An ecogeographic pattern stating that within broadly distributed taxonomic clades of endothermic animals, populations or species in colder climates tend to have larger average body sizes than closely related populations or species in warmer climates.

Principle

Larger body size at colder temperatures reduces relative surface area per unit volume, decreasing heat loss and conferring a thermoregulatory advantage for endotherms in cold environments, all else equal.

Demonstration

Comparative observations showing clinal increases in body mass with latitude or altitude within a mammal or bird species, or interspecific patterns across related taxa distributed along a temperature gradient.

Misapplication

Applying the rule to ectotherms without caution, ignoring phylogenetic history, ecological confounders (resource availability, seasonality), sexual selection, or sampling bias; assuming universality where exceptions exist.

Consequence

Offers a predictive expectation for geographic size clines useful in biogeography, paleobiology, and studies of climate-driven morphological change, while cautioning that multiple processes may produce or obscure the pattern.

Reversal

Converse patterns occur when resource limitation, shorter growing seasons, or other selective pressures favor smaller sizes in colder regions; some taxa show no consistent Bergmann cline.

Boundary

Primarily applies to endothermic vertebrates and to comparisons within clades over sizeable climatic gradients; does not mandate causation, and exceptions occur due to ecology, phylogeny, development, and local adaptation.

Semantic Tension

Tension between a thermoregulatory explanation (surface-to-volume physics) and alternative explanations based on resource supply, life-history trade-offs, or phylogenetic inertia that can produce similar geographic size patterns.

Synthesis

Bergmann's rule characterizes a common tendency for larger body sizes in colder climates among endotherms and provides a thermoregulatory explanation, but it is a pattern with many exceptions whose occurrence depends on interacting ecological, evolutionary, and historical factors.