It is not hard to separate the boy hamsters from the girls, if you know what to look for, though you may have to wait until they’re about a month old to be sure. If you’re getting a new pet, or a pair of them, don’t trust that the pet shop has sexed them properly; it’s always a good idea to check for yourself. Use a bit of caution when handling hamsters, as they may bite, and be careful not to squeeze or drop them when taking a look at their bellies.

Openings
Hold the hamster carefully and look at the underside, near the tail. There will be two openings, one of which is the anus. The other opening will be either the penis or the vulva. The most obvious difference between the two sexes is that there is more distance between the anus and penis, with a small but obvious space between the two. If the hamster is female, there is only a very tiny space, if any, between the vulva and anus. Often the two openings in a female are so close together that they are touching.

Testicles
In male hamsters, testicles will develop and will become quite obvious as he ages. When seen from underneath, they appear as noticeable swellings near the penile opening. As he matures, you can tell a male hamster from above because his testicles are large enough to give his rear end a pronounced pointed appearance. The female’s rear lacks these bulges, and instead looks smooth and rounded. The testicles are not obvious in very young hamsters, and the males also can retract them, so you may need to look closely at your hamster to be sure.

Nipples
Female hamsters typically have two rows of nipples on the belly. These can be seen if you hold her upside-down in your hand and run your finger gently along her underside, parting the hair so that you can see down to her skin. As with other sexual characteristics, the nipples become more pronounced as the hamster ages. If you happen to be looking at a female who already has had babies, the nipples will be quite obvious and much easier to see.

Considerations
The Michigan Humane Society recommends that you house hamsters individually, since adults of some breeds are known to fight and may injure each other. If you do decide to house your hamsters together, it’s best to determine the sex as early as possible if you don’t want to end up with a lot of babies. Females can breed as early as six to ten weeks of age, while males typically will breed by the time they are 10 to 14 weeks old. Young hamsters usually can be housed safely with others of the same sex until they are mature.