Bergmann's Rule

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.

An empirical generalization that, within broadly distributed taxonomic groups of endotherms (mammals and birds), populations or species from colder climates tend to have larger mean body sizes than conspecifics or close relatives from warmer climates; the explanation commonly invoked is thermoregulatory — larger bodies have lower surface-area-to-volume ratios and thus reduced relative heat loss.