The ecogeographical rule known as Bergmann’s rule suggests that there is a positive relationship between body size and latitude when comparing closely related taxa. The underlying mechanism, or mechanisms, to explain this pattern vary as widely as the taxa that seem to follow it, which has led to skepticism over whether Bergmann’s rule should be considered a rule at all. Despite this, Bergmann’s rule is widespread among modern birds, mammals, beetles, and some amphibians, but...