252 DURHAM 



The large size of the animals on the North Rim is in agreement with 

 Bergmann's Rule of larger animals in colder environments. The size of 

 the pocket gophers on the North Rim, however, is correlated not so much 

 with temperature as with compaction of soil and altitude. On the North 

 Rim the relatively small Botta pocket gopher seems to be approaching 

 its limits of toleration of high altitude and compaction of soil. Although 

 the northern pocket gophers found in the center of the Kaibab Plateau 

 where the soil is deep and the temperature low, are unusually large, those 

 taken on the North Rim, not far from the rim of the Canyon (tempera- 

 ture somewhat higher), show a decline in size. However, this decline is 

 probably based on tolerance of range margin (minimum altitude and 

 maximum compaction of soil) (Miller, 1952, p. 442). 



Because most of the nonfossorial rodents in this boreal environment 

 are larger than average, one might expect relatively shorter than average 

 appendages (Allen's Rule). This is true of the tail of the golden-mantled 

 ground squirrel, the least chipmunk and the long-tailed meadow mouse, 

 but the tail of the Say chipmunk and of the canyon mouse is longer than 

 the average. The length of nasals, an indication of the length of rostrum 

 or nose, is also greater in least chipmunks, chickarees, canyon mice, brush 

 mice, bushy-tailed wood rats and long-tailed meadow mice. The longer 

 nasals at least partly account for the greater length of skull found in 

 some of these animals. Of these long-nosed rodents, the brush mouse is 

 unique in that the rostrum is so long that the animal might be mistaken 

 for the long-nosed miouse. Possibly this exaggerated appendage is an 

 adaptation for food getting and air warming by a nonhibernating animal 

 active in a cold climate. A longer than average hind foot appears in 

 rock squirrels, Say chipmunks and canyon mice. 



Considering the short tails of the population of long-tailed meadow 

 mice, we find this character adaptive to the high mountain environment 

 of the Kaibab Plateau. Nevertheless, on examining a series of mountain 

 top populations of meadow mice from Idaho to Arizona we find the 

 unusual correlation of decreasing length of tail w^ith decreasing latitude. 

 This exception to Allen's Rule is also found on the Pacific Coast where, 

 in this same species, the subspecies Microtus longicaudus abditus in Ore- 

 gon has the longest tail (73 per cent of head-body length) and the sub- 

 species 71/. /. bernardinus in Southern California has the shortest tail (52 

 per cent of head-body length). Because the rules of Bergmann, Allen 

 and Gloger deal with adaptive characters, the cline of decreasing length 

 of tail to the southward seems to be an example of a fixed random char- 

 acter. 



