Science Sunday: Bergmann & Allan's Rule (or how the environment influences body size)

By: Meradeth Houston on 7:00 AM

Welcome back to Science Sunday! (I seriously need to get a Bill Nye gif for this series...) Anyhow, how's it hanging with people? I do hope you are all having nice weekends! If you haven't had a chance to check out my current giveaway, may I suggest going here? And tomorrow I have some more awesome fun going on--a lovely cover reveal for THE CHEMISTRY OF FATE. I can't wait to show it off!!

Hmm, so I've been debating about what to talk about today. There's always lots of random stuff I'm reading about and talking about in lectures and since on Friday I was talking about Neanderthals being cold-adapted, well I thought that might be kind of fun :).

Okay, so, there's been a general trend in human body size/shape that's been noticed in most human populations. (It also works well in other species, too, but we'll stick to people for the moment.) People indigenous to areas around the equator (where it's usually much toastier than it is here right now--and yes, I'm totally jealous) are often taller and slimmer, whereas people closer to the poles tend to be more compact and have shorter limbs. It's been suggested that this distribution is because of adaptations to the environment.

What do I mean by that? Well, here's the thing: our bodies like to stay at a very specific temperature. 98.6 degrees on average. So, where the climate is hot, well people need to cool off better, and where the climate is cold, they want to conserve heat. Our bodies over many, many generations have been selected on to help either cool off or warm up. All of this has to do with surface area at its root.

Same volume, but very different surface area!

Think about it this way: if you have some super hot hot-chocolate (or hot liquid libation of choice) and it's in a giant mug where it's spread out really thin along the bottom, well, it'll cool off faster because more of it is exposed to the outside air. A more compact mug with less of your hot-chocolate exposed to the air will stay warmer longer. (See, this blog does have some practical applications! :) Same thing works for bodies. Longer limbs and a longer build have more surface area, so it's easier to lose body heat and keep cooler. Shorter limbs and a more compact frame and it is easier to conserve heat and stay toasty.

Bergmann made this observation when studying the giant mammals that roamed during the last ice age (think mammoths) and his part of the rule deals more with the trunk of the body--it either being more compact or not. Allen's rule deals with the appendages--the arms and legs. Populations from warmer areas often have longer legs and arms than those living in colder environments (with the exception of one particular male appendage, as demonstrated by a former student who decided to try and write a paper on this--that was all kinds of awkward).

Of course, there are exceptions to this, and there's a wide range of variability in every population. And there are some populations who seem to be exceptions to the rule overall (some of which have recently migrated to where they currently live and haven't been there long enough for selection to work its power). Can you think of any exceptions?