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How to breed mice

Breeding (Reproduction)

Does come into heat every three to five days and the gestation period in mice (the time from conception to birth) is 18-21 days. These are obviously averages and your own mice will have their own individual patterns. For example, my does always tend to deliver exactly on time at 21 days, but another breeder's may be slightly later or earlier.

A young litter of blue astrex

Mice get a lot of babies in a litter (anything from one to 32) depending on the age and size of the doe. A mouse is likely to have large litters if her mother had large litters, but owners may not always have access to this information. The first and last litters are usually the smallest (for example, my mouse Eve (below) had a first litter of 10, but after this her litter sizes rose to 14!). Mice can reproduce as early as five weeks old, which is why the babies should be separated by sex when they're around four weeks old to prevent any unwanted or unexpected pregnancies.

Top: Several very young litters share a nest and Bottom: the oldest litter begins to show pigment whilst the youngest babies are the brightest pink

Sexually mature mice must be properly paired to breed successfully. A single male mouse may be included in an enclosure with one or more female mice (the harem system) without difficulty. Including more than one male mouse in this situation can provoke fighting between males, who will try to compete for the right to mate with the females.

At the age of two to three weeks babies will follow mum and 'mug' her for milk

Female mice should not be bred before three months of age. This is because mice are not fully grown until at least 12 weeks of age, or sometimes older. Getting pregnant and raising a litter before this age can stunt the female's growth and impair the progress of the pregnancy and birth. Any accidental pregnancies before the age of 12 weeks should therefore be watched extra carefully and prevented where possible.

Mice are continuously ‘polyoestrous’, which means that they come into heat at fairly regular intervals (every three to five days) throughout the year until they are bred. The period during which the female is receptive to the male and allows breeding is around 12 hours and usually occurs at night. Female mice can come back into heat within l4 to 28 hours after giving birth to a litter. This is called a ‘postpartum oestrus’, which means that mothers can be nursing a litter and pregnant at the same time! This is stressful for both mother and babies and should be avoided.

Squidgey (blue doe) heavily pregnant

Pregnancy lasts an average of three weeks but can be extended by as much as 10 days if the pregnant female is suckling a previous litter. The average litter size is 8-12 pups, though it is not unusual for a female's first litter to be smaller in number. Litter size decreases as breeding females age. Though mutilation and cannibalism of the young are rare, it is wise not to disturb mice for the first two to three days after giving birth, unless you are very sure that the female trusts you completely and is not nervous. Pups are usually weaned at about four weeks of age. The female resumes her breeding cycle two to five days after her pups have been weaned (unless she was bred during her postpartum oestrus).

The gestation period for the mouse ranges from 18 to 22 days. Different strains have different averages within this range but even within a single strain, and even for a single female, there can be significant differences from one pregnancy to the next. Many different factors can have an effect on the length of pregnancy. For example, larger litters tend to be born earlier (Rugh, 1968), as is the case with humans. Non-inbred females tend to have shorter pregnancies than inbred ones, but this may be simply because they tend to produce larger litters as well as larger pups. Birth occurs most frequently between the hours of midnight and 4am when animals are maintained under a standard light-dark cycle; however, it can occur any time of the day or night.

Heavily pregnant doe enjoys some porridge to boost her strength

The gestation period can be greatly extended when the pregnant mother continues to nurse a previous litter. Prolongation up to seven days is not uncommon, and birth can sometimes be pushed back by as many as 16 days (Grüneberg, 1943; Bronson et al, 1966). This fact should be kept in mind when trying to count back from the day of birth to the day of conception in order to determine paternity for females in contact with sequential males.

Most mice will have a sleek body, bright eyes and tails with no kinks in them. If a mouse has a kink in its tail there are two possible explanations; 1) The mouse was picked up incorrectly or was in a fight and injured the tail at a young age or 2) The mouse was born with a kink in its tail. If the mouse's tail was injured and has healed crooked then as long as the mouse is in no discomfort this is fine. However, if the mouse was born with the kink it is not a good idea to breed from this mouse because it may pass on painful spinal defects to its offspring. Although the mouse may not be suffering itself, the kinked tail is an indication of possible problems.

This is what a kinked tail looks like - the tail pictured belonged to a four week old doe. She was not born with the kink but we are unsure how she received it; her mother or siblings are the best explanation.

When thinking about breeding mice there are several things to consider before making an attempt. First you must be sure that you will be able to find homes for all of the babies once the time comes. Remember that selling to pet shops can mean that they will be used to feed reptiles. Also, even if the babies aren't fed to reptiles they can often be kept in dirty, overcrowded cages harbouring diseases. It is best to know that you have willing mouse lovers that will take your babies as pets. That way you can be sure that they are taken care of properly.

Finally, when breeding, it is important to choose both your male and female carefully. Some things to consider are that both parents are healthy and have a pleasing personality. Be sure that they have a good temperament so their offspring will inherit their disposition (if your breeding for food this does not matter). If you are breeding for a certain colour offspring it is important to educate yourself about the dominant and recessive genes of coat collours. You can find more information about colours on the Internet at the Finnmouse site. You will also want to make sure that your female is fully grown before breeding her so that her growth is not stunted. Usually after three months it is ok to breed. After you have made sure that your male and female pass these simple requirements you are almost ready to start breeding.

Two young babies


Reproductive performance can be measured according to several different parameters including age at first mating, number of litters sired, number of pups per litter, and the frequency with which a strain has productive matings. Inbred strains vary widely in their reproductive fitness.

The first important measure of reproductive fitness is the frequency with which a mating pair will produce any offspring at all. With some strains over 90% of all matings that are set up will produce offspring. The C3H/HeOuJ strain is at the extreme end of this group with a 99% frequency of productive matings. At the opposite extreme, among the most well-characterized strains, is BALB/cJ with a frequency of non-productive matings that is over 50%. A second measure of fitness is the age at which females first become pregnant. This can vary from an early 5.9 weeks to a late 8.0 weeks. The third measure of reproductive fitness is litter size. Once again, BALB/cJ performs the worst in this category with an average litter size of just 5.2. All but one of the remaining inbred strains have average litter sizes in the range of 5.4 to 7.0. The one strain that outperforms all others in this category has a much larger average litter size of 9.5. The final measure of reproductive fitness is the average number of litters that a single female can produce in a lifetime. This varies from a low of 2.2 litters to a high of 4.8 litters.

Three of the easily quantitated measures of reproductive performance — frequency of productive matings, litter size, and number of litters — have been multiplied together to give a sense of the overall fecundity associated with any one inbred strain in comparison to the others. The highest fecundity in inbred mouse strains studied was 41.0 and the lowest fecundity 9.3.

The fecundity of female mice declines with both age and number of prior pregnancies. Few inbred females of any strain, with the exception of FVB/N, will produce more than five litters (Green and Witham, 1991). Irrespective of their past reproductive history, most inbred females exhibit greatly reduced fecundity by the age of 8 to 10 months. Male mice, like male humans, can remain fertile throughout their lives. However, older males that have become obese or sedentary are unlikely to breed.

Reproductive performance is among the characteristics most affected by inbreeding. Outbred animals and F1 hybrids of all types will routinely surpass the inbred strains as a consequence of 'hybrid vigour'. With non-inbred animals, the frequency of productive matings is close to 100%, the age of first mating can be as early as five weeks, and litters can have as many as 16 pups. Finally, non-inbred females can sometimes remain fertile up to 18 months of age, and bring as many as 10 litters successfully to weaning.

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