Overcrowded mouse cage with mice climbing the cage walls

Overcrowded Cage Policy

Overcrowded Rodent Cages

When overcrowded cages are identified by DAR staff, an “overcrowded cage fee" is assessed at the time of notification for each cage of breeding mice that is overcrowded as defined by IACUC policy.

The reason for this fee is to ensure that laboratories responsibly managing their mouse colonies are not financially burdened via the per diems in subsidizing the operation of breeding colonies where the colony managers are less than attentive. Historically, overcrowded cage notifications did not incur a fee. However, in a significant number of instances the DAR was "gamed" by mouse colony managers who developed the habit of awaiting notifications and then interceding to redistribute breeding mice and pups just at the cusp of DAR intervention and fee levying. This resulted in increased DAR efforts and costs that were not directly recouped and thus were folded into the per diem rates where these casual breeding colony management practices were subsidized by all investigators.

Regulatory Basis

The IACUC has a Rodent Colony Management policy which includes information on overcrowded rodent cages (link to Policies, Guidelines, Exemptions | Emory University | Atlanta GA) This policy is based upon mouse population density and space requirements provided in the 8th edition of the "Guide for the Care and Use of Laboratory Animals" and clarifying guidance from the NIH and AAALAC International.

Mouse overcrowded cages at Emory generally distribute along two lines:

(1) Pups in excessive numbers either from (a) failing to wean litters in a timely fashion such that numerous animals of multiple ages accumulate in a cage with younger mice often being trampled and out-competed for milk and (b) failing to split harems (e.g., trios) at appropriate times resulting in an excessive pup critical mass. These situations will be the emphasis of the DAR and the notification charges.

(2) simply housing too many adult mice per cage (e.g., 6-7 adult mice in the cage when only 5 are permitted by policy) - this aspect will be managed by the IACUC and not by DAR.

Exceptions to this policy and to these conditions are possible, but must be specifically approved by the IACUC, defined in the protocol, and with a visual designation on applicable mouse cages.

Helpful Advice

To avoid falling prey to DAR notification charges and to remain in compliance with the aforementioned IACUC policy, it is recommended that the following be reviewed with research staff, specifically those managing breeding colonies, at a laboratory meeting or other like forum:

  • Breeding colonies should be checked daily by persons responsible for managing production for several reasons, but in this case to prevent new litters from being vulnerable to the dominant effects of older pups from pre-existing litters.

  • Mouse pups must be weaned between 19 and 24 days of age.

  • Where mice in production are maintained as trios continuously, pup density must be reduced to a maximum of 12 pups per cage where one or more pups is at least 14 days of age.


 Updated: 09.18.2025