Coupled biotopic variation in populations of sympatric rodent species in the Southern Urals

Using geometric morphometric methods, coupled biotopic variation in the shape of the mandible was revealed in cenopopulations of two sympatric rodent species, the pygmy wood mouse (Sylvaemus uralensis Pall., 1811) and bank vole (Myodes glareolus Schreb., 1780), in the Southern Urals. As a rule, heterospecific pairs from syntopic samples synchronously taken in cenopopulations of the two species inhabiting contrasting local biotopes displayed similar (parallel and unidirectional) morphological changes. In an unusually arid year, however, differently directed morphogenetic responses were revealed in young of the year from cenopopulations of the two species in broadleaf forest outliers surrounded by steppe vegetation on tops of the Guberlya low hills. Such a local disturbance of the coupled pattern of biotopic variation in S. uralensis and M. glareolus cenopopulations under ambient conditions sharply deviating from the optimum may be evidence for depletion of their coevolutionary adaptive potential, which is estimated from the range of ecological conditions at which parallelism in the variation of sympatric species is observed.