None of these species is native to the United States. They were all imported. Some knowledge of the rat's habits is essential to intelligent effort toward its destruction. Rats are notoriously prolific. The female rat produces from three to five litters yearly, and has been known to bear young monthly, throughout the year.
Rats are nocturnal in their habits and can see better at night.
The rat is a great traveler, both overland and by transportation. They are not infrequently found in freight cars, and practically nearly every ship that enters a port brings a large number of rats, unless artificial means have been carried out for destroying these undesirable stowaways. The number of rats taken from ships by fumigation and trapping has on frequent occasions amounted to several hundred. Intensive, thorough studies of damage done by rats are lacking, but sufficient random observations are available to make it certain that the cost of maintaining rats is a big economic drain on the Nation. It has been determined that the rat will consume 50 pounds of grain in a year and will damage considerably more.