Allen's rule is an ecogeographical rule formulated by Joel Asaph Allen in 1877,[1][2] broadly stating that animals adapted to cold climates have relatively short limbs.[3] In more detail, it states that homeothermic animals in hot climates have low volume-to-surface ratios due to thermal adaptation while homeothermic animals in cold climates have high volume-to-surface ratios due to thermal adaptation.[3]