Animal User Requirement #1

Title: Requirement for Laboratory Mouse Breeding and Weaning

Purpose: To prevent overcrowded cages and assure litters are weaned appropriately.

Scope and Responsibilities:

  • Animal caretakers—identify overcrowded cages and notify PIs and/or laboratory staff of the situation; separate cages if PIs and/or laboratory staff fail to do so.
  • Investigative staff—wean litters at the appropriate times and manage their colonies
  • Facility management—train animal caretakers to recognize overcrowded cages.
  • Research veterinary staff—provide oversight to ensure adherence to this policy


  • Space requirements for mice will be based on the recommendations for space as outlined in the most current Guide for the Care and Use of Laboratory Animals, unless RARC veterinary or IACUC approval has been granted for a different configuration.
  • Mice are typically bred in male:female ratio of 1:1 or 1:2.
    • If bred in 1:1 ratio, the male, female and the litter may remain together until the pups are weaned PROVIDED the older litter is weaned before the next litter is born
    • If breeding is conducted in a 1:2 ratio in a standard sized cage (i.e. shoebox), it is recommended that the two females be removed to an individual cage when observed to be pregnant.
  • Cage cleaning may be delayed when litters are <7 days old present to prevent undue stress on nursing dams.
  • Date of birth of litters must be recorded on the cage card by the laboratory staff or care staff
  • A cage is overcrowded if a new litter is born before the older litter from the SAME female is weaned (“double litter”)
  • A cage is considered overcrowded if a litter remains with the female beyond 24 days of age, given that for most strains litters can be weaned at 18-21 days
  • Weaning beyond 24 days of age is a special circumstance and requires either veterinary approval (if done sporadically for health reasons) or prior ACUC approval (if done routinely by the laboratory.)
  • If overcrowded cages are not corrected within 24 hours of notification to the PI, animal care staff will separate cages. Fees may apply.
  • Some animal care units may have more stringent requirements than those outlined in this policy. It is the responsibility of the PIs and/or laboratory staff to check with their individual animal care units to determine this.

Author: J. Welter, C. Patten Jr.