VARIATION'S OF RODEN'TS 



249 



TABLE 2 



Subspecies Locality 



mordax 



mordax 



latus 



alticola 



baileyi 



alticola 



Elevation Latitude Tail/Head-Body 



Sawtooth Mts., Idaho 

 Elko Co., Nev. 

 Toiyabe Mts., Nev. 

 Mts. of SE Utah 

 North Rim, Ariz. 

 San Francisco Mts 



Ariz. 



7000' 

 6500' 

 8500' 

 9000±' 

 8000' 

 8200' 

 ?? 



44° N 

 42 

 39 

 38i^± 

 36+ 



3sy2 



32^ 



57 per cent 

 51 per cent 

 50 per cent 

 49 per cent 

 46 per cent 

 45 per cent 

 40 per cent 



leucophaeus Graham Mt., Ariz. 



The relative length of tail to length of head-body for certain sub- 

 species and populations of Microtus longicaudus as obtained from pub- 

 lished measurements. 



Because the altitudes are all relatively high it is assumed that the 

 environments (including temperature) are similar for all these stations. 

 Therefore, in consideration of surface-volume relationships, the tails of 

 the southern populations should be as long as or longer than the tails 

 of the northern populations in order for length of tail to be an adaptive 

 character. This is an exception to Allen's Rule. 



Sexual dimorphism: The males and females on the North Rim are 

 nearly identical in size (Kellogg, 1922, p. 281) except that the skulls 

 of the males are slightly (1 to 2^^ per cent) larger. It was difficult to 

 decide which individuals were adult. One pregnant female was classed 

 as immature because of her subadult pelage and small size. 



Color: The dorsal pelage of one juvenile was unusually reddish. 



SUMMARY 



The measurements (Table 3) of the rodents of the North Rim as 

 compared with measurements of types, topotypes and series of the same 

 subspecies from adjacent regions show that most of the nonfossorial 

 rodents (i.e., rock squirrels, golden-mantled ground squirrels, least chip- 

 munks, chickarees, deer mice and bushy-tailed wood rats) of the North 

 Rim are as large (head-body length) as or larger than the average (or 

 sometimes than the maximum) from adjacent areas. Even the deer mouse, 

 which occurs on both sides of the Colorado River, is noticeably larger on 

 the North Rim than on the South Rim. Although some of these rodents 

 were available to me only in small series, the overall concept of size is 

 that of "large." These animals have correspondingly larger skulls, as 

 shown (Table 3) by length of skull and breadth of brain case. The weight 

 of the animals gives further evidence of their superior size whenever 

 comparative data are available from other collectors. 



