Hamsters are territorial animals by nature and some are much more aggressive towards others than others. The Syrian hamster is a good example. Do not put two Syrian hamsters in the same cage unless you want to wake the dead hamsters. Dwarf hamsters can be kept together, but it's good to understand how hamsters interact with each other. Then you will see a warning sign that things may get out of hand.
Body language is used extensively by hamsters to convey information or signals to other hamsters. Hamsters receive subtle bodily signals from other hamsters' movements as indicators of anger, anxiety or pleasure. As if we could read our hand twisting as a sign of anxiety or tapping our feet as a symptom of restlessness.
Hamsters who have never met can recognize each other and their gender by sniffing the olfactory gland just behind their ears. Related hamsters spend much less time recognizing each other than foreign hamsters, supporting the hypothesis that each hamster has its own unique smell. When two male hamsters meet and sniff, the submissive hamster turns away, but when a female hamster, exhausted from the heat, sniffs the manna, she exposes her back to the lucky one.
Very territorial hamsters spend a lot of time measuring each other to figure out who is the dominant animal. They achieve this by sniffing other odor glands in their abdomen, forming a 'T' and each hamster sniffing in turn. Because of the aggressive sniffing of the dominant hamster, sub-hamsters sometimes find themselves on their hind legs. When this happens, the dominant hamster can exploit the situation by biting the exposed belly of the subordinate hamster. At this moment, the attacking hamster has two options. Submit or fight.
If your subordinate hamster doesn't want to try his luck, he can resign by sticking out his front paw and averting his gaze. If this fails, both hamsters begin rolling fights, an early stage of the fight. This is essentially a bit of a tough wrestling with a couple of stomach bites thrown for good measure. At this time, the hamster can admit defeat by turning its back. On the other hand, any battle that continues beyond this stage becomes very dangerous.
Hamsters who are still fighting at this time are doing significant harm to each other. A defeated hamster may try to leave for the wild, but in a confined environment like ours, the hamster will fight until one side rips the other to pieces. Attempting to separate with bare hands may result in severe finger bites. Your best chance at this point is to spray the hamster with a bottle of water and separate it from the soak while it recovers. You can get a terrible bite if you don't use gloves.