Skull Dimensions of the Great Plains Muskrat, Ondatra zibethicus cinnamominus

Abstract

The importance of cranial measurements is due primarily to their use by taxonomists to distinguish species and in some cases subspecies. The dangers of placing too great an emphasis on the taxonomic importance of cranial measurements were pointed out by Allen (1894). Tryon (1951) says that his studies indicate the need for careful evaluation and perhaps revaluation of the use of skull measurements at the subspecific level. Hollister (1910) in his description of Ondatra zibethicus cinnamominus gives a few of the cranial measurements of the type specimen and the averages of measurements of five skulls of adults from the type locality. These measurements are repeated by Hollister in his “Systematic Synopsis of the Muskrats” (1911). Latimer and Riley (1934) reported on the cranial dimensions of 124 specimens of this subspecies collected in Kansas. Insofar as can be determined, there are no published reports on skull dimensions of known-age animals for any of the muskrat subspecies. All of the skulls reported on here were collected from lakes within the Valentine National Wildlife Refuge which is located in north central Nebraska. These skulls are of known age. The majority of them are from animals that were tagged during the summer months and harvested during the winter trapping period. Females that had borne more than one litter, as evidenced by placental scar counts, were undoubtedly adult females, and hence are included in the knownage adult female group. These skull dimensions are presented primarily to furnish information relative to the variation in skull measurements associated with differences in age and sex of this particular subspecies. The dimensions were taken with a vernier caliper reading to one-tenth of a millimeter. Eight different dimensions of the skull were taken. The points of reference for taking these dimensions are as follows: 1. Total skull length —anterior surface …