Do hamsters need a friend or do they need to be alone?

Syrian Hamsters need to be alone. They are extremely territorial and would fight another hamster.

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Dwarf Hamsters can be alone, or be with another Dwarf Hamster of the same sex and breed. They enjoy company, but it isn't necessary.

If the hamsters begin fighting, they must be separated. If they are getting along well and then you separate them then they can become depressed.