Mice damage household articles by their gnawing activities, and they contaminate food. Besides damaging crops and grain reserves, rats often gnaw insulation from wires and have occasionally been known to cut through lead pipe and concrete dams. The tunneling activity of Norway rats causes structural damage to building foundations and platforms. Rats cause billions of dollars worth of damage throughout the world each year. They are called commensal rodents, referring to the fact that they live at man's expense without giving anything in return.
Rats have very high reproductive rates.
A pair of rats and their offspring could produce 1,500 more rats in only one year if they all survived. 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.