Why are Inuits short and fat, but Africans are often tall and thin?
It's down to a central tenet of biology known as the "Allen Effect", which says that animals in warm environments grow longer legs and appendages (such as ears and tails) than animals adapted to colder climes. The same is generally true for humans and our Neanderthal relatives. Arctic inhabitants, for instance, tend to be shorter than individuals living closer to the equator. However, the biochemical cause of this relationship has never been determined.
The prevailing theory was that the effect was genetic, and that adaptation to the cold leads to reduced blood flow in peripheral tissues (in order to conserve heat), and this in turn restricts the supply of essential nutrients and therefore limits growth. But now, writing in this week's edition of the journal PNAS, Kent State University scientist Owen Lovejoy and his colleagues have shown that something far more fundamentally thermostatic is going on.
First the team reared identical groups of mice in warm (27C) or cold (7C) conditions; sure enough the cold-exposed animals were shorter and stockier than their warmer relatives. Next, to find out whether it was down to blood supply, the researchers grew individual bones in nutrient solutions kept at cold, medium and warm temperatures.
Under these conditions, and without any blood flowing, any difference in bone length must be due to the response of the bone tissue itself to temperature. Remarkably the bones incubated at higher temperatures grew significantly longer than their cooler counterparts, and the warmer bones also showed higher levels of cell division.
This argues that Allen's "extremity size rule" is not a genetic adaptation to cool-living at all, but instead a temperature-driven effect. However, it's not as simple as just turning up temperature to provoke longer legs, the complex web of genes involved and how they too interact with the environment needs to be disentangled. But the bottom line is that if you want long legs move somewhere warm, possibly to a sun-bed!