Standard on Mouse Cage Density


University of North Carolina at Chapel Hill Standard on Mouse Cage Density



The University of North Carolina at Chapel Hill ("UNC-Chapel Hill") standards and procedures described below provide guidance to all researchers and animal handlers to ensure sufficient space for mice with litters that will allow pups to develop to weaning (transfer of pups out of the mother’s cage) without detrimental effects, and to house adult mice in space that is sufficient for their needs. This Standard is a blend of the requirements in the 8th edition of the Guide for the Care and Use of Laboratory Animals and the long-standing, performance-based UNC-Chapel Hill policy.

Multiple factors contribute to successful breeding and welfare of mice. Additionally, different mouse strains can exhibit variations in breeding behaviors and physiological parameters, making it difficult to make a “one size fits all” rule for regulation of mouse breeding. The purpose of this standard is to allocate sufficient space for mice with litters that will allow for optimal breeding and welfare, and to indicate appropriate space for adult mice.


This Standard applies to all Principal Investigators, laboratory members, and Division of Comparative Medicine (DCM) personnel involved in the breeding and maintenance of mouse colonies.


Mouse Breeding Cages

1. Nesting Material

Nesting material must be added to all breeder cages at the time of set-up. Approved types of nesting material for mice can be referenced on the Institutional Animal Care and Use Committee (IACUC) standard: Environmental Enrichment Program for Animals.

2. Documenting Date of Birth

Research staff are required to record date of birth on the DCM Weaning Notice (pink) card within three (3) days of birth. (Note - see the DCM instructions for use of the pink card).

2a. 'Pink Card' Standards

Note: Please see the image of the card in the "Attachments" section of this document.

  • When a new litter is noted in the cage, document the pups' date of birth (DOB) on the pink card using your best conservative estimate of age.
  • Record the wean by date for day 23 using a china marker/wax pencil.
  • This pink card should be used by DCM and the labs to record the litter’s DOB.
  • As each litter is born, record DOB and 'wean by' date on a separate line, if applicable.
  • If there are no litters, remove the pink card and wipe clean, saving for further use.

3. Maximum Number of Animals Per Cage

UNC-Chapel Hill mouse breeding cages are permitted to contain the following maximum numbers of animals:

Maximum Number of Animals Per Cage
  1 Adult 2 Adults 3 Adults **
Single Litter no limit on pups no limit on pups no limit on pups
Multiple litters, after
one litter reaches
14 days of age *
14 pups 11 pups 9

* Pup numbers are not limited until the oldest litter reaches 14 days of age.

** Cages with more than 3 adult mice, plus pups, will be flagged for correction with an ‘Action Required’ (yellow) card. (Please see the image of the card in the "Attachments" section of this document.)

4. Weaning Pups

Wean pups by 23 days old. Typically, pups will have reached 10 grams by 23 days old for most litters.

Do not wean pups that are <10 grams. If pups are expected to weigh less than 10 grams at 23 days, a request for an Exception is required (ACAP Section 8.0).

5. Cage Density Guidelines

After pups are weaned, cage density guidelines are based on body weight as shown in the table below.

Recommended Minimum Space for Mice Housed in Groups - Stock Cages
Animals Weight of Mice (grams) Floor Area of Cage Needed per Animal (inches2) # Adult Mice per Static Micro-Isolator 70 inches2 # Adult Mice per Green/Blue Line 78-82 inches2
Mice in Groups < 10 6 11 13
  Up to 15 8 8 9
  Up to 25 12 5 6
  > 25 ≥ 15 4 5
Female + litter   51 recommended space for the housing group See chart above See chart above

* Non-breeding cages allow a maximum of five (5) adults of the same sex.

