Allen's rule

Allen's rule is a biologicalrule posited by Joel Asaph Allenin 1877. It states that endothermsfrom colder climates usually have shorter limbs than the equivalent animals from warmer climates.

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 boxes of equal volume, 4 units to a side. If you create a stack two boxes wide, one long and four tall, you will have a volume of 64 units with a surface area of 112 units. If you create a stack of boxes in a cube shape two boxes wide, two long and two high, you will have the same volume of 64 units BUT a surface area of 96 units.

In cold climates, the more surface area you expose, the greater the loss of heat and therefore energy. Animals (including humans) in cold climates need to conserve as much energy as possible. A low surface area to volume ratio will be efficient in CONSERVING heat.

In hot climates, the opposite is true. An animal will overheat quickly if it has a high volume with little surface area. Therefore, a high surface area to volume will be efficient in LOSING heat.

The Inuit and the Masai

In anthropology the contrast between the Masai and the Inuit people is often presented as an example of Allen's rule.

The Inuitpeople live and hunt within the Arctic Circle. The sub-zero temperatures encountered in the arctic environment mean that heat conservation is essential. A short, squat, body shape will help retain body heat.

The Masaipeople live primarily in Kenya and North Tanzania. The equatorial temperatures in this environment mean that heat loss is highly beneficial. A tall, long, body shape will help get rid of body heat.

Clarifications

It is important to remember that exposure of exterior surface area is only one of many mechanisms by which humans retain or dispose of body heat. The following mechanisms are also very important:

• Sweating
• Shivering
• Skin colour
• Behaviour(e.g., gathering insulation when it is cold, moving into shade when it is hot)

See also

• Bergmann's_Rule

References

Joel Asaph Allen. "The influence of Physical conditions in the genesis of species." Radical Review, 1877, 1: 108-140.de:Allensche Regel he:??? ??? uk:??????? ?????

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It uses material from the http://en.wikipedia.org/wiki/Allen%27s+rule Wikipedia article Allen's rule.