Effects of latitude and allopatry on body size variation in European water shrews

We studied the intra- and interspecific size variability of 271 water shrews Neomys fodiens (Pennant, 1771) and N. anomalus Cabrera, 1907 from seven sample sites along a latitudinal transect from Bosnia and Herzegovina to Poland. Neomys anomalus was the only water shrew in three Dinaride karst fields, while it was sympatric with N. fodiens in remaining sites. The first principal component scores (PC1; 72.2% of variance explained), derived from principal components analysis of 13 cranial, mandibular and dental measurements, were used as the size factor. One-way ANOVA detected significant interpopulation variation in both species; intraspecific variation, however, was much more pronounced in N. anomalus. No latitudinal size pattern was found in N. fodiens (r = -0.42, p = 0.58), while mean PC1 scores correlated significantly and negatively with latitude in N. anomalus (r = -0.92, p = 0.004). Therefore, along a north to south transect, N. anomalus converged in size towards N. fodiens, which suggests that the former species occupies increasingly more aquatic habitats in the same direction. Individuals from allopatric populations of N. anomalus from Slovenia and Bosnia and Herzegovina were, on average, larger than sympatric conspecific populations from the same latitudinal zone, which is consistent with the hypothesis of character displacement.