Are the western water shrew (Sorex navigator) and American water shrew (Sorex palustris) morphologically distinct?

Abstract

Recent phylogenetic studies applying the mitochondrial cytochrome b (Cytb) gene and nuclear genes revealed divergent cordilleran and boreal lineages in western populations of the Sorex palustris complex that are now recognized as distinct species: Sorex navigator (Baird, 1858) (western water shrew) and Sorex palustris Richardson, 1828 (American water shrew). Their distributions in western Canada are parapatric with a potential contact zone in the foothills of the Rocky Mountains. Using 80 specimens including 69 of known genotype, we assessed morphological differentiation in metric and nonmetric traits among S. navigator, S. palustris, and Sorex bendirii (Merriam, 1894) (marsh shrew), a sister species of S. navigator. Multivariate analyses revealed discrete morpho groups concordant with their genetic lineages and species-level divergence. Three individuals from northwestern British Columbia and one from southwestern Alberta with discordant mitochondrial and nuclear DNA aligned with the S. navigator morpho group showing no evidence for morphological intergradation with S. palustris. Although differentiated in genotype, S. navigator and S. palustris are cryptic species that can only be discriminated from morphology with metric cranio-dental data. We developed diagnostic morphometric criteria to identify specimens lacking genetic data.