Allen's rule
Allen's rule is an ecogeographical rule formulated by Joel Asaph Allen in 1877, broadly stating that animals adapted to cold climates have relatively short limbs. 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.
Allen's rule predicts that endothermic animals with the same body volume should have different surface areas that will either aid or impede their heat dissipation.
The diagram to the right shows two rectangular prisms that are each composed of eight cubes. Each unit cube contains a cubic unit of volume and each of the surfaces of the cubes are a square unit of area. A rectangular prism that is two cubes wide, one cube long and four cubes tall will have a volume of 8 units3 and a surface area of 28 units2. A composite cube that is two cubes wide, two cubes long and two cubes high will have the same volume of 8 units3 but a surface area of only 24 units2.
In cold climates, Allen's rule predicts that animals should have comparatively low ratios of surface area to volume. Because animals in cold climates need to conserve as much heat as possible, Allen's rule predicts that they should have low surface area to volume ratios to minimize the surface area by which they dissipate heat, allowing them to retain more heat.
In warm climates, Allen's rule predicts that animals should have comparatively high ratios of surface area to volume. Because animals with low surface area to volume ratios will overheat quickly, Allen's rule predicts that animals in warm climates should have high surface area to volume ratios to maximize the surface area by which they dissipate heat, allowing them to dissipate more heat.
R.L. Nudds and S.A. Oswald argued in 2007 that there is poor empirical support for Allen's rule even if it is an "established ecological tenet". They said that the support for Allen's rule mainly draws from studies of single species, since studies of multiple species are "confounded" by the scaling effects of "Bergmann's rule" and alternative adaptations that counter the predictions of Allen's rule.
J.S. Alho and colleagues argued in 2011 that, although Allen's rule was originally formulated for endotherms, it can be applied to ectotherms which derive body temperature from the environment. In their view, ectotherms with less surface to volume would heat up and cool down more slowly, and this resistance to temperature change might be adaptive in "thermally heterogeneous environments". Alho said that there has been a renewed interest in Allen's rule due to global warming and the "microevolutionary changes" that are predicted by Allen's rule.
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