Despite their friendly, furry-faced façades, hamsters can be downright vicious to one another. Your best bet to avoid fighting is to house only single breeds and single genders together. Keeping hamsters in a close age range is also helpful. Avoid placing aggressive breeds, like Syrian hamsters, together at all.
Any time you put two unfamiliar hamsters together, or bring a new hammie into the household, don’t physically introduce them right away. Set their cages side-by-side for at least two weeks. This introduces the hamsters by smell and provides a waiting period to ensure the newcomer doesn’t have any diseases or illnesses.
Fighting can happen when you mix different breeds of hamsters together. Dwarf breeds are typically more tolerant of cagemates than Syrian or Golden hamsters.
Some fights happen over territory, so make sure your hamster cage is roomy enough to give each hammie some space. Cages with different sections can be useful, especially if you can easily cap off a tube or tunnel to separate hamsters if they show aggression.
When physically introducing two hamsters, don’t put one into another’s cage -- instead, start them out in neutral territory with a fresh, clean cage neither one has marked with their scent.
Males and Females
Hamsters of opposite genders will fight unless they are together for mating purposes. Even then they should be closely monitored.
Male hamsters tend to fight more than females.
Siblings from the same litter -- especially females -- tend to have less fighting than males. It’s important to know the gender of your hamsters to make sure you don’t unintentionally breed them. Even littermates and parents will reproduce if left together in the same cage.
Signs of Aggression
Hamsters may quibble with warning nips or get into full-on wrestling matches with biting and scratching. A hamster may curl up to defend himself or flip onto his back as a sign of aggression. Since hamsters are nocturnal, you may not see fighting, but telltale signs remain. You may notice bites or sores or a disheveled appearance. Fight wounds may abscess and require medical intervention. A dominant hamster may also keep a submissive one from food or water, so keep an eye on all behaviors.
If hamsters are viciously fighting, physically separate them. Wear gloves and a long-sleeved shirt to avoid getting bitten or scratched.
Tip: Some hamsters, especially older ones who are set in their solitary ways, will always prefer living alone.