Bergmann Size Clines: A Simple Explanation for Their Occurrence in Ectotherms

In general ectothermic organisms grow larger at both lower temperatures and higher latitudes. Adult size in the soil nematode Caenorhabditis elegans reared at 10°C was approximately 33% greater than worms grown at 25°C. Nematode egg size and fish red blood cell size showed similar size increases at lower temperatures. These results indicate that body size differences in many ectotherms may simply be a consequence of developmental processes that cause cells to grow larger at lower temperatures. This would provide a general explanation for the increased size of ectotherms at lower temperatures independent of species-specific ecology.