Standard on Rat Cage Density


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



The purpose of this Standard is to ensure sufficient space for rats with litters that will allow pups to develop to weaning age without detrimental effects, and to house adult rats in space that is sufficient for their needs. Social animals should be housed in stable pairs or groups of compatible individuals. Single housing of social species should be the exception and justified based on experimental requirements or veterinary-related concerns about animal well-being. This Standard reflects recommendations from the Guide for the Care and Use of Laboratory Animals: 8th edition, University of North Carolina at Chapel Hill ("UNC-Chapel Hill" or "University") performance standards, and UNC-Chapel Hill Institutional Animal Care and Use Committee (IACUC) assessment.

Successful breeding of rats is influenced by many factors. In addition, different rat strains can have a variety of requirements for successful breeding and physiological differences, making it difficult to make a “one size fits all” rule for regulation of rat breeding. The purpose of this Standard is to allocate sufficient space for rats with litters that will allow for optimal breeding and welfare and to indicate appropriate space for adult rats.


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

The IACUC expects that anyone involved in animal work at UNC-Chapel Hill will comply with this Standard. Requests for exceptions to this Standard must be reviewed and approved by the IACUC.


Key Action Points for Breeding Rats

  1. Birth of litter: Research staff are required to record date of birth on the DCM Weaning Notice (pink) card within three (3) days of birth.
  2. UNC-Chapel Hill rat cages range from 140 to 143 square inches (individually ventilated cages, static micro-isolator, standard rat caging) and are permitted to contain the following maximum numbers of animals:
    Maximum Number of Animals Table
      1 Adult 2 Adults 3 Adults
    Single Litter
    • No limit on pups
    • Unlimited pups under 14 days
    • 14 pups, 14 days or older
    • Trio mating (1 male with 2 females) - no pups
      (1 female should be removed before litter drops)
    Multiple Litters
    • Not allowed
    • Not allowed
    • Not allowed
  3. Wean pups by 23 days of age. The IACUC will consider exceptions when scientifically justified.
  4. At 14 days, DCM will flag cages that have more than 14 pups and 2 adults. Research personnel will have a 48-hour period after the cage is flagged to address the potential density issue. Cages exceeding allowed density of 14 pups and 2 adults will be considered non-compliant at 17 days.
  5. At 24 days, DCM will flag cages exceeding the allowed density. Research personnel will have a 48-hour period (including weekends and holidays) after the cage is flagged to address the potential density issue and avoid being considered non-compliant.
  6. After pups are weaned, cage density guidelines are based on body weight as shown in the table below.

Recommended Minimum Space for Rats Housed in Groups a

Recommended Minimum Space for Rats Housed in Groups Table
(Rats in Groups)
Weight (grams) Floor Area of cage/Animal (inches2) Height (inches)
7-8 <100 17 7
5-6 Up to 200 23 7
4 Up to 300 29 7
3 Up to 400 40 7
2 Up to 500 60 7
2 >500 b,c ≥70 7
Female + litter   124 (recommended space for the housing group)  

a All labs maintaining rats longer than 8 weeks and/or for ages older than 6 months should document weights at least monthly to ensure adherence to the Standard. Documentation should be available for IACUC review.

b Rats weighing greater than 500 grams but less than 700 grams may be pair-housed in standard rat caging per established UNC-Chapel Hill performance standards which demonstrated no health or behavioral issues associated with these housing conditions.

c As stated in the Guide for the Care and Use of Laboratory Animals, larger animals may require more space to meet the performance standards. PIs that need to house rats greater than 700 grams must justify housing in the standard caging (and require an experimentally-based IACUC-approved exception or a documented veterinary-based justification). Otherwise, PIs must utilize the larger rat cages at the higher per diem (cost per day) to maintain social housing within an acceptable space. Space allocations will be assessed, reviewed, and modified as necessary by the IACUC considering the performance indices (e.g., health, reproduction, growth, behavior, activity, and use of space) and special needs determined by the characteristics of the animal strain or species (e.g., obese, hyperactive, or arboreal animals) and experimental use (e.g., animals in long-term studies may require greater and more complex space). At a minimum, animals must have enough space to express their natural postures and postural adjustments without touching the enclosure walls or ceiling, be able to turn around, and have ready access to food and water. In addition, there must be sufficient space to comfortably rest away from areas soiled by urine and feces.

