56 RODENTS OF IOWA 



Without doubt the rate of increase among rats is more rapid in 

 temperate climates, extremes of heat and cold retarding multipli- 

 cation by decreasing the number of litters and the number of young 

 in each litter. A plentiful supply of stored grain or other nutri- 

 tious food is also conducive to rapid multiplication and maturity. 

 While young may be found throughout the year, early spring and 

 summer are the periods of greatest reproductive activity. Briefly 

 summarized, the breeding habits of the brown rat as stated by 

 Lantz are as follows : ' ' The animals breed from three to five times 

 a year, each time bringing forth from 6 to 19 young. After a ges- 

 tation period of twenty-one days the females give birth to their 

 young in nests built in underground burrows or under floors, stacks, 

 lumber, wood piles, or other shelter. The young are blind and 

 naked when born, but grow rapidly, and young females are .capable 

 of breeding when less than three months old. ' ' 7 In central United 

 States observations show that this rat breeds from six to ten times 

 in a year and produces in each litter an average of ten young. 



Inland, where living conditions 'change with the season, movement 

 of rats from houses, barns, and outbuildings to the fields occurs in 

 spring when the green vegetation lures them forth. Upon the ap- 

 proach of autumn they again seek the shelter of buildings and an 

 abundant food supply in the way of stored grains. While these 

 seasonal migrations are commonly observed, more general move- 

 ments sometimes occur in which great numbers of rats invade, in 

 a body, new territory, taking up their abode and causing heavy 

 damage. It is probable that these extensive movements are brought 

 about by overcrowding and the consequent reduction in the food 

 supply. 



The brown rat is more pugnacious than the black rat, and in 

 places where the latter formerly held sway it has been driven out 

 by its more formidable relative. Rats seldom attack human beings, 

 except perhaps in self defense ; and contrary to a popular opinion 

 the bite is not poisonous, though it may be dangerous owing to the 

 likelihood of infection setting in. 



The sight of rats is not keen in daytime, as is indicated by the 

 fact that they move about slowly and with hesitation. However, 

 the whiskers serve as feelers- and in them the sense of touch seems 

 to be particularly well developed. It is a common observation that 



7 Lantz. D. E., Nat. Hist, of the Rat, in "The Rat and its Relation to the 

 Public Health" : Treas. Dept., Pub. Health and Marine Hospital Service of 

 U. S., 1910. 



