Sexing pinkies can be tricky, especially if you’ve never done it before. If you’re unsure and don’t want to make a mistake, wait until they’re 4 – 4.5 weeks old, and take them to a breeder, a vet or a reliable pet shop to sex for you. If you want to have a go yourself, here’s some tips.
You can actually sex baby mice from day 1, but I find about day 3 to be a good starting point. This is when the genitals become fat and easy to distinguish. When you turn them over, you will note a small pink protuberance, henceforth known as the pink bit. A girl’s pink bit is small and button-like, and sits quite close to the anus, connected by a small line. The boy’s pink bit is longer and has a wider, flatter end to it. It’s also further away from the anus and surrounded by slightly puffy flesh. Here are some pics, kindly provided by Coastal Rodents:
If you find this stage s bit tricky to work out, never fear – when they get enough fur to make them velvety, then the nipples become visible. Only female mice have nipples. They appear high under the arms and legs, and will look like little bald spots. I find this sexing method the easiest of all. If you have babies that are indistinguishable by markings, you could mark the tail of one sex with a non-toxic permanent marker at this point, to make it easier after their fur grows in.
Finally, between 4 and 5 weeks of age, the males’ testicles will start to drop. The best way to see this is by standing the mouse on your palm and pulling the tail gently down. Many people pull the tail up to see the genital area, but this often makes the boys defensively retract their testicles, and thus they’re very hard to see!