Bergmann's rule paper published in J Biogeography

The Hepatic Tanager (Piranga flava; pictured), found at lower and middle elevations in Central America, is larger than its high elevation congener, Flame-colored Tanager (Piranga bidentata). This "low and large" pattern, where lower elevation species are larger than high elevation relatives, is surprisingly common in tropical montane birds.

The Hepatic Tanager (Piranga flava; pictured), found at lower and middle elevations in Central America, is larger than its high elevation congener, Flame-colored Tanager (Piranga bidentata). This "low and large" pattern, where lower elevation species are larger than high elevation relatives, is surprisingly common in tropical montane birds.

Evolutionary ecology is messy; we usually talk about patterns, and only the strongest patterns get labeled as "rules" (and even these "rules" tend to be more along the lines of "strong patterns"). One such example is Bergmann's rule, which describes the pattern that organisms living at high latitudes tend to be larger than organisms living at low latitudes. The likely interpretation is that larger body size is an adaptation to colder environments. In humans, think of the short squat body types of Arctic peoples (good for staying warm!) versus slender and tall East Africans (good for dissipating heat?). But temperature is not the only factor that differs along latitudinal gradients - so does seasonality, species richness, and a host of other abiotic and biotic factors. If Bergmann's rule is really about adaptation to temperature, we should see similar patterns along tropical elevational gradients, where it is cold at high elevations but warm at low elevations. However, I show that in tropical birds, body mass is unrelated to elevational distribution. This lack of pattern holds across regions and comparisons (intraspecific vs. interspecific, and closely related species vs. entire passerine assemblages), suggesting temperature exerts minimal influence on body size evolution in tropical montane birds. At least for tropical birds, Bergmann's rule is not even a pattern!