Guidelines for the Establishment and Use of Mouse Breeding Groups
These ARAC Guidelines are intended to define considerations for the establishment andmaintenance of mouse breeding groups consisting of one male and up to four females who
are reproductively viable. These guidelines are not intended to address breeding groups
with more than four females since these breeding schemes will be addressed by individualInstitutes/Centers.
Pair breeding (defined as one adult male and one adult female) will minimize the chance ofovercrowding, simplify colony management, and allow identification of the dam with litter.
Pair breeding will be addressed by individual Institutes /Centers. Trio breeding groups(defined as one adult male and two adult females) have been used successfully at the NIH
and other institutions for several decades, especially for strains that are difficult topropagate. The use of continuous trio breeding groups can result in a higher breeding
efficiency by permitting breeding at the first post-partum estrus and facilitating pup survival
through cross fostering. The Jackson Laboratory recommends mouse trio breeding groups,
stating that “Most strains produce more progeny per cage if mated as trios because all adultcage mates generally help care for the young.” Harem mating (defined as one adult maleand three or four adult females) maximizes the progeny of an individual male but may
sacrifice the first post-partum estrus if pregnant females need to be removed from the cage
to avoid overcrowding. Harem mating is an option for projects that require an intensive
breeding program.
The discussion of rodent cage size and density in the Guide for the Care and Use ofLaboratory Animals (Guide) makes it clear that institutions should use those space
recommendations as a starting point for determining caging requirements. Furthermore, theGuide identifies examples of performance indices to assess the adequacy of housing which
include the animal(s) health, reproduction, growth, behavior, activity, and use of the space.
Both the NIH Office of Laboratory Animal Welfare (OLAW) and AAALAC International havestated that mouse cages of the size commonly used in the United States may be appropriate
for trio breeding groups, with the caveat that there are several factors that need to beconsidered when assessing the adequacy of cage space. For example, a mouse litter size canvary greatly depending on the age of the female or whether the strain is inbred or outbred.
The Guide recommends that sufficient space be available for mothers with litters to allow
pups to develop to weaning without detrimental effects for the mother or litter. Each
program/facility’s Animal Care and Use Committee is responsible for the review andapproval of a policy for mouse breeding groups that considers the number of adults,
number of pups, and age of various litters to ensure the adequacy of cage space.
Investigators who decide that trio breeding groups or harem mating would be a beneficial
breeding strategy for their research must accept responsibility for any overcrowding in theircolony, and follow these guidelines:
1. Trio breeding groups are best suited for the propagation of inbred, transgenic, orother strains of mice which generate small numbers of pups or are difficult to breed.
Crosses that produce larger litters are best propagated by a pair (defined as one
male and one female) breeding strategy.
2. Investigators using a breeding strategy that includes harem or trio breeding groups
must be aware that they have increased responsibilities to ensure “the adequacy ofcage space” for all animals. These increased responsibilities include the prompt
weaning of mature pups between 18 and 28 days of age and the prompt separationof animals if the cage density becomes greater than that established by the facility
or program. In addition, “difference in the age of the pups of different litters, growthrate, cage dimensions, and husbandry practices such as cage sanitation, etc.” mayrequire an Investigator’s prompt attention and intervention. Many programs
separate litters in the same cage that are born more than 7-10 days apart, with amother or their respective mother, to prevent the older pups from injuring theyounger pups.
References
1. Breeding Strategies for Maintaining Colonies of Laboratory Mice: A JacksonLaboratory Resource Manual. 2009.
2. Guide for the Care and Use of Laboratory Animals, 8th edition. 2011. National
Research Council (US) Committee for the Update of the Guide for the Care and Useof Laboratory Animals. Washington (DC): National Academies Press (US).
3. Office of Laboratory Animal Welfare FAQ. 2015.
http://grants.nih.gov/grants/olaw/faqs.htm#674
4. Trio Breeding (AAALAC FAQ), 2015.
http://www.aaalac.org/accreditation/faq_landing.cfm#Ctrio

Approved - 07/11/2012
Revised – 02/10/16, 2/27/2019, 03/23/2022