Basic climatic parameters (annual average air temperature and annual amount of precipitation) over 12–25 years have been analyzed with regard to their values corresponding to different levels of abundance in three species of forest voles: Clethrionomys glareolus (in 11 areas of the species range), Cl. rutilus , and Cl. rufocanus (in three areas each). The results show that the abundance of all these species correlates significantly with fluctuations of annual average temperature, with the correlation being negative in Cl. glareolus and Cl. rufocanus but positive in Cl. rutilus . In addition, the abundance of Cl. rufocanus shows a significant correlation with annual precipitation. Voles inhabiting different geographic regions may reach high abundance under different conditions. As for the species range as a whole, however, Cl. glareolus is more ecologically flexible than the other two voles. This species is also more warmth- and moisture-loving, while Cl. rufocanus is more cold-loving and xerophilous, with Cl. rutilus occupying an intermediate position between them. Clethrionomys rutilus is most temperature-dependent and stenothermic among them, whereas Cl. glareolus is indifferent to the factors studied.

The analysis of the structure of the chewing surface of the third upper molar (M-3) in Clethrionomys from the European and Siberian parts of the former USSR (3043 ind. of C. glareolus, 319 ind. of C. rutilus, and 314 ind. of C. rufocanus) was performed. The occurrence of animals with simple M-3 morphotypes is higher in the Altai-Sayan mountain system, eastern Siberia (the Yenisei River basin), in the Carpathians, Balkan Mountains, and, probably, in the western regions of Europe. Simple morphotypes occur more frequently in the southern part of European Russia. To the north, the occurrence of simple morphotypes decreases and that of more complicated ones increases. Among Clethrionomys, simple morphotypes occurred more frequently among C. glareolus species that use mainly vegetative plant organs for feeding. The occurrence of simple morphotypes was rare in the cases when seeds and other nongreen plant parts predominated in nutrition of the voles. Based on the structure of the M-3 chewing surface, it is suggested that C. glareolus as the whole genus originated in eastern Asia. The further settling of the species proceeded to the west. The adaptation of the bank vole under new conditions, without competition with the other species of this genus became possible due to the high plasticity of the species under the increasing share of nongreen forage in its nutrition. This fact manifests itself distinctly in settling of C. glareolus in European Russia from the south to the north. This process was accompanied by the complication of the structure of the M-3 chewing surface. The colonization of the West-Siberian Lowland by the northern subspecies C. g. suecicus appeared to occur via the Urals after the sea digression.

To specify the taxonomic rank of form ciscaucasicus (independent species Sylvaemus ciscaucasicus, or intraspecific form of pygmy wood mouse, S. uralensis), a 402-bp the mtDNA cytochrome b gene fragment (402 bp) was examined in ciscaucasicus individuals from six geographic localities of the Caucasus and Ciscaucasus (Krasnodar krai and Adygeya Republic) and 17 S. uralensis individuals from seven localities of the Russian Plain (Saratov oblast, Smolensk oblast, Voronezh oblast, Tula oblast, Moscow oblast, Tver’ oblast, and northern Krasnodar krai). For comparison, the cytochrome b gene was partly sequenced in the samples of yellow necked, S. flavicollis (n = 2, Samara oblast), and Caucasian, S. ponticus (n = 6, Krasnodar krai), wood mice. One Mus musculus specimen from Western Europe, whose nucleotide sequences were deposed in the GenBank, was used as intergeneric outgroup. Phylogenetic trees for the forms examined were constructed based on the mtDNA sequence variation and using the neighbor joining and maximum parsimony methods. The network of the cytochrome b haplotypes was also constructed. The level of genetic divergence was evaluated using Kimura’s two-parameter algorithm. Based on the data on the sequence variation in a 402-bp mtDNA cytochrome b gene fragment, the hypothesis on the species status of the ciscaucasicus form was. The mean intergroup distances (d) between the geographic groups of S. uralensis varied from 0.0036 to 0.0152. At the same time, the distances between the pygmy wood mice and the group of S. flavicollis-S. ponticus varies in the range from 0.0860 to 0.0935, and the level of intergeneric genetic differentiation (Sylvaemus-Mus) is higher than the latter index (d = 0.142). Ciscaucasicus should be considered as geographic substitution form of S. uralensis. Furthermore, the Caucasian populations of S. uralensis (= ciscaucasicus) were characterized by a threefold lower value of intergroup genetic divergence (d = 0.0062) than the East European populations (d = 0.0179). This finding pointed to some isolation of Caucasian populations of pygmy wood mouse and depletion of their gene pool. However other molecular genetic data (similarity of nucleotide composition and consistence of the levels of intra-and intergroup distances) suggest the absence of geographic subdivision between Caucasian and East European populations of S. uralensis relative to the molecular marker examined.