The brown rat is large (head-body length seven to 10 inches, tail length ﬁve to eight inches) for a rat. It has a salt-and-pepper look with brown, black and gold hairs. There are darker hairs down the middle of the back. The belly fur is gray- or cream-colored. The feet have white fur. The ringed, scaly, one-colored tail is nearly hairless.
The brown rat may be found statewide in Illinois. It lives in buildings, barns, houses, dumps and other areas associated with humans. This rodent will eat almost anything. It does eat food intended for human use and can contaminate food supplies. It is usually associated with poor sanitary conditions and livestock areas. This rat will carry food to its nest instead of eating it where the food is found. The brown rat is known to spread diseases. This nocturnal mammal is a good climber. It produces some sounds. Mating may occur at any time throughout the year. The average litter size is seven. Young are born helpless but develop rapidly. They are able to live on their own in about one month. Females begin reproducing at the age of about three months. If conditions are favorable, a female may reproduce once per month. The average life span of the brown rat is about one and one-half years. This species was introduced to the United States from Europe by humans.