BREEDING MICE

Breeding mice to exhibit takes a lot of effort but is also very rewarding. Whether you only keep a few mice or have already begun your show career, there is always more to learn. Find out where to start, what to look for in your mice and how to progress with your stock.

Breeding for exhibition

AN INTRODUCTION TO BREEDING

When breeding fancy mice, the usual foundation stock is a trio (or two) of your chosen variety. This comprises a buck and two does. These are mated together when they reach a suitable age (usually around 12 weeks, but this can vary with some varieties doing better if slightly younger). When housing pregnant does, some people keep each doe separate and some prefer to leave two does together. There are risks and benefits associated with each method, and in the end it all comes down to what works best for the fancier and their mice.

Mouse fanciers having a chat during a mouse show

Gestation is approximately 19-21 days for mice. Bucks should be removed before the doe gives birth to avoid reimpregnation just after she gives birth. It is not because bucks will harm their offspring – they’re actually pretty good at keeping the litter warm and so on in their mother’s absence – but because back to back litters can be hard on the doe’s body, and you will usually not achieve good quality litters this way.

It is standard practice to reduce litters by culling. This gives the best young ones the chance to reach their full potential. In order to choose which mice to rear, fanciers must practice selection. As the litters mature, they should be re-evaluated regularly to check progress and colour/markings. Many fanciers use large running on boxes to allow groups of young (weaned) mice to develop before making their final choices of which to keep, show and breed from.

Selection is key

SUCCESSFUL BREEDING

In order to breed high quality mice you must constantly select the best examples from your stud to pass on desired traits to their offspring. There are many things that breeders must consider and balance in their pairings, such as type, size, colour, markings and so on. Choices may also depend on a particular feature you are trying to improve, such as tail set or colour coverage on the feet. No mouse is perfect, so, in general, select for the best all-round candidates; in good condition, healthy, of reasonable type and size and at least a fair example of their variety.

We must also bear in mind that not all breeding animals are fit for the show bench, especially in marked varieties. This does not necessarily mean that they cannot produce winning mice in their litters. In fact, if we only bred from marked mice good enough to show we’d end up losing them altogether relatively quickly!

The selection process starts with the buck and doe to be bred together. Ideally this pair should be good quality stock from established exhibition lines. All being well, approximately three weeks later the doe will give birth to her litter and the fancier’s next round of selection begins.

Culling

REDUCING LITTERS

For exhibition mouse breeders culling is a necessity. It is scientifically and anecdotally proven that reducing the number of mice in a litter at a young age gives the remaining young ones the chance to reach their full potential. For those who are interested, see Grüneberg (1943), which is also quoted by Silver in Mouse Genetics (1995); “For litters of four or more pups, the amount of milk produced by a lactating mother will increase with the number of young, but the increase will not be proportional”. With this in mind, show breeders generally choose to reduce their litters to enable them to rear the best possible animals.

Litters should be reduced as early as possible to gain the maximum benefit (this will differ depending on what traits you need to be able to distinguish before doing so). Ask the breeder you get your stock from for advice on how to reduce the litters.

THE BASICS OF BREEDING

Does

Generally speaking, does can be bred from the age of 12 weeks. Some varieties that tend to become obese and therefore suffer from fertility problems are often bred slightly younger to counteract this – say from ten weeks or so. A doe will give you the best litters when she is still in her prime, so if you are going to breed from a doe then it is advisable not to leave it too long after 12 weeks before her first breeding.

Bucks

Bucks can breed from the age of five weeks or so and remain fertile until they die. Only one buck should be used per cage when mating up, or the bucks will fight over the does. Each buck can be placed with up to three or four does at a time and should be removed when the does are seen to be pregnant and before they give birth.

Gestation

Pregnancy in mice lasts 19-21 days.

Litters

Litter size varies with the age of the doe, the variety and particular strain. While mice can have over 20 babies in one litter, the average size is more like 7-12. The young are born without fur and with their eyes and ears closed. The ears will gradually open over the first four days, and the eyes will open at 12 days. Mothers with litters should be kept in solid-sided enclosures to avoid the young escaping. Young ones are weaned at four weeks of age and bucks should be separated from their mother and sisters at this point to avoid pregnancies.

