Determining the gender of mice can be a difficult or an easy process. The main thing to keep in mind is that it takes practice to be good at telling the gender, but once you get the hang of it, you will be an expert, too. The above two images are the rear view of two mice. The one on the left is a female mouse. Her genitals look like an exclamation point. The anus almost touches a slash, which is the vagina. There is hardly any distance between the genitals. The male, on the left, does not display his testicles all the time. When he is stressed, cold, or dangled by his tail, his testicles will be drawn into the body cavity, leaving behind some wrinkled fur covered skin. This skin is what separates the anus from the rest of the genitalia. there is a large separation between both, compared to hardly any separation on a female. This is the largest giveaway in telling the gender of mice. When a male mouse is relaxed, dangling by his forelimbs, or around exciting female mice, he usually will wear his testicles so they are prominently displayed, and this is of course a dead giveaway.

Determining the gender of baby mice is much more complicated. The easiest way to become adept at determining gender on sexually immature mice is to begin with baby mice which are between 9 and 12 days old. At that age the baby mouse is fully furred on the back and his or her tummy fur is growing in. The nipples on female mice will create small discs of fur free tummy. This is only obvious when the belly fur is just beginning to grow in. On baby male mice, they don't have these prominent nipples. As you can see in the picture above to the left, the purple arrows note how close the genitalia is on a female 10 day old mouse. The four red arrows point out some of the baby's nipples. They have three on each side on the bottom near the hind legs, and two more nipples up in their front "arm pits". This is a total of ten nipples. Mice are designed to have large litters! On the right side you can see the same picture without the arrows to complicate the picture, and below you can see the same girl mouse.

Once you become expert at telling the difference in gender in 10 day old baby mice your expertise will extend in both directions. You will be able to tell the difference between both genders by simply looking at the distance between the genitals. It takes practice, but consistently working on learning the differences will help you to get the hang of it. One day you will suddenly know how to tell the difference!