding): One adult male and one adult female
    • Pros/Cons:
      • Preferred method to minimize overcrowding
      • Allows for identification of the dam and sire of the litter
      • Utilizes post-partum estrus
  • Polygamous: One adult male and multiple females
    Trio: One adult male and two adult females
    Harem: One adult male and more than two adult females
    • Pros/Cons:
    • Produces the maximum number of offspring per male mouse
    • Females may share task of rearing offspring
    • Utilizes post-partum estrus
    • More complicated record keeping
    • May not know which pups belong to which female
    • Can be more time consuming and difficult to manage due to complications of record keeping and the need to separate multiple pregnant dams or dams with their litters to prevent overcrowding.

Harem mating of mice is often used for strains that are difficult to breed or if intensive breeding is required. When a harem mating system is used, one male is housed with multiple females. Unless the pregnant females are removed prior to giving birth, the multiple litters produced result in severe overcrowding within the cage. The cage becomes soiled rapidly, resulting in an unhealthy intracage environment and the number of mice in the cage exceeds NIH guidelines for mouse housing.

In order to provide a healthy intra-cage environment and to meet the recommended minimum space requirements in the 2011 revision of the Guide2, trio mating schemes (one male with 2 females) can be used with the following provisions:

  1. If females are visibly pregnant, one female must be removed to a separate cage prior to parturition.
  2. In the larger cages, the male may be left with the female and litter provided that the litter is weaned at 21 days.
  3. For litters that can’t be weaned at 21 days, the male must be removed from the cage prior to delivery.

Policies for Colony Management

  • Contact information: All cages must have a cage card with the Principal Investigator’s name, current protocol number, and the name & email address of the contact person. Investigators are responsible for updating this information. The facility animal care staff use this information to contact the laboratory if a cage is overcrowded or needs attention.
  • Maximum mice/cage: The maximum number of adult mice per cage is five*. This is based on the average weight of an adult mouse and the size of the caging used in our animal facilities. Recommended spaces for commonly used group-housed animals are indicated in the Guide for the Care and Use of Laboratory Animal2.
  • Birth/wean dates: DLAR staff are responsible for recording the birth and/or weaning dates on their breeding cages. These dates are critical for breeding programs with tight timelines for the weaning of pups and the birth of subsequent litters. The typical weaning age is 21 days. However, in some cases, genetically modified mice may need to remain with their mothers for a longer period of time due to inability to use the lixit or to consume the chow; thus separation may need to be delayed up to 28 days of age. In breeding mice with these constraints on weaning the male must be removed from the cage during the female’s pregnancy and a Husbandry Deviation form must be completed for each strain.
  • Post-partum estrus pregnancy: If the dam has a litter and is pregnant due to breeding during post-partum estrus, then toward the end of gestation, daily monitoring by the lab is required to watch for the birth of the litter and welfare concerns, such as trampling of neonates and fighting. Where this breeding scheme is used, the current litter of the pregnant dam must be weaned at 21 days of age to avoid overcrowding and high levels of filth in the cage.
  • Weanling housing: Weanlings must be housed at a maximum of five per cage*. In some cases, it is beneficial to provide weanlings with gel packs to ensure hydration. These are available in the animal facilities.
  • Responsibilities: Research personnel are responsible for the following: mating set ups, separation of adults in adherence to this policy, weaning, updating contact information. DLAR personnel are responsible for recording the birth and/or weaning dates.

Trio or harem breeding: If using trio or harem breeding schemes, and more than one female has a litter, each dam with her pups must be removed to another cage if any animal welfare concerns are observed, such as trampling, fighting, very high levels of filth, or if overcrowding is inevitable. Also, the mice must have enough space to express normal postural adjustments2.

*Specific racks and rooms will be identified to indicate acceptable cage density.

1. Association for Laboratory Animal Science 2011 Reference Directory
2. Guide for the Care and Use of Laboratory Animals, 8th edition