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. The average life span of a rat is one year. Mice mature rapidly and by 1.5 to 2 months of age are old enough to reproduce. They carry their young for 19 to 21 days and have 5 or 6 young per litter and 5 to 8 litters per year.