Syrian hamsters are solitary animals and should always be kept singly; that is, one hamster per cage. But if this is the case, why do you see groups of Syrian hamsters apparently living in peaceful coexistence at pet stores? Why can't owners do this at home?
Simply because most hamsters seen at pet stores are quite young, and even Syrian hamsters can be kept together at a young age. Usually by about 8-10 weeks, they must be separated, or serious fighting (often fatal) may occur.
Trying to keep adult Syrian hamsters together is just asking for trouble.
It is also worth mentioning that just because they appear to be okay at the pet store at any given moment, that doesn't mean there hasn't been or won't be problems. Within older groups of hamsters, those that had been in a fight would probably have been removed from the cage. Ideally, hamsters would also be separated into same-sex groups well before the age when fighting is likely to be a serious issue in order to prevent unwanted pregnancies.
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