Latitudinal patterns in the size of European butterflies

The importance of seasonality in shaping latitudinal size patterns was investigated in 16 species of lycaenine and satyrine butterflies with distributions in Sweden and continental Europe. Species without shifts in voltinism within the study area generally displayed clear size clines, increasing in size from north to south within Sweden. These species were also larger in continental Europe than in Sweden. In northern temperate areas a linear model for mean size at different latitudes seems to be a reasonable approximation for most species in this category. The size patterns for three species with shifts from univoltinism to bivoltinism at latitudes within Sweden were more reminiscent of the "saw-tooth"-pattern predicted by theory for such cases. The results suggest that size patterns in European butterflies to a large extent is governed by the length of the favorable season at different latitudes, affecting selection for development time and size. Altitude and drought also affect the effective season length and may amplify or complicate latitudinal patterns.