Rodent Breeding: Mice

Last updated on July 25, 2016 10 min read Breeding Animals - Rodent Breeding: Mice

Boston University is committed to observing federal policies and regulations and AAALAC international standards and guidelines for the humane care and use of animals.

Mice have short gestation times and large litters. Therefore, cages may become overcrowded quickly if the individual responsible for managing the breeding colony and separating animals at the proper times does not do so in a timely fashion.

When overcrowding of cages happens, the animals become uncomfortable and stressed. Pups can die from being trampled. In static micro barrier cages, the air quality quickly deteriorates with a high density of animals and may predispose them to respiratory disease.

Overcrowding of cages is an animal welfare concern and can have a deleterious effect on research.

This policy provides guidelines for mouse breeding colonies to define standards and responsibilities for mouse housing and mouse breeding cages. These policies assign responsibility for action when mouse breeding cages are overcrowded.





Any recurring problems with mouse breeding colony management will be brought to the attention of the IACUC.

Breeding Schemes Procedures

Two different breeding schemes are acceptable. In either, the designated colony manager (research staff or BU ASC staff) is responsible for carefully monitoring pregnancies.





Required Documentation and Cage Cards




Weaning Pups






Number of Mice Per Cage

Cage sizes vary in the different facilities; the colony manager is encouraged to ask BU ASC staff about the size of the cages in which their mice are housed.



Defining Responsibility for Separating and Weaning Mice



BU ASC Actions in Cases of Overcrowding

  1. The BU ASC staff checks for overcrowding (O/C) and pregnancy when performing daily health checks and when changing cages. Any cages that are overcrowded according to standards defined above are marked with a Problem Notification—O/C Card, dated, and initialed.
  2. When overcrowding is noted, the responsible individual, the designated colony manager, is given up to 48 hours to correct the problem, depending on the severity of the overcrowding. Note: Weekends and holidays count as days and are not exempt.
  3. If overcrowding is not addressed within the allotted time, BU ASC staff separates the mice and Special Service charges will be incurred at a minimum of 15 minutes.
  4. Males that are fighting or have fight wounds: ASC staff will separate promptly.
  5. When a harem-housed mouse is noticeably pregnant (usually 14 days gestation), 48 hours. However, if a female seems to be about to give birth, she is promptly separated.
  6. When two litters, one newborn and one previous litter, are in one cage, separation is performed as soon as possible. In such a case, the BU ASC staff separates the older pups into separate cages and gives a few food pellets on the cage floor. Female and newborn pups are left in the breeding cage. The cage is marked with a Problem Notification—O/C Card and any other information necessary to identify the mice, dated, and initialed.
  7. Any time a cage is significantly O/C and the welfare of the animals is at stake (Emergency O/C), the animals are promptly separated into acceptable group sizes.
  8. Adding/updating the number of cages is documented on the census sheet by the colony manager or the person who separates the mice.
  9. When the overcrowding is corrected, the O/C Card is removed.

In all cases where ASC staff are performing separations, there will be a Special Service charge for a minimum of 15 minutes.

BU IACUC Approved October 2010, Revised January 2014, Reviewed June 2019

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