Geographic Variation in the Body Size of Some Japanese Leptocarabus Species (Coleoptera, Carabidae) : The "Toppled-Domino Pattern" in Species along a Geographic Cline

We analyzed the geographic pattern of body size variation in five species belonging to the subgenera Leptocarabus and Adelocarabus in the genus Leptocarabus (Coleoptera, Carabidae) in the Japanese Archipelago. Except for L. (L.) hiurai in Shikoku, these species are parapatric or partially sympatric with each other. In all four species, the northeastern populations had larger bodies than the southwestern populations, while in any two parapatric species the southwestern species had larger bodies than the northeastern species. We called this type of geographic variation the "toppled-domino pattern". Within each species, body size was positively correlated with the annual mean temperature of the locality (the converse of Bergmann's rule). The toppled-domino pattern resulted from (1) interspecific differences in the altitudinal range at the boundary zone, where southwestern species occurred at lower altitudes than northeastern species, and (2) intraspecific regional differences in body size, in which northeastern populations have larger bodies than southwestern populations under the same temperature conditions. In L. (L.) procerulus and L. (A.) arboreus from Honshu, the interspecific difference in mean body size at sympatric sites was greater in cooler habitats. This trend was more evident in central Honshu, where there are alpine zones, than in northern Honshu, where there are no alpine zones. We hypothesize that the observed geographic patterns of intra- and interspecific body size variation resulted from (1) climatic adaptation during colonization and range shift by Leptocarabus through the glacial and inter-glacial periods, and (2) the effects on body size evolution of interspecific interactions in a contact zone.