[souffle]

You could also mention 'bullying'. A hamster that is constantly being harassed, picked on, not allowed in certain areas and is being kept away from food will exhibit stress related symptoms such as hiding away, weight loss, keeping away from the others, timidness. There hamsters are constantly on high alert as they are constantly secreting stress hormones which lead to loss of condition as well as stress on the organs.
Wild groups are not quite the same as captive kept animals as wild groups are a mixture of ages and sexes. Single sex groups are not a natural situation and this alone can lead to tension within the group.
I would say bullying and the mental health of the hamster is equally as important as physical injuries in some cases.