Latitudinal gradients in butterfly body sizes: is there a general pattern?

We tested for the existence of latitudinal gradients in the body sizes of butterflies in North America, Europe, Australia and the Afrotropics. We initially compared body sizes (measured as male forewing length) of all butterflies found in 5° latitudinal bands in each region, and then evaluated the relationship between body size and latitude statistically using the latitudinal midpoint of each species' distribution. Trends were examined for species in all butterfly families together and for each family separately. We found that gradients in body sizes were inconsistent in different geographical regions and butterfly families; in some cases species were larger towards the tropics, in some they were smaller, and in other cases there were no relationships. Most of the gradients, when they existed, reflected between-family effects arising from changes in the relative numbers of species in each family across regions. We conclude that general ecological explanations for geographical trends in butterfly body sizes are inappropriate, and gradients largely reflect historical patterns of speciation within and between taxa in each biogeographical realm. Thus, the robustness of body size gradients found in other insect groups should be confirmed in future studies by including more than one geographical region whenever possible.