Dept. of Streets and Sanitation Rats in Chicago Chicago's rat population is predominately the Norway Rat . It has coarse grayish hair, small ears, a blunt nose and the ability to gnaw through materials as tough as cinderblocks. They ordinarily avoid humans, but will fight when cornered. Litter sizes increase when food is plentiful and decrease when food is scarce.
Females can become pregnant again within several hours of giving birth.
As the food supply dwindles, the stronger rats in the colony will kill and eat the younger and weaker ones. Cut off the food supply and this killing will continue until there is only one survivor. Rats are extremely anti-social toward one another, existing in small colonies that usually range from one to five dozen individuals. This constant killing holds down the rat population through death and injury, but the overall size of any rat colony will depend upon the amount of available food. Rats prefer fresh food, but can survive on rotting edibles or even dog feces.