Mouse Breeding
Before you even begin mouse breeding, make sure you plan properly and think ahead. Why are you breeding? Do you have homes for the offspring, or a pet store that may buy them? When breeding mice, your breeders must be in good health, because unhealthy mice produce unhealthy offspring. Always provide fresh, clean water and a proper diet—don’t cut corners on feeding to save money. As with any rodent colony, quarantine all incoming mice before adding them to your breeding program. Strict isolation of all newly acquired rodents for at least 4 weeks greatly helps prevent disease, which is especially important because some pet mice and rats can carry serious illnesses without showing obvious signs.
Mice can be bred most successfully when they are mated within preferred age ranges. These can vary by strain, but some general guidelines apply. Typically, males can be mated from about six weeks of age and have a reproductive lifespan of 12 to 18 months. Females can be mated a little younger, at around five weeks of age. Pregnancies may occur earlier, but this is variable and should not be relied on in a planned breeding strategy. Females may be continuously mated up to 10 months or more, but litter size usually starts to decline after 7–8 months of age. Virgin females not mated before 3 months often have reduced fertility and may breed poorly or not at all, so it’s important to plan your mouse breeding strategy so females are not held back too long before being paired.
In general, female mice reach sexual maturity and begin estrous cycles at around 35 days of age, though inbred strains often mature a bit later than outbred strains. For this reason, many breeders begin pairing females at 5–6 weeks of age. Males mature slightly later than females and are typically placed into breeding at 6–8 weeks of age. The age at which male mice are weaned can affect their later breeding performance; some breeders recommend weaning inbred males intended for breeding at around 28 days to improve their reproductive success.
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.
Drawing by: Lorryta Bowler
2-day-old baby mice. The female on the left, the male on the right. Photo: Craig Robbins.
Possible weaning variances
Pups are weaned at three weeks of age. At three weeks, they should be fairly active, eyes open and eating food. How ever they may still be suckling. A good test for weather or not there ready to wean is their reaction when you remove the lid from the cage. If they stay perfectly still, there too young. If they poke around during this test, there old enough to be removed. If pups are weaned too early, they will not survive, so it is pivotal that pups are weaned at an appropriate time. If the animals are runted or otherwise underdeveloped, a positive benefit in remaining in the parental cage for one week (possibly more) until they are better able to thrive. Pups need to be weaned by the time the next liter is born. This will prohibit “double littering”, and the risk of older pups trampling the younger litter, competition for milk and undesirable ammonia build-up.
Disease implications in breeding
Mice can suffer from a variety of illnesses. The spectrum of diseases is dependent on strain, housing conditions and a wide variety of other conditions. The health of your mice should be important, if you feel that there might be something be wrong seek a veterinarian. Observed problems brought to their attention to allow timely intervention.
We hope you find this information helpful.
Best of luck,
Living Art Reptiles™.
Source Information:
Living Art Reptiles, Gary Brown, Aurora Burds Connor.
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