xxv. 3 BEAVERS 659 



societies. The gophers (Geomys) and kangaroo rats (Dipodumys) of the 

 United States are also sometimes placed here. They are jumpers, 

 paralleling the jerboas, gerbils, and jerboa-rats. 



Possibly also in this group are the beavers, Castoridae, found 

 throughout the Holarctic. These are aquatic rodents (Fig. 426) that 

 show remarkable habits in preparing a house and store of food for 

 the winter (Fig. 427). The house ('lodge') is built on a mass of debris 

 so as to be surrounded by water. Sticks are built up to make a wall 

 round the platform and the whole finally closed by a dome of sticks 



Fig. 428. Jerboa, Dipus. (From life.) 



and mud, which is carried by the beavers with their fore-paws. When 

 this damp structure freezes it makes a strong protection against bears 

 and other enemies, and the beavers keep warm inside it. Food is 

 obtained from the bark of branches kept in a store under water and 

 brought up to the lodge through a plunge hole. The beaver dams are 

 made by the beavers during the summer in order to deepen the streams ; 

 they may reach a height of 12 ft and a length of several hundred. The 

 lodges and dams are the result of cooperative work by successive 

 generations. The beavers work compulsively, repairing the structures 

 whether they need it or not. Like other rodents they mark their 

 territory by smell, there being large anal oil-glands. When an animal 

 smells a deposit it visits it and adds its own 'castoreum'. 



The Myomorpha is a very large group, including the rats, mice, 

 voles, jerboas, and other types, all having the medial portion of the 

 masseter running through the infra-orbital canal, but probably not 

 really closely related. They include many families and genera, with 

 specializations for individual ecological niches. The jerboas, Dipodidae 

 (Fig. 428), are members of this group, with limbs specialized for 

 hopping, but in other respects with somewhat primitive characteristics. 

 There has been a great elongation of the metatarsals of the three 

 central digits, which alone are well developed. The gerbils (GerbiHus) 

 are yet another family of jumping animals, living in deserts. The 

 Muridae (Fig. 429) are among the most successful of all mammals and 

 are an ancient family, recognizable back to the Pliocene and invading 



