Allen's Rule

An ecogeographic observation that endothermic animals from colder climates tend to have relatively shorter and more compact appendages (limbs, ears, tails) than related animals from warmer climates.

The observation that, within broadly distributed endothermic species or clades, individuals from colder climates tend to have relatively shorter extremities or appendages (ears, tails, limbs) than conspecifics or close relatives from warmer climates; this pattern is commonly attributed to thermoregulatory optimization because shorter appendages reduce exposed surface area and heat loss.