VARIATIONS OF RODENTS 241 



and possibly then only one litter per year, so that there would be but 

 few mobile j-oung in early summer. 



Temperature and humidity, with the resulting flora, appear to be 

 factors in the distribution of pocket gophers in the Grand Canyon. In 

 1947 maximum and minimum temperatures were taken for the few days 

 spent at each of the several collecting stations. Standing alone these few 

 data have but little significance ; but they agree with the temperature 

 gradient for July obtained from the three official weather stations in 

 the Grand Canyon National Park. 



TABLE 1 



Average daily temperatures for the month (from weather stations) 

 or fractional part (from field data) of July, 1947. Names of official 

 weather stations are in capital letters. Areas of occurrence of the Botta 

 pocket gopher are boxed. 



Temperature 

 Station Elevation Min. Max. Av. 



INNER CANYON 2400 ft. 76° 106° 91°F. 



The similarity of temperatures at the three collecting stations on the 

 North Rim where Botta pocket gophers were taken and that of the 

 South Rim where this animal is common, is evident. The hot air moving 

 up the canj^on and the canyon wall produces Sonoran vegetation and 

 gives higher local midday temperatures than obtain otherwise at the 

 given altitude. This is especially well illustrated in the maximum tem- 

 perature for Swamp Point, which is in the path of the hot air cur- 

 rents. The Botta pocket gopher, the only one on the South Rim, is now 

 known to occur on the North Rim at certain places where the environ- 

 ment is suitable. It is assumed to occur over much of Powell Plateau and 

 possibly on such arid points as the tip of Walhalla Plateau. 



