636 The Animal Kingdom 



spending the day in the protection of heavy grass or other shelter. The 

 first young are produced in March, and there are four to seven young 

 per Htter. During a season three or four htters are produced. Since 

 cottontails feed on vegetation, they may do considerable damage to 

 gardens or young orchards when they get too abundant. They are of 

 considerable importance as game animals. In the West, the long-eared 

 jack rabbit, Lepus, replaces the eastern cottontail. 



Order Rodentia. — In both numbers and kinds, the rodents ex- 

 ceed all other animals. Of the estimated 932 genera of living mammals, 

 344 are rodents. Not only are there a great number of genera and 

 species, but also there are tremendous numbers of individuals. Some 

 of the species of rodents are among the most destructive of man's com- 

 petitors. Rodents may be found in nearly all habitats, and some of 

 them have even successfully learned to live in man's homes. 



The most characteristic feature of the rodents is their development 

 of the incisors for gnawing. Unlike the teeth of man, these incisors 

 grow throughout the life of the animal from a persistent pulp. Only 

 the anterior surface of the tooth has enamel, the posterior surface is 

 dentine and wears more rapidly than does the front surface. As a 

 result, the teeth become sharp and chisel-like. 



The most numerous of the rodents are those groups called the 

 rats and mice. Of the New World mice, one of the commonest and 

 most widespread genera is Peromyscus, the deer mouse, or white-footed 

 mouse. These handsome mice in general have large ears and eyes, 

 a gray or brown upper coat, and a white belly and legs. Most species 

 feed on insects and weed seeds and are seldom of any economic impor- 

 tance. Their significance rather is due to the fact that they occupy an 

 important position in the food chains of many carnivorous vertebrates. 

 Of more economic importance are the members of the genus Microtus, 

 the prairie voles. These gray mice make extensive runways through- 

 out fields and meadows. At times they become extremely abundant 

 and may do damage to orchards by girdling young trees and may destroy 

 stored grains. They are kept under control only by the voracious ap- 

 petites of hawks, owls, snakes, and foxes. 



The generally bad reputation of rodents is due to the Old World 

 genera, Rattus and Mus of the family Muridae, which have been in- 

 troduced into this country. No doubt their arrival coincided with that 

 of the first white men. The house rat, Rattus norvegicus, does enor- 

 mous amounts of damage by destroying all types of food, gnawing holes 