Rebreeding

It is best to give does a break of a week or two after they have weaned their litter before they are bred again. Condition is key however; some does may regain condition quickly and handle gestation again relatively soon, while some may never quite reach their pre-litter fitness again.

But what if… ?

How do I avoid removing a potential winner if I cull at a young age?
You can never know 100% how any mouse you cull would have turned out. With varieties where a whole litter can be showable, such as pale selfs, you will inevitably have to remove mice with potential in order to benefit the litter as a whole. It is easier to see show potential in marked mice (in darker colours) when they are young, as there is a more tangible ‘right’ and ‘wrong’. Talk to experienced breeders of your chosen variety to get tips on what to look for in the nest and rear only strong individuals, that way you will have good foundations to build upon while you are learning the finer points of selection.
I’m only just starting out with this variety and I need to build up my stock. Is it ok to keep more young ones per litter for a while?
Stock can go downhill very quickly – it only takes a couple of generations. If you are worried then try to start with more than one trio – two trios or a buck and three or four does should be plenty to start with. One option would be to foster some of the young ones onto another doe, but of course this depends on other does being available to you.
I can’t see any difference between the young ones in my litters. Am I doing something wrong?
It may be a while until you can spot the minute differences that more experienced fanciers can. If possible, it may help you to visit the shed of a successful fancier. If you can’t do this, have a chat to other fanciers at shows to pick up some tips, especially those who breed your varieties.
What if there are three or fewer worth keeping in my litter – can I reduce to less than four?
Reducing to very low numbers (one or two, possibly three) may not provide enough stimulation for the doe to continue to produce milk. If it is a case of not wanting to keep more than one or two young ones from the litter, leave an extra doe or two to ensure the survival of these.
Skills

SEXING MICE

Sexing mice is one of the most important skills to learn if you are serious about breeding or keeping large numbers of mice. You need to be able to identify and remove unwanted bucks from the nest at only a few days old, as well as to separate baby bucks from their mother and sisters at four to four and a half weeks old to prevent them impregnating the does.

Females (does)

Female mice are known as does. They have ten nipples that they use to feed their litter until it is fully weaned at around four weeks. The nipples can be seen when a doe is nursing (at other times they are usually covered reasonably well by the belly fur) and are a good way to identify female mice since males do not have nipples. Nipples are most visible in young mice or in does who have had a litter. If the nipples are not obvious then you must rely on checking the mouse’s genitals alone to sex it. A female mouse’s vagina is close to its anus, with little space in between the two.

Mouse fanciers having a chat during a mouse show

Males (bucks)

Male mice are called bucks. They do not have nipples, but just because you cannot see a mouse’s nipples does not mean they are not there. If a buck is old enough to leave its mother (four weeks or older) then it should have fairly obvious testicles. Mice have large testicles in comparison to their body size, which will ‘drop’ as the mouse matures into adolescence.

Mice can retract their testicles into their body, so they may be there even though they are not totally obvious. If you are unsure there are several ways to deal with this. A simple way is to give the mouse a high sided food bowl to eat from and watch it perch on the side – if it has testicles they should drop at this point for you to observe. Alternatively, when you lift the mouse to inspect underneath, you may still be able to see the slightly baggy scrotum even if the mouse is retracting its testicles. This is more obvious in older mice who are fully developed. You should also check the anogenital distance. The distance between the anus and the genitals of a male mouse is larger than it is in a female mouse. If you compare the two you will see that while the female’s anus and vagina appear almost joined, the male’s penis is spaced quite far apart from the anus.

Babies

Lots of beginners find it difficult to sex baby mice, but all you need is a bit of practice – and ideally someone to show you how. Below are some photos of pinkies and furred babies that may help

©Cait Walgate
©Cait Walgate
©Cait Walgate
©Cait Walgate