Where Are We Now? Bergmann's Rule Sensu Lato in Insects

Bergmann's rule states that individuals of a species/clade at higher altitudes or latitudes will be larger than those at lower ones. A systemic review of the known literature on inter- and intraspecific variation in insect size along latitudinal or altitudinal clines was done to see how often such clines appeared and whether they reflected classwide, species-specific, or experimentally biased tendencies. Nearly even numbers of studies showed Bergmann clines and converse-Bergmann clines, where insects get smaller as latitude/altitude increases. In fact, the majority of studies suggested no clines at all. Small ranges may have obscured certain clines, while giant ranges may have introduced artifacts. Researchers examining interspecific patterns found clines less frequently than those examining intraspecific patterns because of variation among species within the clades, which renders interspecific studies unhelpful. Bergmann's rule does not apply to hexapods with nearly the same consistency as it does to endothermic vertebrates. The validity of Bergmann's rule for any group and range of insects is highly idiosyncratic and partially depends on the study design. I conclude that studies of Bergmann's rule should focus within species and look at widespread but contiguous populations to account for all sources of variation while minimizing error.