Allen's rule is a biological rule posited by Joel Asaph Allen in 1877. The rule says that the body shapes and proportions of endotherms vary by climatic temperature by either minimizing exposed surface area to minimize heat loss in cold climates or maximizing exposed surface area to maximize heat loss in hot climates. The rule predicts that endotherms from hot climates usually have ears, tails, limbs, snouts, etc. that are long and thin, whereas equivalent endotherms from cold climates usually have shorter and thicker versions of those body parts.