Hot weather for longer legs

There's an old rule in biology known as Allen's Rule, which states that warm-blooded animals from cold climates are likely to have shorter legs, and other appendages such as ears or tails, than the equivalent animals from hotter climes.  A good example is the difference between Inuit people from the Arctic, who tend to be short an squat, compared with Maasai warriors from Kenya that are usually much taller.

It's been thought that this is hard-coded into animals' DNA. But now researchers at Pennsylvania State University have shown that temperature can directly affect cartilage growth, providing a biological explanation for this rule.

It was a simple experiment. They raised mice either at 7 degrees, 21 degrees or 27 degrees centigrade and then measured their tails and ears. They found that they were significantly shorter in the mice raised in the cold, compared with the mice raised at warmer temperatures, even though their overall bodyweights were the same.

They found that the mice raised in the cold has less blood flow in their extremities.  And 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.

It's true to say that these animals are warm-blooded, so they may just be maintaining a constant body temperature.  But as anyone will know, especially given the chilly temperatures we've had recently, the temperature of your extremities can be a fair few degrees lower than your core temperature, which is kept at 37 degrees.  In the experiments testing bone samples grown in the lab, the researchers grew them at 32, 37 or 39 degrees, which would reflect the temperature of a mouse's tail, or toes when kept at cold, medium or hot temperatures.

All this means that while Allen's rule still holds, the research tells us that the length of limbs, at least in response to temperature, isn't necessarily hard-coded in the genes.  There's still a lot more research to be done - for example, to find out if the changes in limb and tail length can be inherited, and to discover the genes that are affected by changes in temperature.