Updated Policy on Mouse and Rat Breeding and Cage Densities

November 14, 2016

As of Monday, November 20, 2017, this policy has been retired and transitioned into a guideline.
All procedures and requirements remain in effect and can now be found in the

Guidelines on Mouse and Rat Breeding and Housing Management.

Questions or concerns should be directed to your ULAM Faculty Veterinarian.
If you don't know your ULAM Faculty Veterinarian, please send an inquiry to ulam-questions@umich.edu
and your question will be routed appropriately.



Table comparing per female production index in pair versus trio breeding

To ensure the continued health and well-being of mice utilized in research at the University, and to remain consistent with federal standards governing animal use activities, the Policy on Mouse and Rat Breeding and Cage Densities has been updated. The updated policy will go into effect on December 1, 2016.

In order to allow adequate transition time, the policy's implementation will begin with a five-month grace period. During this time, no fees will be assessed for veterinary technician time to perform separations and no non-compliances recorded. Several educational opportunities outlining proper compliance with the new policy will also be made available to research staff throughout the grace period.

On May 1, 2017, the grace period will end. From that date forward, failure to adhere to the new processes and procedures outlined in the policy will result in fees for technician time and could lead to non-compliance notifications.

All laboratory staff involved in the care and use of mice and/or rats at the University of Michigan should read the Policy on Mouse and Rat Breeding and Cage Densities in its entirety to familiarize themselves with all new policies and procedures.

  • The updated Cage Density and Breeding Policy is in effect and should be followed
  • In accordance with the updated policy, cages found to be overcrowded will be flagged by husbandry staff for evaluation by the veterinary technician
    • The veterinary technician will notify laboratory personnel of cages that need to be separated by a specified deadline. If the cage is not separated by that date, the veterinary technician will perform the separation and encourage that laboratory staff reach out to their Faculty Veterinarian
  • Repeated violations will result in required meetings with the Faculty Veterinarian for further education regarding the policy
  • During the grace period, lab members are strongly encouraged to attend one of the free Breeding Workshops offered by ULAM, take the online Breeding course, and/or meet with their Faculty Veterinarian to review breeding practices.

The new Policy on Mouse and Rat Breeding and Cage Densities seeks to minimize overcrowding conditions in breeding cages by requiring research staff to house no more than 2 adult mice and 1 litter (regardless of the number of pups in the litter) per standard mouse cage, unless appropriate scientific justification has been provided and approved by the Institutional Animal Care & Use Committee (IACUC).

The easiest way to adjust to this policy change, with the least risk of non-compliance, is to house monogamous pairs (pair breeding) in a standard mouse cage. The policy still allows for the following:

  • Trio or harem breeding (e.g., up to 4 or 5 adults in one cage, where multiple females are bred to the same male) provided all but 1 pregnant female is removed from the cage such that only 1 litter is born in the cage; in other words, trio breeding is acceptable, but trio birthing is not. Please note that this breeding strategy requires the laboratory to take full responsibility for appropriate, timely litter separation of the pregnant females. Failure to do so will result in more than 1 litter being born in the cage, which will result in levying a fee for technician time to perform separation of the litters to bring the cage into compliance, and a possible non-compliance notification once the grace period has expired.
  • Post-partum estrus can be used as long as the first litter is weaned prior to, or upon birth of, the second litter to prevent the presence of two litters in a cage (i.e., no extended weaning is allowed if post-partum estrus is used).

Graphic detailing the current and new processes for rodent breeding and cage densities

Laboratories who have poor breeding lines and who wish to apply for scientific justification in order to support continued trio breeding (i.e., housing 3 adults and 2 litters in a standard cage) should first consult with their ULAM Faculty Veterinarian to assure that they have taken all necessary steps to optimize their breeding.

After careful consultation and implementation of best practices to optimize breeding, laboratories may take one of the following actions:

  1. If the optimization steps correct the poor breeding problem, but the Principal Investigator would still prefer to house 2 females, 1 male, and 2 litters in a cage they may, permitting available housing space, pay to house their mice in larger (rat) cages.
  2. If the optimization steps do NOT correct the poor breeding problem, then breeding records demonstrating poor breeding must be submitted to the IACUC as scientific justification for exceeding allowed cage densities. The laboratory should collect breeding records from a minimum of 3 pair-bred cages in order to document poor breeding. These records must show that one or more of the following criteria were met:
  • More than 90 days have elapsed from mate date with no pups born,
  • 4 or less pups have been born in each of the last 2 litters,
  • Inconsistent breeding has occurred; 2 of the last 4 litters did not survive until weaning. 

Once this documentation has been approved by the IACUC, a Special Treatments or Procedures Form will be generated and placed in the room. The strain of approved mice will also be recorded in a database maintained by ULAM Faculty Veterinarians to denote that future rodents of this strain have been approved for overcrowding.

Questions or concerns about this new policy should first be directed to your ULAM Faculty Veterinarian. If you don't know your faculty veterinarian, please send an inquiry to ulam-questions@umich.edu and your question will be routed appropriately.

list of Frequently Asked Questions is also available via PDF download.