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. Thousands or millions of rats will try to swim across wide rivers. Many will drown. Men have always tried to kill off rats, and for good reasons.
Rats are often in horror stories and movies because they are associated with death and disease.
Rats can be very noisy as well by when they squeak, whistle, and chirp. They can keep people, who live in buildings where there are rats, awake at night.