242 DURHAM 



The Botta pocket gopher of the North Rim is similar to the sub- 

 species absonus to the northeastward in the small size, reduced sexual 

 dimorphism and conservative skull characters. In color, the grayest of 

 the series from Saddle Canyon of the North Rim compares favorably 

 with the average absonus. A close genetic linkage between these two 

 subspecies seems reasonable because the intervening land is probably in- 

 habited by Botta pocket gophers. The surprising similarity in general 

 coloration and markings between the typical boreorarius and the sub- 

 species fulvus taken on the opposite side of the Grand Canyon cannot 

 be explained so easily. Genetic differences in skull characters make these 

 two subspecies readily separable. Further discussion of the similarity 

 in color pattern between the pocket gophers on the two sides of the 

 Grand Canyon is to be found in the account of the northern pocket 

 gopher. 



Thomomys talpoides kaibabensis Goldman 



Habitat: The northern pocket gopher is the common one of the 

 North Rim and occurs widely over the Kaibab Plateau. It is found in 

 the deep soils of the mountain meadows and in the forests of pine, fir 

 and aspen, where it attains a large size; and it may be found also on 

 adjacent ridges in stony soils which support only thin timber. 



Size: The most obvious environmental response of this pocket gopher 

 is the adjustment of body size to the depth and texture of soil. Clines 

 of decreasing size from easily tilled, deep soils to shallow, stony soils 

 can be easily demonstrated in a given valley or from the west end of 

 the North Rim to the east end. For example, the largest specimen is 

 from the deep alluvium of Swamp Lake (the most westerly station for 

 the species) and the smallest adults are from the stony soil of Walhalla 

 Plateau (the most easterly station). About midway between these two 

 stations, on a ridge of shallow, stony soil, two immature northern pocket 

 gophers were taken. Because they were unusually pale and had small 

 ears these two specimens passed for Botta pocket gophers until their 

 skulls had been cleaned and examined. A series of average northern pocket 

 gophers were taken a few rods away in a typical boreal environment. 



The population of the Kaibab Plateau was formerly assigned to 

 fossor but later Goldman ( 1938, p. 333) renamed it kaibabensis. Judging 

 from the thirty-one adults which I took on the North Rim, I think the 

 average size is much smaller than that given in the original description 

 of the subspecies. In head-body length, my males and females average 

 159 mm and 155 mm, respectively, whereas those few selected specimens 



