RODENTS IN SOIL FORMATION GRINNELL 341 



Of the five types of bursowing rodents in California, the most 

 widespread, in the aggregate the most abundant, and certainly the 

 most effective in its equipment for turning over the soil, is the pocket 

 gopher (Thomomys) ; and upon this type I propose chiefly to dwell. 

 An examination of a pocket gopher shows its structure throughout 

 to be remarkably specialized for burrowing into and through the 

 ground. A study of its habits shows that in all probability a pocket 

 gopher spends at least ninety-nine one-hundredths of its existence 

 below ground. Its world is limited by the earthen walls of a cylin- 

 der. In one direction this cylinder brings safety from enemies; at 

 the other end it brings accessibility to food. We find that the gopher 

 is deficient relatively to other rodents with respect to eyesight. Its 

 hearing is likewise below the average and seems to be keenest for 

 sounds of very low rate of vibration, such as jarrings of the ground. 

 Its sense of touch is localized not only in the nose and surrounding 

 vibrissa?, but also at the tip of the tail. The animal moves quite as 

 well backward in its burrow as forward : it needs to be apprised of 

 conditions in both directions. 



The body as a whole is short, thick through, with a notable mas- 

 siveness anteriorly — just the opposite of the litheness of structure 

 characteristic of, say, the squirrels. The head of a pocket gopher 

 is larger in proportion to its body than is that of any other land 

 mammal in California. The head is joined to the body without any 

 obvious neck constriction, and the shoulders are broad. The bigness 

 of the head is accounted for both by the thicker and more ridged bones 

 of the skull and by the greater mass of the muscles attached to them. 

 These are correlated with the structure, position, and operation of 

 the relatively huge incisor teeth. 



The mouth is a vertical slit, guarded by furry lips which are ap- 

 pressed so as to keep out. the earth loosened by the projecting incisor 

 teeth or pushed ahead of the animal by means of the face and fore- 

 feet. The pocket gopher is our only mammal in which the incisor 

 teeth can not be concealed within the lips. 



Comparison of the pocket gopher, as an extreme type of digger, 

 with the California ground squirrel, which is also a digger but to a 

 far less degree, shows some significant differences. An average adult 

 California ground squirrel weighs 681 grams ; an average adult male 

 pocket gopher weighs 170 grams, close to one-fourth as much. But 

 the weight of the skull of the ground squirrel is 7.8 grams, while 

 that of the gopher is 7.2 grams, practically the same. In other 

 words, the skull of a gopher is four times as heavy as that of a 

 ground squirrel, total weights considered. The brain case, however, 

 seems to have relatively about the same capacity in the two animals. 

 The skull and teeth of the pocket gopher, together with the muscula- 



1454—25—23 



