Rats breed at 3 to 5 months of age, carry their young for 21 to 33 days and have 6 to 12 young per litter, and 4 to 7 litters per year. The actual figures within these ranges depend on the availability of food, the density of rats in an area and the age of the female rat. Although one female rat may produce up to 84 young in a year, on the average about 20 young per female per year survive to maturity. Female rats can breed again 1 to 2 days after giving birth. Generally, rat populations are lowest in the winter and, on the whole, control will be most effective at this time.
Rats usually construct their nests near food and water.
Nests are generally hidden in quiet, secluded places and are made of soft materials such as paper, cloth, feathers, grass, etc. They are bowl-shaped and approximately 20 cm in diameter. Newborn rats are completely helpless and are born naked with their eyes and ears closed. They develop rapidly, however, and by 9 to 14 days are covered with hair and beginning to explore their environment. They nurse until they are about 4 to 5 weeks old and by 3 to 5 months are independent and sexually mature.