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.
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.
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.
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.
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.
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.
Although it is illegal to sell mice as live food for reptiles etc in the UK, you must bear in mind that this does NOT mean that no one feeds their herps live mice: some breed their own or buy from pet shops without publicising their intentions. I know for a fact that understanding staff and the attitude of the pet shop can save mice from being fed to other animals; the staff in the pet shop where I occasionally take some of my mice told me that they refused to sell mice to someone who they had good reason to believe was going to use them as live food. You can imagine how grateful I was that I had checked out the pet shop properly - they were alerted by several things, including that the lady trying to buy the mice as 'feeders' regularly bought frozen mice and that she asked if they sold 'live mice'. How many people do you know who go to buy a pet mouse and refer to it in that way?