Q: My mouse is pregnant, and I know I have to remove the male. I’ve heard that mice are very social, though and I don’t want him to get lonely. What should I do? Also, I’ve heard that you can’t always tell the gender easily with baby mice. Is there a way you know?
A: While female mice are extremely social, your male mouse prefers to have his own space and will fight with other males. If you’re concerned that he will be bored, give him an exercise wheel, some extra toys and a few places to hide and he’ll be completely content. But be sure to remove your male mouse before the female gives birth, because she can become pregnant the same day the babies are born.
Sexing baby mice can be a bit tricky, so I recommend that people try this at different ages and mark the babies when you’re sure of the sex. A little bit of permanent marker on the tail usually works.
First of all, females have nipples and males do not. This is the easiest method for sexing, but it only works until they get fur on their belly, then it’s harder to see the nipples.
After that you’ll need to look at their genital area. Females have a small space between the openings and males have a larger space. Do some comparisons between them and take the time to find some images online. As the baby mice get older, it can be tricky because the males can pull their testicles inside and they look very much like a female.
When you separate them by sex when they reach 5 weeks of age, check each group daily to make sure you haven’t mis-sexed any mice. When doing this, it’s easiest to put them in a clear container (like a critter keeper) and look at them from underneath. When the males are running around they are more likely to have their testicles exposed, which makes sexing much easier. Best wishes to you!