VARIATIONS AND ADAPTATIONS OF THE RODENTS 



OF THE NORTH RIM OF THE GRAND 



CANYON, ARIZONA 



By 



Floyd E. Durham 



Allan Hancock Foundation 



The Kaibab Plateau, a high, isolated dome in northern Arizona, is 

 bounded by the Grand Canyon of the Colorado River on the south and 

 east, by canyons and ravines on the west, and by high desert on the north, 

 as well as to the eastward and westward (Map 1). The higher part of 

 the Plateau is an isolated unit of Transition and Canadian forest, an 

 island at whose margins strike waves of desert winds. The zone of transi- 

 tion between Sonoran vegetation and heavy timber is narrow, especially 

 on the North Rim of the Grand Canyon. This intermediate strip is 

 where the ranges of most of the forest dwelling mammals meet the ranges 

 of the desert mammals. Few kinds cross this zone. 



Within the confines of this secluded forest there evolved the endemic 

 Kaibab squirrel, well known to mammalogists and to the casual visitors 

 of the Grand Canyon National Park. The powerful factors of isolation 

 and selection which produced this striking pattern of black underparts 

 and white tail in the squirrel presumably influenced the other rodents 

 there. In order to study these variations, I made arrangements for limited 

 collecting on the North Rim of the Grand Canyon (Map 2), particularly 

 in the wild, unfrequented western area of the Park, during one month 

 of early summer in 1947 and again in 1949. Thirteen species of rodents 

 were observed, collected and studied (Table 3). Variations within the 



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