Is diet quality an overlooked mechanism for Bergmann's rule?

Bergmann's rule (body size increases with latitude) has long interested biologists; however, its mechanism remains unclear. An overlooked mechanism (latitudinal variation in plant quality) might help explain Bergmann's rule. We studied three herbivores. In the field, the planthopper Prokelisia and the sea hare Aplysia, but not the long-horned grasshopper Orchelimum, were larger at high latitudes, following Bergmann's rule. In the laboratory, all three species grew larger or faster on high-latitude plants. High-latitude diets increased Prokelisia size and Aplysia growth rates by 8% and 72%, respectively, enough to explain the increase in field body size toward high latitudes. Therefore, latitudinal variation in herbivore body size could be influenced by latitudinal variation in plant quality, which may directly or indirectly also affect body size in detritivores, parasitoids, and predators. Studies of Bergmann's rule should consider the influence of biotic factors on body size in addition to abiotic factors such as temperature and precipitation.