It can go through the hollow walls of houses, gnawing out wood. If not controlled by man or natural enemies, the Norway rat can easily have a population explosion. A female can mate with a male just hours after giving birth, and mating goes on between males and females all year round. The 7 to 11 young are born hairless and blind but can open their eyes at two weeks of age. They are well enough developed to leave the nest at three to four weeks of age.
Females can breed at three months of age.
They can have 2 to 22 young, but 7 to 11 is much more usual. Few Norway rats live out their full three years. At two years of age, females stop breeding and males are not as interested in mating. A female can have up to 12 litters a year if conditions are very good, but usually has five. If there are too many Norway rats in one area, they can start a mass migration or movement.