Brown Rat

The brown rat is found worldwide, with the exception of the polar regions. It can survive in almost any environment, but it is most commonly found near farms, in garbage dumps, and in sewers. It likes dense cover, where it will dig a series of linking burrows in sloping ground in the side of a ditch. It also prefers to live near water and is a good swimmer. The brown rat lives in colonies where every member recognizes each other by smell. There is a social structure in a colony, but the dominant rats are tolerant of others. Where there is plenty of food available, the rat may need to colonize only small areas no more than several yards in length. In large colonies, such as those found sewers and garbage dumps, the highest-ranking rat will live in the choice spot, close to the food source. The low-ranking rats must often struggle to survive.

Brown rats are capable of producing ultrasonic vocalizations. As pups, young rats use different types of ultrasonic cries to elicit and direct maternal search behavior, as well as to regulate their mother's movements in the nest. Although pups will produce ultrasounds around any other rats at 7 days old, by 14 days old they significantly reduce ultrasound production around male rats as a defensive response. Adult rats will emit ultrasonic vocalizations in response to predators or perceived danger; the frequency and duration of such cries depends on the sex and reproductive status of the rat. The female rat will also emit ultrasonic vocalizations during mating.

A female brown rat is ready to breed when she is 11 weeks old and weighs four ounces. After mating, the male plays no part in rearing the young.

The brown rat feeds at night and sleeps through the day. It is most active at dawn and dusk. Although its eyesight is poor, the brown rat has a very keen sense of smell which it uses to locate food. The rat prefers to eat stored or cultivated cereal grains but also eats meat. It eats various types of poultry, including ducklings.