Theory
The theory behind Allen's rule is that endothermic animals with the same volume may have differing surface areas, which will aid or impede their temperature regulation.
Consider eight cubical boxes of unit volume and unit area of a side. A rectilateral stack two boxes wide, one long and four tall will have a volume of 8 units and a surface area of 28 units. A cubical stack two boxes wide, two long and two high will have the same volume of 8 units but a surface area of only 24 units.
In cold climates, the greater the exposed surface area, the greater the loss of heat and therefore energy. Animals in cold climates need to conserve as much energy as possible. A low surface area to volume ratio helps to conserve heat as there is a smaller surface area for the heat to pass through.
In warm climates, the opposite is true. An animal will overheat quickly if it has a low surface area to volume ratio. Therefore, animals in warm climates will have high surface area to volume ratios so as to help them lose heat.
R.L. Nudds and S.A. Oswald (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds claimed that there is poor empirical support for Allen's rule despite being an "established ecological tenet". They claim the support for Allen's rule mainly draws from single-species studies, since multiple species studies are "confounded" by the scaling effects of "Bergmann's rule" and alternative adaptions that go against the predictions of Allen's rule.
According to J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, although Allen's rule was originally formulated for endotherms, it can be applied to ectotherms which derive body temperature from the environment. The reason for this is that ectotherms with less surface to volume would heat up slower and cool down slower which they claim might be adaptive in "thermally heterogeneous environments".
According to J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, there has been a renewed interest in Allen's rule due to global warming and the "microevolutionary changes" it predicts.
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