Breeding Data

This data is taken from 'The Biology and Medicine of Rabbits and Rodents', 2nd ed,. by JE Harkness and JE Wagner. The book is mainly a laboratory guide; it is well known, and well-respected, but note that some of these figures are averages or estimates, rather than absolutes, and even this wonderful book contains some data that may be outdated or based on guesswork rather than research:

With caution! Most mice are very, very fertile - they have up to 20 babies (called 'pinkies' at first) per litter, although 8-10 is more normal. If a pair are left together, they can produce a litter every 3-4 weeks; they do not 'take a break' or plan their family. Mice do not have incest taboos; they will mate with their parents, children, brothers, sisters etc..

It is hard to find enough good homes even for very pretty mice, so don't breed unless you're sure you can house the babies. Giving them to a pet shop may seem like the easy way out, but if you do so, please remember that:

- many pet shops sell mice as live or dead food for snakes

- you will have no control over the home where your baby mice end up. They may be kept in cramped housing, poorly fed, and badly treated.

Unless there is a fertility problem with a particular mouse, you can expect it to conceive within a week of the male and female being placed together. The female comes on heat approximately every 4-5 days, usually overnight. The gestation period is 17-21 days, although mine have all taken 20 or 21 days. You cannot normally tell that a mouse is pregnant until the last week of her term, when her belly starts to bulge.

Sometimes pregnant mice will behave uncharacteristically, eg a previously friendly mouse may suddenly nip. Don't worry about this - she'll get over it when the babies are a few weeks old, if it even lasts that long.

When introducing the male and female, she should always be put into his cage, and not the other way around - female mice can be very aggressive to males who pester them, and even more so if the female is in her own territory.

This is not usually necessary - remove any male cage-mates to avoid re-mating, but other female mice will usually keep the mother mouse company, and may help her to look after her litter. For example, they may take turns keeping the babies warm while the mother goes to eat.

If two female mice are due to give birth within a few days of each other, they should certainly be left together. They often share childcare, with each mouse feeding both litters.

You cannot normally tell that a mouse is pregnant until the last week of her term, when her belly starts to bulge. As a rough guide, this could happen after day 15 or so - but some mice will 'bulge' earlier, and others may show little sign of pregnancy before giving birth. She may start to build a nest a couple of days before giving birth. If you are trying to decide when to remove the father, do it when you can see her belly bulging, or when she starts to build a nest. If it turns out that she wasn't pregnant after all, you can always put them back together later.

Sometimes pregnant mice will behave uncharacteristically, eg a previously friendly mouse may suddenly nip. Don't worry about this - she'll get over it when the babies are a few weeks old, if it even lasts that long.

As soon as she starts to look pregnant, make sure she is in a suitable cage. It must have a secure nesting area which is free from drafts and preferably enclosed - the darker it is inside the nestbox, the better.

The best housing for a mouse to give birth and rear a young litter in is probably a customised breeding box - a secure wooden cage with wire mesh panels for ventilation.

You can find instructions on how to make these in the books 'Fancy Mice' by Chris Henwood and 'Exhibition and Pet Mice', by Tony Cooke.

Alternatively, you can keep the mouse in a tank or cage with very narrow bars, and with a secure nestbox. Remember that when the baby mice start running around they will be able to wriggle out between bars only 1/2" apart with ease.

Another alternative might be a large plastic bowl, as described in the housing section, covered with very fine mesh wire.

Do not disturb a mouse when she appears to be in labour unless it is absolutely vital. The easiest way to cause complications in a mouse or rat's labour is to keep checking up on the mother. Disturbance can stop or slow down labour, as can being in a brightly lit place. There are a number of studies on this subject - I'll add references when I've tracked them down. If the mouse has been straining for more than a couple of hours with no results, or if she is losing a lot of blood, then consult a vet; otherwise, leave well alone, and be very discreet when you check up on her.

When your mouse has given birth, you will usually be able to hear squeaking. Now see the next section on 'when can I look at the babies?

The mother mouse may move her babies around, or bury them under the bedding or litter. She may do all kinds of wierd things. They are her babies, and she will rear them how she wishes! Follow the golden rule of animal breeding: leave well alone, unless it is an emergency. Even if you think your mouse is abandoning her babies, she may well just be hiding them in a warm place while she rests. Since hand-rearing newborn mice is virtually impossible, the best thing you can do in any situation is give the mouse privacy, nesting material, food and water, and leave her to get on with it.

Regular handling should start before the eyes open, and as soon as the mice start eating solid food, you can get them used to eating from your hands.

Mice are born looking like frozen prawns, in a very underdeveloped state. They are hairless, blind and deaf. Usually they are pink, but some dark varieties show pigment at birth. Baby mice are called 'pinkies' generally until their fur grows.

Over the next 3 days pigment cells come to the surface of the skin and markings become visible. Hair starts to grow at about 1 week.

The babies start moving around more, and nibbling solid food, and opening their eyes, from about 10 days. They do not need purees or soft food - they have teeth, unlike human babies, and can eat hard foods as soon as they start on solids.  As soon as your babies start to eat solid food, you can try to encourage them to eat from your hands.

Soon after this mice enter the 'flea' stage when they are very hard to handle - they literally jump like fleas. Be very careful if you pick up mice at all between 10 days and 3 1/2 weeks of age.

Between 3 and 4 weeks of age they calm down.

At 4 weeks they should be fully weaned from the mother's milk. At 5 weeks, males and females should be separated and they are ready to go to new homes.