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Guidelines for Efficient Mouse Colony Management

(These guidelines were adapted from a document provided by the Baylor College of Medicine.)

The goal of managing any mouse colony at UCI should be to maintain adequate numbers of animals in as little shelf space as possible, while adhering to the university’s policies regarding health and well-being of the mice, and minimizing labor costs.  What constitutes “adequate numbers” will, of course, depend on a number of factors, including:

o       experimental needs,

o       breeding characteristics of a given strain,

o       genotypes and phenotypes (morbidity/mortality) of individual mice,

o       limits of approved animal use protocol. 

Therefore, these guidelines will address in detail only the minimum effort and cage space needed to keep a normal strain of mice “on the shelf”.  However, the principles of mouse numbering, culling, and replenishment can be utilized on any scale to minimize cage space.

Different strains of mice vary in fecundity, and certain mutant strains can be difficult to breed, due to a variety of factors such as small litter sizes, low fertility, poor mothering instincts, high rates of cannibalism of newborns, and higher morbidity or mortality resulting from the genetic mutation/alteration.  When acquiring a new genetically modified strain, therefore, it is always a good idea to consult with someone who has direct experience with maintaining that strain.

• Cages required to maintain a strain: Simple strain maintenance generally requires no more than 2-3 mating cages and 3-4 additional cages to hold weaned pups that will be used to replace old breeders.  To predict the cages needed to produce mice for experiments, figure about one litter per month from young breeders, and about 6-8 pups/litter, until experience proves otherwise.  Maintaining these production levels requires consistent replacement of older breeders.
• Breeding: House breeders in pairs of one male and one female. Gestation lasts 19-21 days.  Check cages at least twice each week to flag pregnant females and record approximate birth dates.  Pups should be weaned (removed from breeding cage and separated by sex) at 3 weeks of age.  Identification can be accomplished prior to weaning by any of several methods (see below).  Tissue for genotyping is usually collected at the same time that ID numbers are applied.
• Weaning: At 3 weeks of age, male and female pups are moved to separate holding cages, with no more than 5 mice per cage.  Females of any age or breeding status can be housed together.  To minimize fighting, weaned males should be group-housed only with their littermates, and only until they are exposed to females.  Males from different litters can be grouped together only if they are no more than 4-5 days apart in age.  Even littermates may have to be separated as they age, to prevent fighting.  Males that have been used as breeders must ALWAYS be housed singly, because they will kill each other.
• Replacement of breeders: Replace a breeder pair if:

o       they have not produced a litter in two months,

o       they are producing small litters (1-3 pups per litter),

o       they are killing their pups.

New breeder pairs should be 8-12 weeks of age.  Although both males and females can breed up to 8 months of age and beyond, if new breeders are available there is little point in continuing to use breeders older than about 6 months.  Also, a mouse that has not been allowed to breed prior to about 4 months of age may never breed successfully.  As breeders age, both litter size and the frequency of litters decrease.  It may be wise to wait for new breeders to produce a litter, before euthanizing old breeders, if the strain presents any unusual breeding problems.

• Cull older progeny: Euthanize older offspring as new litters are weaned.  It is easy to neglect weaned pups, since they are not breeding.  Periodic evaluation of all cages is essential to minimizing cage usage.  If progeny are more than 3 months old, and you have new litters to replace them, and you don’t need them for experiments, then take them off the shelf.  The only reason to keep more than one or two weaned litters for a strain that is not being used in experiments is if that strain exhibits unusual morbidity or mortality, or is otherwise difficult to breed.
• Pup identification: Weaned pups from different litters may be housed together if they are uniquely identified (or if they don’t need to be identified).  This helps to minimize the number of holding cages.  (See above, however, for caveats about males.)  The following table lists the common methods of pup ID, with pros and cons of each.

An example of low-level maintenance

Week 1:

The progeny from mating cage 1 are ready to be weaned.  Three females and four males are found.  They are numbered and placed in two cages.  The two cages of older progeny are discarded (shaded boxes).  Result: no change in cage number.

Week 2:

The progeny from mating cage 2 are ready to be weaned.  Two females and three males are found and numbered.  The females are added to the cage of 3 females created in week 1, while the males are put into a new cage.  The oldest cage of male progeny is discarded (shaded).  Result: no change in cage number.

Week 3:

The oldest male progeny can be discarded (shaded).  No other action is necessary.  Result: cage number reduced to five.