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Allen's rule is a ecogeographical 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.
A contributing factor to Allen's Rule may be that the growth of cartilage is partly dependent on temperature. Temperature can directly affect the growth of cartilage, providing a proximate biological explanation for this rule. Experimenters raised mice either at 2 degrees, 26 degrees or 48 degrees Celsius and then measured their tails and ears. They found that the tails and ears were significantly shorter in the mice raised in the cold in comparison to the mice raised at warmer temperatures, even though their overall body weights were the same. They found that the mice raised in the cold had less blood flow in their extremities. When they tried growing bone samples at different temperatures, the researchers found that the samples grown in warmer temperatures had significantly more growth of cartilage than those grown in colder temperatures.