Any estimate of the rodent population of the United States can be nothing less than a conjecture, but a guess of one rat per person would probably be conservative. Such an estimate would be supported by the intensive studies of the United States Public Health Service in its sanitary campaigns in several large cities, in which the rodent population was calculated on trapping percentages. 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.