VARIATIONS OF RODENTS 253 



Males are larger than females in the rock squirrel, bushy-tailed wood 

 rat and both species of pocket gophers. However, the females of the rock 

 squirrel weigh more than the males — a possible adaptation to reproduc- 

 tion and/or hibernation. Females are larger than males in the Say chip- 

 munk and deer mouse, except that in the latter the males have larger 

 skulls. 



The limited trapping done below the Rim in the Sonoran Life Zone 

 indicated that desert mammals tend to move into the boreal zone more 

 than boreal mammals into arid places. The reason for the appearance 

 of the rock squirrel and the cliff chipmunk, both immature, in lower 

 Transition seems to be population pressure. The Botta pocket gopher 

 on Powell Plateau seemed out of place in the pine forest but apparently 

 no competition was being offered there by the northern pocket gopher. 

 The Botta pocket gopher and the bushy-tailed wood rat seemed as versa- 

 tile as any of the other rodents in occupying both desert and boreal 

 habitats. The ubiquitous deer mouse seemed to shun the arid Muav 

 Saddle and vicinity — the brush mouse was taken there. Although the 

 canyon mouse was taken at an unusually high altitude for the species, 

 it was still in its Sonoran Life Zone. 



Possible close genetic relationship between species occupying both 

 North and South Rims has been pointed out in the discussion of the size 

 of the least chipmunk, of the white patches on the Botta pocket gopher, 

 and of the short tail of the long-tailed meadow mouse. 



Rodents of the North Rim support Gloger's Rule of adaptive colora- 

 tion. The Kaibab squirrel, with limited range in the high, cold, moist 

 forest, shows areas of dense melanins. Those rodents with wide ecologi- 

 cal tolerances and more extensive ranges (e.g., the rock squirrel of the 

 North Rim) tend to be paler than one might expect. This paleness, pre- 

 sumably affected by the nearness and potency of the extensive deserts, 

 is particularly pronounced in certain juvenile pelages of the mountain 

 pocket gopher and the bushy-tailed wood rat. The young of the long- 

 tailed meadow mouse appear to be as dark as the adults, and one immature 

 specimen from the Canadian forest has a definitely reddish dorsum. Im- 

 mature pelages may be indicative of ancestral environments, whereas 

 adult pelages seem to reflect the present local environment of the par- 

 ticular animal. 



Paleness resulting from fading and wear of pelage of such sun-loving 

 animals as the rock squirrel and the golden-mantled ground squirrel is 

 the result of exposure to intense insolation. Such seasonal variation 

 appears not to detract from the animal's protective coloration on pale 

 soils and dead tree trunks. 



