Before you even begin breeding make sure you plan properly; think ahead. Why are you breeding? Do you have homes for the offspring? Do you have a pet store that you can possibly sell to? When breeding mice you want to make sure that your breeders are in good health. Mice in poor health will produce unhealthy offspring. You definitely want to make sure that they have fresh clean water all the time and are fed appropriately; do not skimp on you feeding to save money.  As with anything quarantine all incoming rodents before they are introduced to you breeding program. Strict quarantine or isolation of all newly acquired rodents for at least 4 weeks greatly helps prevent disease among pet mice and rats. This recommendation is especially important for pet rodents because of the severity of certain diseases that they may harbor without showing signs of illness. 

Mice can best be bred when mated within preferable age ranges. These can vary strain by strain, but broad generalizations can easily be made. Typically, males can be mated from six weeks of age on, and have a reproductive lifespan of 12 to 18 months. Females can be mated a little younger than males, at five weeks of age. Pregnancies may occur earlier, but this is quite variable and should not be relied upon in an effective breeding strategy. Females may be continuously mated up to 10 months or more, but the size of litters produced will diminish after 7 or 8 months of age. Of note, virgin female mice not mated before 3 months of age often have a diminished fertility, and may breed poorly or not at all after this milestone. It is therefore important to adjust breeding strategy accordingly, such that females are not retained for the purposes of mating and line retention much past this age.

In general, female mice reach sexual maturity, and estrous cycles commence around 35 days of age. However, inbred strains often reach sexual maturity a little later than outbred strains. Consequently, many labs begin breeding females at 5-6 weeks of age.
Males mature slightly later than females. Males are typically placed into breeding at 6-8 weeks of age. The age male mice are weaned may affect their ability to successfully mate, and some groups recommend weaning inbred male mice intended for breeding at 28 days to improve their reproductive performance.

 Reproductive Biology

Gestation time 19 - 21 days
Age at weaning 3 weeks
Age at sexual maturity 6 - 8 weeks
Approximate weight at birth 1g
Approximate weight at weaning 8 - 12g
Approximate weight at adult 25 - 35g (male > female)
Lifespan 1.5 - 2.5 years Average litter sized 4 -12
Total number of litters per breeding female 4 - 8
Useful breeding life of females 6 - 8 months
Useful breeding life of males 18 - 24 months

Breeding Systems

 

Monogamous - One male and one female are selected and paired together for the duration of their breeding life. This system simplifies record keeping and lends itself well to maintaining inbred or outbred colonies.

 

Polygamous - Also referred to as harem breeding, it is a system where one male is housed with two or more females (keeping in mind mouse housing density and overcrowding guidelines). This system results in the large number of young from the least number of breeding animals. It is the most economical method of laboratory animal production.

 

Inbreeding - Brother/sister or parent/offspring matings for a minimum of 20 generations. This type of system is used to produce animals that are very genetically similar. The reproductive performance and behaviors can vary depending on the strain.

 

Out breeding - Also referred to as random breeding, this system avoids the mating of close relatives and produces the maximal amount of genetic heterogeneity and large litters. Animals of the same stock are mated, producing a more vigorous animal by maintaining genetic diversity. Accurate records are necessary in order to prevent breeding animals that are related to each other.

 

Line Breeding - In this system, the mating of animals by specific trait is performed. This is usually done to propagate mutant or transgenic lines, or because the trait is needed for research.

 

Cross Breeding - The mating of animals of different breeds or strains, also called "hybrid cross."

 

Backcrossing – Breeding of successive offspring to pure (wild type) mice generation after generation so that your mutation or phenotype is on a “pure” background. Backgrounds are not pure unless 20 generations of backcrossing has occurred. For example, each new generation of mice is bred to a pure C57B6 mouse.

Cage Density

Overcrowding of cages may result in estrous cycle suppression and trampling of neonates. In addition, overcrowded cages also become soiled quickly and may lead to detrimental levels of ammonia and other waste gas accumulation within the cage.

To reduce overcrowding of cages, litters should be separated by sex at weaning. Continued housing of male and female littermates after weaning may result in unwanted pregnancies, unnecessary neonatal losses and overcrowding.

Housing Requirements

Male mice are typically housed singly, or with one or more females for breeding. Male mice from the same litter may be housed together, but fighting often occurs as the animals reach sexual maturity. Once a male mouse has been used for breeding, he should not be returned to group housing as fighting often ensues and may result in loss of a valuable breeding animal. Prior to breeding, males should be placed in their own cage for several days prior to introducing females to allow the male to mark the cage and establish his territory.

 Health

Many infectious agents (bacterial and viral) of rodents can cause decreased reproductive performance, neonatal mortality or result in unhealthy pups.

 Sexing Mice

Determining the sex of mice is best accomplished by utilizing two major differences. The first is comparing the distance between the anal opening and genital papilla (anogenital distance). This distance is shorter in females and greater in males. The second takes advantage of the fact that in mice over 12-14 days old there is an absence of fur in a band across this anogenital space. Effective sexing often is achieved by using both of these differences in combination. Determining the sex of very young mice may be difficult for the beginner, and it is often helpful to compare a number of littermates to one another. Additionally, it should be noted that in older mice, the presence of nipples indicates a female, as male mice do not have nipples.