Breeding Mice - The Basics

Some Facts About Mouse Reproduction

- Scientific name: Mus musculus
- Weight: 20-60 g (varies according to type)
- Life expectancy: 1-3 years (varies according to background, "strain")
- Sexually mature: 4-7 weeks (both males and females)
- Cycle of heats: 4-5 days, continuing all year round
- Gestation period: 19-21 days
- Weight at birth: 1-1,5 g
- Size of litter: 4-14 (even more), average 12 babies
- Pigment starts to show: from 3rd day forward
- First coat has developed: at 10 days
- Eyes open: 12-13 days
- "Flea age": 10 days to 3,5 weeks, can be less.
- Nursing period: approximately 3-3,5 weeks
- Weaning age: males 3,5 weeks, females may be weaned at 4 weeks
- Separate to single sex groups: 3,5 weeks
- Ready to leave for new homes: 1 month of age
- Proper age for first mating of female: 3-4 months
- Proper maximum age for first mating of female: 5 months
- Resting period between matings: At least 2 weeks, preferably over 1 month between weaning of one litter and mating for another.
- Litters for one female: 3 is generally enough!
- Leave the female out of breeding: at 1 year of age.

Some Further Notes

Sexually mature means that the mice can reproduce at that age, not that they are physically or mentally ready for it. The situation is similar to other mammals - you wouldn't mate a dog (bitch) at the first heat, or consider 12-13 year old girl to be ready for pregnancy, now would you?

As a doe ages, the risks involved with first pregnancies get higher. The pelvic bones of older females who have not given birth aren't as flexible as those of a younger doe. Thus, the doe may not be able to give birth. In the best case, you can spot the difficulties in time and get the doe to a vet, which gives her the potential of surviving. In the worst case scenario (which, taking into consideration that mice tend to give birth at night when it's quiet, tends also to be more common), the doe suffers a painful death.

The male mouse will mate with the female very soon after she has given birth. This results with another litter being born when the first one is still only 3 weeks old. Although male mice are good fathers and help the female with the babies, it is better to separate him few days before the female will give birth. If you don't know when the mating has happened, separate him at 2-2,5 of them moving together. It may be a better idea to move the female in with an experienced female, if you feel that the mother needs help. There are risks involved, but moving the pregnant female (at 2 weeks of pregnancy) and the other female together into thoroughly cleaned tank reduces the risks of fighting. Give only one nest and loads of nesting materials.

Nesting materials consist of shredded paper - preferably tissue paper, high quality dry hay or similar matter. Do not use "hamster cotton", fabrics or cloth. These can result in choking a baby or amputating a limb!

"Flea age" is a stage of mouse babies' development, when they jump blindly away from danger - including human hands. When handling flea-age babies, you should always handle them just above their tank. Otherwise there is great risk of one of the babies literally flying from your hands and landing on the floor - even causing death. Some flea-age babies are also known to bite hands rather vigorously.

Well fed and very healthy female mouse can cope with two succeeding litters, but it is not advisable to put her under that stress. Do not believe, if someone tells you that it is the normal state for female mice to be constantly pregnant and/or nursing. This is definitely not true.

Avoiding unwanted pregnancies and litters is easy - keep the female and male mice apart!