Sexing Mice

by AppleCheeks Sat 09 Apr 2016, 9:27 pm

The content on this page was first published on The Fun Mouse website and has been reposted with permission. Please do not reproduce in any way.

Better safe than sorry.

If you are unsure of sexing, the following method may help you. Set up 3 cages; one for the females, one for the males, and one for the mice you are not positive of sex. Once you have separated them into these 3 tanks, go back to the ones you are unsure of and look again. If you are not 100% positive of the sex of any, place them *each* into a tank alone. You can't risk a buck in with your does or a doe in with your bucks. This can result in unwanted babies which could prove devastating to the other and her young. If you are unable to sex them, there is no shame in finding a good breeder to do it for you or ask a vet. You can even take bum pictures and post them to reputable mouse groups for assistance. It is better to ask for help than have accidents happen. It is not recommended to ask a pet shop to sex your mice for you. While some may be able to sex mice well, they are notorious for miss-sexing mice.

Sexing, How-to.

There are a few things to look for when sexing mice, as well as a few ways to go about looking for those things. One way to sex your mice is to lift them gently by the base of the tail. You should never pick up a mouse solely by the tail (lifting all feet off a surface). Always remember that tails are not handles placed there for the convenience of humans. The tail can give you assistance, but should be respected as a valuable body part of the mouse. The tail is used by the mouse to balance as well as help them regulate body temperature. As you lift the tail, allow at least their front feet to rest on a slick surface. To prevent the mouse from gripping something while you are lifting the tail, place the mouse on a slick surface so they can't pull. A mouse can suffer sever injury if held wrong. They can snap their back (causing a mouse to become paralyzed, or worse, death), break their tail, the skin can come completely off the tail, and their tail can actually break completely off as well (mouse tails do not grow back like some lizards). Alternatively, you can also place the mouse in an empty fish tank (no bedding). Hold the tank above your head, preferably resting on a secure surface to prevent it from falling. If you do not have a tank, you can use a glass dish, drinking glass, jar (no lid!), etc to look up from below. Anything with a clear bottom will work. Looking up at the mice from below may help them relax (as opposed to being held, if they are not accustom to that), potentially allowing males to relax their testicles as well as giving you a good view of their over all genital area without having to tail them. Do note that males can hold their testicles in for long periods of time, therefore looking from below for testicles is not a guarantee you have sexed them properly unless you actually see testicles drop. Only then will you know you have a male. Failure to see testicles relax does NOT mean you have a female. It only means you have a female or a non-relaxed male.

Sexing Adult mice using the Genital method.

Below are pictures of 2 adult mice, one buck and one doe. You will notice that there is a larger gap between the anus and penile shaft of the buck than that between the anus and vagina of the doe. The distance between the anus and genitals is called the "anogenital distance." You will also notice that the female has a Vaginal Orifice, whereas the buck does not. Checking for the Vaginal Orifice, or lack there of, is the best method of sexing possible.

Sexing Juvenile mice using the Genital & Nipple method (Early age).

Babies can be sexed at any age, though it can prove to be very challenging depending on the line and experience of the caregiver. Don't feel silly if you are unable to accurately sex babies at a very young age.
Note that with some lines the bucks and does have less of a anogenital distance (gap between the anus and genitals) difference, especially in youth. To assist you with sexing your own pups I have provided pictures from ages 3 days old until 23 days old below.

Always remember that pinkies can get cold extremely fast. They should only be handled for very brief periods of time until they fur up. They are also very fragile, especially their tail. If you chose to lift the tail to view their genitals, be sure to very gently grip the very base of the tail. Alternatively, you may be able to hold the pup upside down in your hand.

Around 5 days you'll see noticeable flaking. This is due to fur starting to come in.

Sexing Juvenile mice using the Genital & Nipple method (Early age).

If you are extremely observant, have fantastic eyes, and have the right lighting you can see nipples as early as 3 days old, when you first check on the litter. To have the best results when looking for nipples hold the pup so there is a slight glare from a light (such as a lamp) on their belly and gently move the pup around so the glare moves among the tummy. When the reflection goes over the nipples, you should be able to see them. They look like little indentations. You will have the best luck finding the nipples farthest back, by the genitalia. Depending on your monitor, you may be able to see the nipples on the 6 day old doe below, as well as in other pictures.

Sexing Juvenile mice using the Nipple method (Standard age).

At around 1 ½ weeks old, when the fur starts coming in on the belly, you can noticeably see the nipples on female fuzzies. Males will not have nipples. As the fur continues to grow it will "cyclone" around the bare nipples. As a female ages the nipples may become less noticeable. Some adult does will not have very noticeable nipples at all. Therefore, it is wise to keep track of the mice with and without nipples, if you rely heavily on the nipple method.

The dimple in the middle of the belly is their "bellybutton."

Around this age you will notice that the genitalia on both sexes are becoming more defined. On the doe you can see her Vaginal Orifice, behind the Urethral Orifice (the "flap of skin" between the anus and nipples). The male does not have those orifices, but rather a penile shaft. Noting the difference between the penile shaft and the vaginal orifice is the best method of sexing.

Notice that the nipples, while still visible, are becoming less apparent now.

Due to their growth slowing, from here on I have stopped the day-by-day pictures. As you've seen in the last few pictures, their development has slowed down remarkably in comparison to the first 2 weeks of life.

You may notice at this point that the buck in these images is angora.

Comparison with a 23 day old relaxed male.