Since we got into the discussion of Bergmann’s Rule, it is only fitting that we also talk about its kissing cousin, Allen’s Rule.

It states that endotherms from colder climates usually have shorter limbs (or appendages) than the equivalent animals from warmer climates. Endotherms are organisms who produces heat through internal means, like say through muscle shivers or raising of one’s metabolism. Mammals and birds classify as endotherms but reptiles and fish are classified as ectotherms,

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. This means that two people of the same volume but the person who has less surface area through short limbs is will lose less heat.

We take an excerpt from the Wikipedia Article on Allen’s Rule HERE.

“”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.

(From a physics point of view, it does sound very reasonable.)

“”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 cartilage growth, providing a biological explanation for this rule….were significantly shorter in the mice raised in the cold, compared with the mice raised at warmer temperatures….when they tried growing bone samples at different temperatures, the researchers found that the samples grown in warmer temperatures had significantly more cartilage growth than those grown in colder temperatures”” (!!)

When Allen’s Rule is applied to humans, the researchers seem to mostly agree that Allen’s Rule is valid. Verne Troyer who played Mini-Me in the Austin Powers movies is an example of someone who has short limbs relative to their body. He is 2′ 8″ tall and suffers from cartilage–hair hypoplasia dwarfism He is pictured above.

“”According to William R. Leonard of the Department of Anthropology at the University of Florida, human populations follow Allen’s rule. As evidence, Leonard cites a study by D.F. Roberts of the Anthropology Laboratory at Oxford University that showed human populations follow Allen’s rule. 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 found the rats with narrow nasal passages, broader faces, shorter tails and shorter legs survived the best in the cold – Wikipedia”

Me: My final conclusion to the Allen Rule is that it is a useful “rule of thumb” to be considered when we do make arguements about limb and height deviations seen between different ethnic groups located across the world. Limb length relative to height is often a good determinant factor on how the human body has evolved against nature over time.

Note: You can find more information about Allen’s Rule by clicking HERE which gives a great understanding of the Principle by Palomar University. 

Tyler’s Notes:

Here’s an interesting new study regarding Allen’s rule.

Allen’s Rule Revisited: Temperature Influences Bone Elongation During a Critical Period of Postnatal Development.

“Limbs of animals raised at warm ambient temperature are significantly and permanently longer than those of siblings housed in the cold. These highly reproducible lab results closely parallel the ecogeographical tenet described by Allen’s extremity size rule, which states that appendage length correlates with temperature and latitude. It is unclear what mechanisms underlie these differences and in what pattern they emerge, since the morphology is traditionally thought to reflect naturally selected genomic adaptations for thermoregulation. This study tests the a posteriori hypothesis that adult extremity length is subject to substantial modification by temperature during a brief but critical period of early postnatal development{this may explain why being born in the summer increases height}. Weanling mice were divided into three groups and housed at 7°C, 21°C, or 27°C for eight weeks. Tail lengths and body mass were measured weekly. Mass did not differ at any age. Analysis of tail elongation curves revealed two distinct phases: an initial period of rapid temperature-sensitive growth in which elongation rate was directly impacted by temperature; and a second phase of continued growth in which rates were identical among groups. Comparable growth reactions occur in response to other environmental variables such as exercise, suggesting that the skeleton is most responsive to external stimuli during a window of heightened sensitivity when growth occurs most rapidly.”

Couldn’t get the full study yet.