ANIMAL BEHAVIOR — WALKER 307 



power. Badgers {Taxidea) probably surpass other mammals in the 

 quantity of earth they excavate in digging out their prey. 



The little desert creatures such as the pocket mice and kangaroo 

 rats pack the sand about their houses with their forefeet, patting it 

 so rapidly that a buzzing sound is produced ; but the motions are too 

 rapid to enable the human eye to catch the details. 



The teeth of rodents protrude considerably, and are important 

 for cutting wood, as weapons of defense, and as tools for digging. 

 Often the burrowing rodent uses his teeth to remove stones or to 

 cut hard earth. The lips of certain groups are closed behind the large 

 front incisoi-s so that the front teeth actually protrude outside of 

 the mouth proper. Thus, dirt is kept out of the mouth by the 

 lips which make curtains or doors close behind the teeth. The 

 mouths of rodents are situated rather definitely on the underside of 

 the head in a position well adapted for working below the level of 

 the nose. They would thus appear to be poorly adapted for working 

 above that level. These animals have, however, effectively solved the 

 problem of cutting out the top of a tunnel or cutting a root or 

 other object obstructing the upper portion of a runway. They merely 

 throw themselves on their backs and hold the underside of their 

 heads close to the top of the burrow so that they can cut upward as 

 easily as downward. 



USE OF TOOLS AND MATERIALS 



The use of tools and materials is more common among animals than 

 is generally supposed. The best known use of materials is seen in 

 the making of birds' nests. In this work each species of bird follows 

 a very definite pattern of procedure. Many birds choose a special 

 kind of material for one part of the nest, and a different type of 

 material for another part of it. 



Among mammals the making of the nest, particularly the lining 

 of a cavity with material that is dry and insulating, is well known. 

 It is common to see squirrels carrying dry leaves or shredded bark to 

 their homes, particularly after an unusually cold night or a bad rain. 

 Apparently the little fellows have not been comfortable through the 

 night and their first thought in the morning is to improve their nest. 



The habit of the so-called pack rat or trade rat {Neotoma and 

 Teonoma) of carrying small articles away from a camp and leaving 

 something in place of the objects taken has been frequently described. 

 Apparently the rodent's idea is to carry material to its home for 

 protection against invasion, or perhaps for ornamentation. For ex- 

 ample, a nest in a crevice of a cliff or at the base of a cliff may not 

 provide adequate protection on th^ exposed sides. Therefore the 

 clever rodent carries sticks, dried cow dung, dried cactus pads, and 



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