Posted by Elise Lockton on April 18, 2006

Q: I have never clearly understood the concept of how small mammals have a greater surface area to volume ratio compared with larger mammals. It seems like it would be the opposite? How do large mammals (moos, caribou) endure the exposure of the AK winter?

State: Alaska

A: I'm glad to see that you're thinking about this. It's not an easy concept to grasp fully, even when we have a physics explanation. But I want to try another way that may help explain this concept - at least it does for me.

Let's compare a mouse and an elephant. I like to work with extreme examples when trying to understand a new concept so I've chosen two very different-sized animals. Try to imagine this as I explain and see if it helps. First, picture a mouse sitting next to an elephant. Now picture in your mind the imaginary creation of a "new" elephant of exactly the same size, made up of lots and lots of those mice. It's going to take an awful lot of those mice but if we pack them all together in just the right way, we'll have a solid elephant. Some of those mice will be on the outside but many more will be on the inside of our imaginary elephant. Those many, many mice on the inside won't lose body heat directly to the air; only the relatively fewer mice on the outside will. So, although the new area on the outside is much much larger than on that original mouse, the total area that the imaginary elephant will lose heat from is less, by comparison. Larger animals certainly lose heat but not as quickly as the smaller animals.

One other way to look at this is to reverse this image. If I have a large bowl of peas that are too hot to eat, I can speed up the cooling by spreading out all of the peas on a large plate. This dramatically increases the surface area (ratio of surface to internal volume) and the peas cool much more quickly. So, larger animals in the north are much more capable of maintaining their own body temperature than are smaller animals. Of course, there are mice in the north but they don't live out in the open during winter; they're in burrows and under the snow where it isn't so cold.

I hope this helps and doesn't confuse you even more.

John Wiessinger ( April 20, 2006)