Allen's rule and the physics of evolution

A century-and-a-half-year-old theory holds that transplanting a species into a cold climate does more than make them grow a fur coat. It might change the dimensions of their bodies. We still think that theory might be right -— but for the wrong reasons.

Joel Allen was a zoologist at what must have been an intoxicating time for the profession. On the Origin of Species was published when he was 21. When he was 27 he went to Brazil, searching for past ice ages and the fossilized relics they left behind. When he was 33, he got a job at the Museum of Comparative Zoology. These three intellectual influence, evolution, ice ages, and comparative zoology, are apparent in the scientific rule that still bears his name. Allen's rule states that animals that live in extremely cold temperatures will not just gain fat and thick fur, or even an adapted circulatory system, but will rearrange their bodies, shortening all their limbs in response to the cold.

Allen's rule and the physics of evolution

Allen's rule was framed as an evolutionary adaptation. And although it was a biological phenomenon, he believed it was in response to physics. A squat, wide mug of coffee stays warm enough to grab and absentmindedly swig from for a good hour. Pour the coffee into a long, thin champagne flute and it will cool quickly. Long limbs leave the maximum amount of surface area exposed to cold temperatures. A stockier build keeps all that lovely warm circulating near the center of the body, with a minimum surface area exposed to the cold. There's plenty of evidence for this. Polar bears are most well known for their stockiness and short ears, leaving the least possible amount of core temperature exposed. Sea birds that live in cold climates tend to have shorter legs. When it comes to people, Allen's rule has had a couple of challenges — especially a lot of people who spent the last few thousand years in the hot climates of North and South America, and didn't grow proportionally longer arms and legs.

Still, researchers have found a lot of evidence to back Allen's rule, but not necessarily for the reason that Allen set down. A study on mice raised them at three separate temperatures. The extremely hot mice were longer in the tail and the legs than the mice that had spent their lives shivering, even if they had the same overall body mass. But this was not a generational adaptation. It only took one lifetime for mice to be stubbier about the limbs. How did it happen so quickly? When warm-blooded animals are exposed to cold temperatures the blood vessels constrict. This is why we see so many pale faces on winter mornings. The body pulls blood away from areas where it can cool too much and cause hypothermia. As the blood and nutrient supply to the area is cut back, bones and cartilage don't develop as readily, and the overall area doesn't grow as well.

What does this say about Allen's rule? Perhaps this is all part of adaptation. An immediate response to cold might be one pathway to an enduring response to cold. It's also possible that the mouse experiments be unrelated to a generational, hereditary Allen effect? Or is it all just the lack of growth of a nutrient-starved body? If it's the former, animals in the extreme north and south might be even worse off due to climate change than we expect. If short limbs are a cold adaptation, and the cold is receding, they're not just facing loss of habitat, but invading hordes of incoming animals better evolved to warm temperatures.

Top Image: Alan D Wilson

Second Image: Andreas Trepte
Via Journal of Evolutionary Biology, An Interspecies Test of Allen's Rule, The Naked Scientists, NCBI.