Date of Degree

8-1993

Document Type

Thesis

Degree Name

Master of Arts (MA)

Program

Biology

Advisor

Arthur G. Cleveland

Advisor

Donald D. McLain

Advisor

Lance E. Sandberg

Abstract

Standard external measurement data was recorded from study skins at four Texas institutions. These collections also provided the specimen skulls from which twelve standard cranial measurements were made with digital calipers to the nearest 0.01 mm. This study included 111 Mexican ground squirrels (Spermophilus mexicanus) and 76 spotted ground squirrels (Spermophilus spilosoma). Most specimens had been collected in Texas and Mexico. Fifteen spotted ground squirrels had also been captured in four other states. An adaptation of the USDA Plant Hardiness Zone Map was used to subdivide the study area, so that geographic variation could be determined. Comparison of mean and standard deviation for males and females of both species revealed some sexual dimorphism. Female spotted ground squirrels appear to be consistently larger than the males in body size, while the opposite appears to be true for Mexican ground squirrels. Patterns of geographic variation differed for these two species. Climatic gradients were found for both species, although, for certain characteristics, an unexplained size increase occurs before the start of the gradual cline for the spotted ground squirrel. This study concludes that factors besides temperature are influencing geographic variation in these species. Future studies on the life histories and sexual dimorphism of these ground squirrels are needed.

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