240 DURHAM 



an animal would traverse its tunnels to spring a trap or close a burrow 

 opening. The northern pocket gopher, in the deeper, moister soil of the 

 forest, was still active and readily trapped at this season. 



Botta pocket gophers were obtained at the following localities: (1) 

 Swamp Point where the soil is thin, drainage excessive and chaparral 

 dominant; (2) Muav Saddle where the soil is shallow and stony and 

 contains but little moisture in early summer, the exposure to insolation 

 is maximum and the vegetation is limited to Upper Sonoran chaparral 

 by the up-canyon and up- wall drafts from the hot deserts and valleys to 

 the westward and below; (3) Saddle Canyon, north of Muav Saddle, 

 where insolation is reduced because of the north exposure, where the 

 hot desert winds are deflected overhead, where some alluvium has accu- 

 mulated and seepage from highlands supplies some soil moisture, and 

 where scattered pines grow in the chaparral; and (4) the northeastern 

 part of Powell Plateau, which is typical Transition Zone with a good 

 stand of yellow pine. Two Botta pocket gophers were taken at the latter 

 station although the environment seemed more appropriate for the north- 

 ern pocket gopher. 



Color: Botta pocket gophers from Muav Saddle and Swamp Point 

 are buffy golden; those from Saddle Canyon are huffy gray; those from 

 Powell Plateau are darker than either, tending to rufous brown. In gen- 

 eral the brightest buff pocket gophers are found in environments of high 

 insolation and thin stony soil; the grayer animals in less exposed places 

 which contain some alluvium ; and the browner animals at the margin of 

 the forest where the soil is deeper and moister. 



Size and sexual dimorphism: As the smallest individuals were taken 

 from the stoniest soil, sexual dimorphism in size on the North Rim seems 

 to increase with depth of soil and ease of excavation. 



Age and sex ratios: Of the eighteen specimens of the Botta pocket 

 gopher taken, six were immature ; and of the twelve adults, only two were 

 males. The low percentage of adult males of both the Botta pocket 

 gopher (18.2) and the northern pocket gopher (16.1) is unexplainable. 

 The percentage of immature specimens taken in the two species is 36.1 

 and 11.4, respectively. The difference in ratios of the two age groups in 

 the two species suggests differences in breeding cycles. It may be that 

 the Botta pocket gopher, adapted to warm climates, bears young late in 

 the summer and/or early in the spring, so that the young are mobile by 

 early summer ; whereas the northern pocket gopher, adapted to the short 

 summer season of high mountain areas, may have young late in the spring 



