Geographic Variation Analysis of Dipodomys ordii Using Nonmetric Cranial Traits

Abstract

Geographic variation in 18 nonmetric cranial traits was assessed for 11 populations of Dipodomys ordii from Oklahoma, Texas, and New Mexico. Frequencies of all traits were examined for sex, size, and side dependencies in a geographically restricted sample of 272 crania. Two traits were significantly affected by sex, two by cranial size, and two exhibited different frequencies on the left side of the skull than on the right. Relatively few within-individual and among-population intertrait correlations were found. The distance measure used in testing for interpopulation trait frequency differences was the Mean Measure of Divergence (MMD), as modified by Green and Suchey (1976). Nonmetric multidimensional scalings of distance matrices derived from each of four trait subsets and a principal components analysis based on one subset gave similar results. The 11 samples were displayed in three-dimensional summaries approximately as expected on the basis of their geographic distribution. Because nonmetric traits may be adaptively neutral, frequency differences between populations may result from genetic drift. Nonmetric traits are clearly of value in studies of geographic variation in Dipodomys. I follow Sjøvold (1977) in suggesting that consideration be given to each trait with regard to how strongly it is influenced by dependencies (e.g., sex and size) that may confound interpretation of results.