How do hamsters show they're jealous of another hamster?

Hamsters usually show that they are jealous of another hamster by being aggressive to the hamster in question. Example, scratching, biting, hissing, snapping, or taking possessions away.

Why do hamsters become jealous? Well, they are like children, in a way. If you spoil one hamster, acquire another, and then completely forget about your previous spoiled hamster, it's bound to become jealous of the new "guy\girl in town."

Usually it's not a good idea at all to bring a male and a male hamster, a girl and a girl hamster, or a boy and a girl hamster unless (in the case of a boy and a girl) you wish them to mate. Males are territorial while females are also so. Boy and girl hamsters will fight, even if confronted with perfect mating conditions.

Just remember that it's all based on the hamster's personality and breed. Other than that, you'll do just fine. :)