Allen's rule

Allen's rule is a biological rule posited by Joel Asaph Allen in 1877. It states that endotherms from 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 only 96 units.

In cold climates, the greater the exposed surface area, 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 helps to conserve heat.

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.

In a nutshell, it simply means there is a ratio between body surface to body mass. For example, Fur or Tutsi people of Africa release body heat more readily because their ratio is high. However, Eskimos and Inuit retain body heat and therefore have a lower ratio.

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

The Inuit people 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 Maasai people 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
 * Human skin color
 * Behavior (e.g., gathering insulation when it is cold, moving into shade when it is hot)