Hamsters have longer, harder teeth which, in the long run can hurt you ball...notice I said can, not will....I am just saying it is possible. Also, if you feed hamsters, your ball might imprint onto the hamsters and then he won't eat anything else (what if you change your mind down the road and want to feed rats?) Hamsters are solitary creatures and if you breed them and have a whole bunch, they willeventually fight whereas mice and rats like to live in communities. Keeping a breeding male and female hamster may cause fights. even if the ones you have now get along, they will not live more than a few years and then you will need to start over with new breeders who may not be so nice towards each other.