The brown rat is a supreme generalist; its opportunistic lifestyle, agility and prolific breeding potential have helped it to colonise practically every part of the world. The fur of this rat varies in colour but is generally brown or greyish-brown with some black hairs, becoming paler on the underside. The tail is long and largely hairless, the naked ears are prominent and the eyes are relatively small.
The Brown Rat is usually active at night and is a good swimmer, both on the surface and underwater, but unlike the related Black rat (Rattus rattus) they are poor climbers. Brown rats dig well, and often excavate extensive burrow systems.
Reproduction and life cycle
The brown rat can breed throughout the year if conditions are suitable, a female producing up to five litters a year. The gestation period is only 21 days and litters can number up to fourteen, although seven is common. Brown rats live in large hierarchical groups, either in burrows or subsurface places such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.