Norway and roof rats have litter sizes of six to 12 young, but up to 22 pups have been recorded. The gestation period ranges from 21 to 25 days, with a female typically producing three to six litters in her lifetime. Under ideal conditions, females may give birth every 24 to 28 days. Young rats become sexually mature at about three months, with most living six months to a year.
In comparison, under optimum conditions, a house mouse may live two years or more, and can breed throughout the year. A female has a gestation period of 18 to 21 days, and produces a litter ranging from two to 13 mice. A long-lived female may produce six to 10 litters.