Population Variation in Dipodomys ordii from Western Texas

Abstract

Morphological and habitat variation was studied in four populations of the kangaroo rat ( Dipodomys ordii ) in western Texas. Twenty-six external, cranial, and ratio characters were analyzed in 536 specimens. The number of characters exhibiting secondary sexual dimorphism varied from 14 at Tahoka to five at Canadian and Kermit. In the two northern populations (Canadian and Needmore) females were larger than males, whereas males were larger than females in the southern populations (Tahoka and Kermit). Many of the characters studied had low coefficients of variation (CV) for mammals. Least supra-occipital width, however, showed an abnormally high CV, not only in D. ordii but also in three other species of this genus ( D. merriami, D. spectabilis , and D. deserti ). Despite large sample sizes and precautions against sampling bias, many variates did not fit a normal distribution but were skewed. Large correlation coefficients between certain variates suggest many of the characters studied represent a single adaptive complex and should not be considered independently. Factors that may have influenced the existing evolutionary diversity are discussed.