Character Displacement and Biogeography of the Pygmy Shrew in Northern Europe

Two ecologically similar shrew species, Sorex minutus and S. araneus, were studied in sympatry on the Swedish mainland and in allopatry on Irelend, the Outer Hebrides, and two islands in the Baltic Sea, in order to examine competitive character displacement in skull morphology. Eight skull measures were compared on a total of 228 specimens. S. minutus had significantly smaller jaws in sympatry. It was concluded that this difference is a result of interspecific competition; alternative explanations were tested and rejected. S. araneus did not exhibit morphological differences between sympatric and allopatric populations. The difference in response of the two species, it was concluded, is due to their different relative abundance in sympatry, which implies an asymmetry in their competitive effect on one another. It is suggested that late—Quaternary zoogeographic patterns during periods of low sea level can explain the present distribution of allopatric pygmy shrew populations.