Skull Sizes and Proportions in Western Palearctic Wood Mice (Sylvaemus, Muridae, Rodentia) from Eastern Europe: 1. Interspecific Variability

Craniometric features (measurements, indexes) in several species of the genus Sylvaemus (S. sylvaticus, S. flavicollis, and S. uralensis from Eastern Europe and some adjacent territories, as well as S. ponticus and S. witherbyi from the Caucasus) have been studied using multivariate statistical analysis. For the first time, craniometric characters of the genus Sylvaemus are considered in relation to the feeding ecology of species. A total of 614 skulls were studied. Based on original material, discriminant keys have been developed using skull measurements. The keys appear to be suitable for the diagnostics of the studied Sylvaemus species, including sibling ones. The largest skulls are shown to be characteristic of S.flavicollis, and the smallest ones, of S. uralensis. Cluster analysis of the absolute measurements of the skull cumulative selection has revealed the greatest isolation of S. flavicollis and the similarity of coexisting but phylogenetically remote S. ponticus and S. witherbyi. Factor analysis showed that, within the genus Sylvaemus, the general sizes are less essential than the characters related to food smashing (the length of the lower jaw and rostrum, the width of the occipital part of the skull), and possibly also to the thickness of the incisors. Comparative analyses of the relative skull measurements (the indexes related to the food preferences of the animals, granivory or folivory) allow us to conclude that the food adaptations to granivory in the structure of the skull and teeth increase in the series S. uralensis–S. sylvaticus–S. witherbyi–S. flavicollis–S. ponticus. The cranial evolution in species within the genus Sylvaemus is suggested to have been directed to increased adaptations in the mechanisms in seed peeling and processing.