Home: Breeding

Sexing Mice: Part Two

By Cait McKeown

Sexing mice is one of the most important skills to learn if you are serious about breeding or keeping numbers of mice. Even casual pet owners will benefit from learning how to sex mice because when buying a pair of mice or a companion for a single mouse they can check that they have not been sold the ‘wrong’ sex. You will also be able to tell whether the shop keeps the sexes separate or just puts them all in one big tank together. This way you may be able to avoid purchasing a pregnant mouse by accident, or unintentionally purchasing an opposite sex friend for your current mouse.

Females (does)

Female mice are known as does. They have eight to twelve nipples that they use to feed their litter until it is fully weaned at around four weeks. These nipples can sometimes be seen and are a good way to identify female mice since males do not have nipples. However, nipples are usually most visible in young mice or in does who have had a litter, and cannot always be seen through the mouse’s fur. If the nipples are not obvious then you must rely on checking the mouse’s genitals alone to sex it. A female mouse will have a tear shaped vagina close to its anus, with little space in between the two.

  • Look for nipples
  • Check distance between anus and reproductive organ – if they are close together you probably have a female.

Doe who has previously given birth and is still nursing pups

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 will probably have fairly obvious testicles. Mice have large testicles in comparison to their body – each testicle is the size of a peanut minus the shell. These testicles will ‘drop’ as the mouse matures into adolescence and can be seen as early as two and a half weeks in very early developers.

Testicles can clearly be seen on this three week old cream buck.

Mice can retract their testicles into their body, so they may be there even though they are not totally obvious. There are several ways to deal with this, since the mouse will not be able to hold them there for long. 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 will 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, so if you are sexing a young mouse you will also have to 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 reproductive organ is well spaced from the anus.

  • Look for testicles
  • Look for the scrotum
  • Check the distance between the anus and reproductive organ – if the distance is quite large you are probably looking at a male.

Cream buck - note the distance between and the separation of the anus and genitals in males, whereas does' may appear to be 'connected' and closer together.

Babies (both sexes and comparisons)

Left 2-3 day old buck Right: 2-3 day old doe

Left: Doe at a few days old. Right: Buck at a few days old. Note how the buck's genitals are more pronounced whereas the doe's are somewhat flatter.

Left: Doe aged 8 days. Right: Buck aged 8 days. Note the prominent nipples in the doe and smaller distance between genitals and anus as compared to the buck.

Above: Two 11 day old bucks.

Different views of 11 day old doe.

Two 11 day old does.

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