Rat Aggression Toward Other Rats
If they are from the same litter, or have already been introduced from the place of purchase/adoption, you shouldn’t have any problems with fighting amongst rats.
Sometimes males will go through testosterone driven ‘phases’ at several months of age, though this rarely turns out to be a problem, and generally sorts itself out. If it doesn’t, neutering often does the trick (cost and experience varies from vet to vet).
If you get two rats from separate sources, and you follow PROPER quarantine and intro procedures, then fighting should not be a concern.
It is easiest for rats to get along if they’ve grown up together already. The easiest rats to introduce are neutered males with females. The most difficult tends to be two adult males.
Rat Aggression Toward Humans
Will they Bite Me? My Child?
Rats are actually the least likely rodent to bite. They are very tolerant and social creatures, and tend to only bite if very frightened. Rats make excellent first pets, so long as parents are willing to treat their rats the same as they would a cat or dog, and realize that just because they’re smaller animals, does not make them disposable pets.
Can Rats Live with Other Animals?
As with many cats and dogs, rats can get along with many animals if they’ve grown up together. It is however extremely unsafe to ever let your rats play with other pets in the house, as they can be easily injured. If you introduce your rats to cats or dogs, do so slowly, and never leave them unattended.
You may think you know your loving dog or cat, but they are predators too, and a rat can die very easily from a single bite from either of these pets. Ferrets and rabbits will kill a rat.
Never let your rat near smaller rodents like hamsters, mice, or gerbils, or near any birds. These are prey to your rats, and are likely to end up as food.
Their predatory instincts are still exceptionally strong and present, and putting a smaller rodent or bird near your rat is a huge risk. Some people have even said they’ve caught their rats swimming in their fish tanks, hunting fish, or in one case, catching and eating a small turtle.