THE MAMMALS 



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incisors are long and curved. This is the most easily recognized charac- 

 teristic of the group. The teeth continually grow from their roots and 

 the rodents wear them down on the ends by gnawing on hard objects, so 

 they will not become too long. The beavers are even able to gnaw down 

 trees. 



The Squirrel-like Rodents. This large group includes the squirrels, 

 ground squirrels, flying squirrels, chipmunks, gophers, ground hogs, 

 prairie dogs, beavers, and others. The beavers are the largest rodents in 

 the United States. They gnaw down trees and place them across 

 streams to form dams. Cracks between the logs are filled in with 



Photos by Winchester 



Fig. 28.23. The beaver. This large rodent gnaws down trees which are used to build 

 dams across streams. The broad, flat tail acts as a support while gnawing and is also 



valuable as a rudder while swimming. 



mud. Then they build their homes over the lakes which are formed 

 above the dams. These homes are above the water line, but have only 

 underwater entrances. Thus they serve as an excellent retreat from 

 enemies. The beavers are beneficial because these dams act as an aid in 

 controlling erosion ; but the popularity of their fur threatened their 

 extinction before protective measures were put into effect. 



The Rat-like Rodents. These are the many species of rats and 

 mice which are so abundant the world over. They do tremendous 

 damage to the food supply, during its growth and after it has been 

 harvested and placed in storage. They also help spread some of the 

 world's most serious diseases, such as bubonic plague and typhus. 



