How can you tell the difference between a girl or boy hamster?

It is impossible to tell by physical examination alone when they are babies since their genitals are mostly internal, and the testes in the males often don't descend for a couple of months, if at all. Because of that issue and aggression - which can be fatal, it is usually safest to house hamsters individually. If you try to separate a large litter by gender, you may still end up with unintentional inbreeding. That is because the testes in the males sometimes don't fully descend or they descend late, thus leading to erroneous conclusions about their gender. Now, if you observe them attempting to mate, there is no mistake (though it may be too late to prevent pregnancy by the time you catch them), since the male's genitalia will be visible.