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 while equivalent animals from cold climates usually have shorter and thicker versions of those body parts.
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 (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds said that there is poor empirical support for Allen's rule despite Allen's rule being an "established ecological tenet". Nudds and Oswald 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 adaptions that counter the predictions of Allen's rule.
J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, said that, although Allen's rule was originally formulated for endotherms, it can be applied to ectotherms which derive body temperature from the environment. Alho said that ectotherms with less surface to volume would heat up slower and cool down slower, 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.
Katzmarzyk and Leonard said that human populations appear to follow the predictions of Allen's rule. They said that there is a negative association between body mass index and mean annual temperature for indigenous human populations, meaning that people who originate from colder regions have a heavier build for their height and people who originate from hotter regions have a lighter build for their height. They said that relative sitting height is negatively correlated with temperature for indigenous human populations, meaning that people who originate from colder regions have proportionately shorter legs for their height and people who originate from hotter regions have proportionately longer legs for their height.
A.T. Steegman of the Department of Anthropology at State University of New York investigated the assumption that Allen's rule caused the structural configuration of the "Arctic Mongoloid" face. Steegman did an experiment that involved the survival of rats in the cold. Steegman said that the rats with narrow nasal passages, broader faces, shorter tails and shorter legs survived the best in the cold. Steegman said that the experimental results had similarities with the "Arctic Mongoloids", particularly the "Eskimo" and "Aleut," because these "Arctic Mongoloids" have similar features in accordance with Allen's rule: a narrow nasal passage, relatively large heads, long to round heads, large jaws, relatively large bodies, and short limbs.
R.L. Nudds and S.A. Oswald (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds did a study of the exposed lengths of seabirds' legs that said that the exposed leg lengths were negatively correlated with maximum environmental temperature, supporting the predictions of Allen's rule.
J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, said that tibia and femur lengths are highest in populations of the Common Frog that are indigenous to the middle latitudes and this is consistent with the predictions of Allen's rule for ectothermic organisms.
A contributing factor to Allen's Rule may be that the growth of cartilage is partly dependent on temperature. Researchers at Pennsylvania State University have shown that temperature can directly affect the growth of cartilage, providing a proximate biological explanation for this rule. In an experiment, 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.
- Bergmann's rule - that correlates latitude with body mass in animals.
- Gloger's rule - that correlates humidity with pigmentation in animals.
- Hesse's Rule - that correlates climate with heart weight (as a percentage of body weight).
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- Alho, J. S., Herczeg, G., Laugen, A. T., Räsänen, K., Laurila, A. And Merilä, J. (2011), Allen’s rule revisited: quantitative genetics of extremity length in the common frog along a latitudinal gradient. Journal of Evolutionary Biology, 24: 59–70. doi:10.1111/j.1420-9101.2010.02141.x
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- "Hot weather for longer legs". The Naked Scientists. December 2008.
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