The Institutional Animal Care and Use Committee (IACUC)
UNC-CH Policy on Cage Density of Mouse Cages
Introduction:
Updates in the 8th edition of the Guide for the Care and Use of Laboratory Animals have prompted re-evaluation of the
UNC-CH mouse cage density policy. The revised policy below is a blending of new Guide requirements with the long
standing, performance based UNC policy.
Mouse Density in Breeding Cages:
Mouse strains have a large variety of requirements for successful breeding. Many of these depend on acknowledgment of
the natural behaviors of mice in the wild, and include communal rearing of litters, seasonal differences in fecundity and
innate physiological differences among “normal” but inbred strains of mice. As a result of this diversity, it is impossible to
make a “one size fits all” rule for regulation of mouse breeding. The purpose of this policy is to allocate sufficient space
for mothers with litters that will allow for the pups to develop to weaning without detrimental effects.
Key Action Points for Breeding Mice:
1. Birth of litter: Research staff record date of birth (on investigator breeding card)
2. UNC mouse breeding cages range in size from 70 to 82 square inches (static micro-isolator 70, green line 78, blue
line 82), are permitted to contain the following maximum numbers of animals:
Single litter
Multiple litters*
1 Adult
no limit on pups
2 Adults
no limit on pups
3 Adults
no limit on pups
14 pups
11 pups
9 pups
* no need to limit the number of pups until pups reach 14 days of age
3. Wean pups at > 10 gm and approximately 21 days old.
4. 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:
Animals
Weight,
gram
Mice in Groups <10
Up to 15
Up to 25
>25
Female + litter
Floor Area of
cage/animal,
inches2
6
8
12
≥ 15
51 recommended
space for the
housing group
# Adult mice
per static microisolator 70
inches2
11
8
5
4
See chart above
# Adult mice
per green/blue
line 78- 82
inches2
13
9
6
5
See chart above
Revised October 9, 2003, July 16, 2004, January 12, 2005, January 26, 2005, January 27, 2006, April 17, 2009, October 8, 2010, June
6, 2012; July, 10, 2015; April 8, 2016.
Other Essential Information:
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DLAM will mark cages not meeting above requirements with a yellow “Action Required” card which will list the
day indicating when corrections must be made. Corrections must be made by the end of the day listed on the card
(two calendar days). Example- DLAM flags a cage with a yellow “Action Required” card on Wednesday,
corrections must be completed by end of the day Friday (two calendar days). On Saturday DLAM staff will
separate according to the policy. For a cage marked on a Friday, DLAM will separate the following Monday.
Cages that exceed the densities listed in this policy are considered out of compliance when the date on the “Action
Required” card has expired.
Options for reducing numbers: remove female, remove male, move mother/ litter, cull pups w/ unusable
phenotype, move to larger cage
After the expiration date, DLAM is authorized to correct the overcrowded cage, separate animals, and assess a
separation charge without notifying the investigator. DLAM will correct by removing a male or separating litters.
If there is no space in which to separate, DLAM will euthanize animals to correct the cage density.
In addition to weekdays and weekends, the end of day date policy applies for extended weekends and holidays.
If investigators are unable to meet the end of day date requirement, they should contact DLAM in advance to
make the appropriate arrangements.
DLAM supervisors will inform the IACUC of ongoing compliance problems with this policy.
The IACUC will consider requests for an “Exception” to this policy on a case-by-case basis if the investigator
provides scientific justification with documentation. The exception will not be in effect until the PI has received
written notification from the IACUC.
Non-breeding cages allow a maximum of 5 (25g) adults of the same sex (note: 6 adults if < 25g)
Research staff should avoid placing mice prone to fighting in the same cage. As persistent fighting is known to be
common in certain mouse strains, older animals, males not weaned together, or males that are not littermates,
these conditions should 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.
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 IACUC policy: Environmental Enrichment Program for Animals
Note: Breeding Software Program - Conditional Allele Mouse Planner (CAMP) – created at 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
http://helixweb.nih.gov/CAMP/
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Revised October 9, 2003, July 16, 2004, January 12, 2005, January 26, 2005, January 27, 2006, April 17, 2009, October 8, 2010, June
6, 2012; July, 10, 2015; April 8, 2016.