Pests Old-World or European rats, Rattus rattus and Rattus norvegicus , have been living in association with human populations for centuries. Once transported into a new geographic area, they generally establish breeding populations quickly, since they are adapted to living in conjunction with human population centers. The Norway rat has a short tail, shorter than the head and body combined, with a blunt nose and small ears. The Norway rat thrives in unsanitary conditions, and is an opportunistic scavenger. Norway rats are much more likely to infest structures than are roof rats.
Roof rats have larger ears than Norway rats, and tails longer than their bodies.
Roof rats prefer natural foods such as seeds, nuts, fruits and vegetables, but will scavenge if necessary. Both species are nocturnal, foraging at night, and usually stay in or close to burrows during the daytime. Both species are rarely seen during the day, unless the competition for food and habitat is significant. Both require heavy vegetation for burrowing, and reliable source of water. This allows infestations to expand rapidly, provided adequate habitat is accessible.