VARIATIONS OF RODENTS 245 



the Rim to dense Canadian forests. It was taken at every station except 

 Swamp Point, Muav Saddle and Powell Spring. It probably occurs at 

 these places also, although the brush mouse was the dominant white- 

 footed mouse at the latter two stations. 



Population fluctuation: Normally the deer mouse occurs in con- 

 siderable numbers in such areas, but 1947 was a lean year and only four 

 adults were obtained in a month of trapping on the North Rim. The 

 mouse was common in 1949 when enough more were obtained to make 

 a series of seventeen adult males and fifteen adult females. This fluctua- 

 tion in population of Peromyscus agrees with Kelson (1951, p. 79) who 

 reported the species scarce in eastern Utah in 1946 but abundant in 

 August, 1948. Also, Quick (1953, p. 257) stated, "During the fall of 

 1947, populations of . . . Peromyscus were low [in British Columbia]. 

 In the following autumn, 1948, [Peromyscus] made marked advances 

 . . . and the natives . . . called it a 'mouse year.' " 



Size and sexual dimorphism : From external measurements it is evi- 

 dent that the sexes of the deer mouse on the North Rim are almost 

 identical in size of appendages (i.e., length of tail, hind foot and ear). 

 In length of head-body the females are 4.35 per cent longer than the 

 males, and their weights (corrected for embryos) are correspondingly 

 heavier. The skulls of the males are slightly larger than those of the 

 females. The tail and hind foot of the specimens from the North Rim 

 are slightly longer than those of the topotypes from the San Francisco 

 Mountains (Merriam, 1890, p. 65) but the skulls of the two populations 

 appear identical in size (Osgood, 1909, p. 263). However, Durrant 

 (1952, p. 312) for Utah and Warren (1942, p. 199) for Colorado 

 list a smaller size for this mouse and their measurements agree with 

 those obtained from a series collected by G. P. Ashcraft, formerly of 

 the Hancock Foundation, on the South Rim of the Grand Canyon. 

 Averages for this series, consisting of 18 males and 8 females, are 145, 

 153; 61, 63; 20.3, 21; 16, 17; skull: greatest length, 24.9, 25.6; zygo- 

 matic breadth, 12.5, 12.8; breadth of cranium, 11.6, 11.9; length of 

 nasals, 10.2, 10.6, respectively. There is little difference in skull size in 

 these various populations, but in length of head-body both those from 

 the San Francisco Mountains and those from the North Rim exceed 

 other populations by 6 mm. 



Peromyscus boylii rowleyi (Allen) 



Habitat: The brush mouse occurs sparingly on the North Rim. Only 

 three adults were trapped, one in the cabin at Muav Saddle and the 

 other two, along with five immature specimens, from rock cliffs near 



