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URMC / Animal Resource / Services / Vivarium / Mouse Cage Density Policy

Mouse Cage Density Policy

Overcrowded mouse cages represent a significant animal welfare concern. Such cages are noncompliant with Public Health Service (PHS) Policy and our Assurance to PHS. The Guide for the Care and Use of Laboratory Animals states the PHS recommendations for housing densities. In order to standardize housing densities and prevent or eliminate the possibility of overcrowding within cages, the University’s Animal Resource has adopted the following UCAR-approved policy. Ventilated cages accommodate up to five mice greater than 28 days of age. Static cages accommodate four mice greater than 28 days of age. Cage densities exceeding these numbers represent clear policy violation..


  • Standard Breeding: 1-3 females: 1 male; pregnant females housed individually before birth of pups.
  • Continuous Breeding: 1 male: 1 female; not separated before birth of pups. This strategy takes advantage of the post-partum estrus which occurs within 14 -28 hours after parturition.
  • The breeding strategy utilized must be described in the UCAR protocol.
  • Justification is required for continuous breeding and/or for cage densities which exceed those described above.
  • Continuous trio breeding results in overcrowded cages (e.g. male and two females and litter(s)) which must be cleaned more frequently. For that reason, this strategy is discouraged and requires scientific justification. If necessary (and justified and approved by UCAR), PIs must submit a special request describing the continuous trio breeding strategy and obtain cage cards/stickers that identifies cages as continuous trio breeders. These cages are subject to a higher per diem associated with more frequent cage changes which are required for these higher density cages.


Investigators who choose to manage their own breeding colonies are responsible for timely weaning. Most mice may easily be weaned at 21 to 23 days of age, but this period may be extended to 28 days for fragile mice. The Animal Resource staff reports the date new pups are found on the cage card. Litters not weaned before their 29th day of age will be reported to the investigator on day 29 with a requirement for the PI or staff to wean that day. DLAM will separate unweaned litters the following morning for a charge of $50.00 per cage. If the investigator weans the cage but fails to inform DLAM, a fee will be charged. Delayed weaning protocols must be approved by UCAR with specification of actual weaning ages (up to 35 days of age) for extremely fragile mice. Additionally, a special request must be submitted to the Animal Resource office identifying the group of mice approved for delayed weaning. Continuous breeding with delayed weaning is not permitted.

If a breeding strategy results in two litters from the same dam, the older litter must be weaned when the new litter is born. This means that litters may require weaning between 17 and 20 days to prevent overcrowding and trauma to newborn pups. Cages identified with two or more litters will be reported to the PI. If the younger litter appears to be unaffected by the presence of the older pups, the investigator must wean the older litter by the following day. If not, DLAM will wean the older litter for a fee. If the younger litter is being harmed by the older pups, DLAM will immediately wean the older litter for a fee and notify the investigator. The investigator is expected to provide supportive care (moistened chow, Hydrogel®, small house in vent racks) and daily observation until early weaned pups are self-sustaining.

The DLAM veterinary staff provides training in the management of rodent breeding colonies for investigators and their staff. DLAM also offers colony management services to PIs for a fee.

Overcrowded Cages

Overcrowded cages (> five mice over 28 days of age in ventilated cages, > four mice over 28 days of age in static cages) will be reported to investigator. DLAM will remove mice from overcrowded cages if the investigator has not done so by the day following notification. There is a fee for this service.


A completed cage card must be present on all mouse cages. Please refer to the Barcoding page for information on cage card activation. The information on the card should include: the investigator's name, the approved UCAR protocol number, an animal identification number (if applicable), the mouse strain/stock and the account number. Individual animal identification such as ear punches, ear tags, toe clips, tattoos and implantable transponders is encouraged, especially in cases where animals are group housed and/or appear identical. All methods of identification must be described in the animal protocol and approved by UCAR.

The DLAM and Vivarium staff is available to discuss any questions you may have regarding this policy.  Please do not hesitate to contact the Animal Resource Office at (585) 275-2651.