‘Action Required’ Standards

  • General “Action Required” yellow card practices:
    • DCM will mark cages exceeding the recommended minimum space requirements with a yellow “Action Required” card and will note the date research personnel must make the corrections at the top of the card. Laboratory personnel must make corrections before the end of the expiration date listed on the card (i.e., two consecutive calendar days), including weekends and holidays.
      • Example - DCM flags a cage with a yellow “Action Required” card on Wednesday and corrections must be completed by end of the day Friday (two calendar days). On Saturday morning DCM staff will separate according to the standard. For a cage marked on a Friday, DCM will separate the following Monday.
    • Each day DCM will make a list of cages flagged with “Action Required” cards. If the lab corrects the density by the end of the day the card is placed, there will be no charge assessed. If the lab does not correct the density by the end of the day the card is placed, DCM will assess a flagging charge per cage on the following day.
      • Note:  action by the laboratory may be required on a weekend or holiday. It is the responsibility of the lab to monitor their colonies to avoid flagging charges on a weekend or holiday.
    • On the morning after the expiration date, DCM is authorized to separate animals to correct the overcrowded cage and will assess a separation charge per cage. If there is no space in which to separate, DCM may euthanize animals to correct the cage density.
      • Options for reducing animal numbers are provided on the “Action Required” card.
    • DCM is not required to notify the investigator prior to marking the cage, separating the cage, or euthanizing animals.
  • The IACUC expects that research staff will manage colonies such that yellow "Action Required" cards are not required on a frequent or excessive basis. The IACUC will be notified about labs which chronically rely on DCM to manage colonies and when greater than 10% of the investigator’s  total breeding cages are flagged with “Action Required” cards at any given time.
  • After correcting the density issue in a flagged cage, researchers should flip the ‘Action Required’ card over and place it in the back of the card holder. This will alert DCM staff that the issue has been addressed. DCM will verify the cage is compliant and will remove the card.

Additional Standards

  • Research staff should avoid placing mice prone to fighting in the same cage. Housing conditions such as certain mouse strains known for persistent fighting, mice of different ages, males not weaned together, or males that are not littermates are to be avoided. The IACUC considers incompatible housing resulting from fighting/injury an animal welfare concern that must be remedied as soon as it is identified.

Note: Breeding Software Program - Conditional Allele Mouse Planner (CAMP) - created at National Institute of Health (NIH) National Heart, Lung, and Blood Institute and published in Transgenic Res. 2011 Aug 26 to help scientists set up a breeding strategy and calculate the costs of generating mice with conditional alleles. To access the free application, go to the NIH's CAMP website.


Requests for exceptions to this Standard must be reviewed and approved by the IACUC.


Yellow “ACTION REQUIRED” Card: Yellow card placed by DCM technicians on rodent cages that are out of compliance for cage density, indicating a specific date that the issue must be remedied by. Please see the image of the card in the "Attachments" section of this document.

Pink Wean Card: This pink card is placed by either DCM or research staff, whomever identifies a new litter first. The date of birth (DOB) and the wean date (DOB + 23 days) is to be written on the card as soon as the new litter is noticed. Instructions for this card’s use, and an image of the card are included in this Standard. Please see the image of the card in the "Attachments" section of this document.

Cage Density: The maximum number of rodents allowed in a given cage space based on the animal’s weight and cage type.

Related Requirements

External Regulations and Consequences

The Guide for the Care and Use of Laboratory Animals, 8th ed. 2011, pp. 56-58

University Policies, Standards, and Procedures

For more detailed guidance, please refer to the University Policy on the Care and Use of Vertebrate Animals for Research, Training and Teaching Purposes.

Contact Information

Contact Information Table
Subject Contact Telephone Email
Protocol or Standard Questions Office of Animal Care and Use 919-966-5569
Rodent Housing Questions Division of Comparative Medicine 919-962-5335  

Important Dates

  • Effective Date and title of Approver: 10/09/03, IACUC
  • Revision and Review Dates, Change notes, title of Reviewer or Approver: Revised 07/16/04, Revised 01/12/05, Revised 01/26/05, Revised 01/27/06, Revised 04/17/09, Revised 10/08/10, Revised 06/06/12, Revised 07/10/15, Revised 04/08/16; Revised 03/01/2019 Veterinarians reviewed, new pink card and yellow action card added with descriptions on utilization.

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Article ID: 132203
Thu 4/8/21 9:26 PM
Wed 2/14/24 11:51 AM
Effective Date
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06/06/2022 12:00 AM
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Vice Chancellor
Last Review
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12/01/2023 12:00 AM
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06/06/2022 12:00 AM
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12/01/2024 12:00 AM
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09/12/2019 2:52 PM
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Research-Institutional Animal Care & Use Committee