Response to environmental factors and competition: skull, mandible and tooth shapes in Polish water shrews (Neomys, Soricidae, Mammalia)

Environmental factors and competition in closely related sympatric species may result in character displacement or phenotypic convergence. We studied the effects of sympatry on size and shape of the skulls, mandibles and molars in two species of water shrew, Neomys anomalus and Neomys fodiens . We studied populations from six localities in Poland, three where the species are sympatric (Pomorze, Białowieża, and Bieszczady Mts) and three where only N. fodiens is found. We studied shape and size using geometrical landmarks. The three morphological structures that we studied are expected to respond differently to environment and competition, because they are controlled by different numbers of gene loci, have different developmental patterns and different functional roles. We found strong evidence that shape in all three structures was influenced by local environment and that both species responded to geoclimatic factors in the same way. Indirect evidence suggests that the parallel response of the two species is likely to be ecophenotypic in the case of skulls and mandibles but selective in molars. We found no evidence for character displacement in either size or shape, although significant differences were found between the two species as a whole, and within species between localities. Not only was there any evidence for displacement but also the species were actually more similar when they occurred in sympatry, probably because of the similar ecophenotypic responses to their shared environment.