236 DURHAM 



Size: Specimens obtained from the North Rim, particularly those 

 from Swamp Lake, are large. In most mass measurements of animal and 

 skull, the North Rim specimens are as large as or larger than the average 

 of the topotypes of S. L lateralis. They also average larger than those 

 listed from Utah (Durrant, 1952, p. 132). Those from the margin of 

 the Kaibab Plateau are somewhat smaller; e.g., an adult female from 

 Jacob Lake, near the periphery of this population, had worn teeth, was 

 suckling young and had a head-body length of only 173 mm. Squirrels 

 of this size from the North Rim were immature, with unworn teeth and 

 juvenile pelage. 



The tail of the North Rim specimens is short, 48.5 and 46 per cent of 

 head-body length for males and females, respectively, against averages 

 from Howell (1938, p. 193) of 51 and 53,5 per cent for squirrels from 

 Colorado and New Mexico. In this character of relatively short tails 

 (usually less than half head-body length), the North Rim population 

 resembles the subspecies chrysodeirus more than lateralis. From the 

 measurements of Davis (1939, p. 203) and those of Hall (1946, p. 322) 

 of the subspecies trepidus, it appears that an adaptive cline of decreasing 

 size and increasing length of tail extends southward from Idaho into 

 Nevada. The short tails and large size of these squirrels on the Kaibab 

 Plateau are in agreement with Allen's Rule and Bergmann's Rule, 

 respectively, that shorter than average appendages and larger bodies 

 appear to the northward and at higher altitudes. 



Comparative weights from other populations of the subspecies latera- 

 lis are lacking, but Hall {op. cit.) gave the weights of 181 and 199 grams 

 as averages for ten males and ten females, respectively, of S. I. chryso- 

 deirus, the linear measurements of which correspond closely with those 

 of lateralis of the Kaibab population. 



Sexual dimorphism : I found the females of the North Rim heavier 

 than the males (averages of 208 and 185 gr., respectively). This is in 

 agreement with statements from Hall {op. cit.) but not from Hatt (see 

 Howell, op. cit.). Males have larger hind feet. Although one was a sub- 

 adult, the two males taken on the North Rim had hind feet 45 mm long, 

 whereas 44 mm is the maximum foot length for the females and 42.4 mm 

 is the average. Tails of the two males average 90.5 mm in length against 

 85 mm for the females, and 48.5 and 46 per cent of head-body length, 

 respectively. Of the fifteen adults I took on the North Rim, only one 

 was a male. I cannot explain the reason for this uneven taking of sexes 

 (Linsdale, 1938, p. 178). 



Color and molt: Adults molt during June and July. At that time it 

 is not uncommon to take in the same trap line one specimen in bright 