Other Essential Information

  • DCM will mark cages not meeting above requirements with a green “Action Required” card which will list the day indicating when corrections must be made. (Please see the image of the card in the "Attachments" section of this document.) Corrections must be made by the end of the day listed on the card (two calendar days).
    • Example - DCM flags a cage with an “Action Required” card on Wednesday, corrections must be completed by end of the day Friday (two calendar days). On Saturday DCM staff will separate according to this Standard. For a cage marked on a Friday, DCM will separate the following Monday.
  • Cages that exceed the densities listed in this Standard are considered out of compliance the day after the date on the “Action Required” card has expired. (Therefore, DCM will correct the issue at this time so that the density is in compliance.) After the expiration date, DCM is authorized to correct the overcrowded cage, separate animals, and assess a separation charge without notifying the investigator. DCM will correct by removing a male or separating litters. If there is no space in which to separate, DCM will euthanize animals to correct the cage density.
  • Rat cages cannot be off the ventilated rack system for extended periods of time. Please see UNC’s Standard on Animal Transport and Biosecurity for more detailed information.
  • The IACUC recognizes two breeding schemes:
    • Monogamous pairing (one male and one female): This is the preferred breeding scheme.
    • Trio breeding (one male and two female): This scheme is allowed but requires frequent monitoring. Prior to parturition (giving birth to offspring), one of the females should be removed. If continuous breeding is used, older litters should be weaned prior to parturition or as soon as the new litter is noted.
  • Under the current set of guidelines two females with two litters may not occupy the same cage. However, the IACUC recognizes that future genetically engineered rat strains may have greatly reduced reproductive capability and maternal behaviors. Scientifically based exceptions to this Standard may be requested on an as needed basis.
  • Research staff should avoid placing animals prone to fighting in the same cage (e.g., males-other than those weaned together, older animals, different strains). Incompatible housing resulting in fighting/injury is an animal welfare concern that must be remedied as soon as it is identified.
  • Nesting material must be added to all breeder cages at the time of set-up. Approved types of nesting material for rats can be referenced on the IACUC's Standard on Environmental Enrichment for Research Animals.
  • DCM supervisors will inform the IACUC of ongoing compliance problems with this Standard.
  • The investigator is responsible for contacting the IACUC if they cannot comply with this Standard.


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


Green “ACTION REQUIRED” Card: Green card placed by DCM technicians on rodent cages that are out of compliance for cage density, indicating a specific date that the issue should be remedied by. 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.

Weaning: The transfer of pups out of the mother’s cage

Related Requirements

External Regulations and Consequences

University Policies, Standards, and Procedures

Contact Information

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

Important Dates

  • Effective Date and title of Approver: 09/22/00; UNC-Chapel Hill IACUC
  • Revision and Review Dates, Change notes, title of Reviewer or Approver: Revised 03/18/04; 05/16/06; 04/17/09; 10/08/10; 12/07/12; 04/29/13; 04/08/16; Placed on University Standard template, Reviewed by veterinarians, clarification on when rats are separated after “Action Required” card is placed on the cage; UNC-Chapel Hill IACUC

Approved by: UNC-Chapel Hill IACUC

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Article ID: 132207
Thu 4/8/21 9:26 PM
Mon 3/11/24 12:59 PM
Responsible Unit
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Research-Institutional Animal Care & Use Committee
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Vice Chancellor
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10/20/2024 12:00 AM
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03/11/2024 12:00 AM
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07/12/2022 12:00 AM
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07/12/2022 12:00 AM
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09/16/2019 4:31 PM