An external and cranial mophometric study of altitudinal variation in Microtus arvalis in Switzerland

Cranial and external measurements in 422 museum specimens of the common vole, Microtus arvalis, from Switzerland were examined to determine whether they varied with altitude in accordance with either (1) Bergmann's Rule, or (2) subspeciation. Correlation coefficients between altitude and size were calculated on 32 dimensions, each of which had been divided into sex segregated age groups. In not one of the 108 tests was correlation significant. The lack of adherence to Bergmann's Rule was investigated by (1) reviewing the basic concepts of the Rule, as applied to homeotherms; (2) identifying the major abiotic and biotic selection pressures that might affect growth in Microtus arvalis, including climate and competition with sympatric congenerics; and (3) indicating deficiencies in data derived from museum specimens which might have influenced the computations. There were several references in the literature to the occurrence of two subspecies of Microtus arvalis in Switzerland - the nominate subspecies M. a. arvalis (Pallas, 1779) and a montane subspecies called either M. a. incertus (Selys-Longchamps, 1841) or M. a. rufescentefuscus (Schinz, 1845). The lack of evidence in this study for the existence of two phenotypically and distributionally distinct subspecies was considered by (1) examining the general appropriateness of discussing variation in Microtus arvalis in terms of subspeciation; and (2) reviewing the literature on the diagnostic characters and distribution attributed to the montane morph. Two major conclusions were drawn. The first was that Bergmann's Rule should not be considered a "rule" until firm definitions are established on at least two of its founding precepts - the groups of animals to which it applies and the taxonomic level at which it applies - and until it is found to apply to a majority of the cases for which it is intended. The second conclusion was that for species like Microtus arvalis, in which growth is highly variable and distribution is by and large continuous, the usefulness of formal recognition of in-fraspecific populations is questionable until an overview of the geographic variability of the species as a whole is well documented.Science, Faculty ofZoology, Department ofGraduat