Science has proven that if you leave an adolescent hamster in a cage with an aggressive adult male hamster for as little as just one hour a day, it will grow up to be a bully.
It will, however, be fearful of hamsters its size or larger. What? Don't believe me? Here's the study by Craig Ferris, published in 2003.
Having raised hamsters, I can say that this is definitely true. They will seem like nice sweet boys as long as the dominant hamster is around, but if you remove the dominant hamster, inevitably at least one that had been bullied as juveniles turn to bullying the others.
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Hamster abuse!
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