30 March 2010

Beaver's tale

In their 2003 book The Beaver, Müller-Schwarze & Sun call the beaver's (Castor canadensis) tail a "multipurpose tool" and list its functions:

1. To support the beaver's body when it's cutting a tree.
2. To help the beaver maneuver in the water.
3. To slap on the water to signal danger.
4. To store fat.
5. To regulate heat loss to the surroundings.

If we were to rate a single-function organ, like the eye, on a scale from 1 to 5, we could perhaps give it a 4.5, if not a 5. Our eyes are pretty damn good at doing what is required of them. Granted, they may develop various illnesses, become near- or far-sighted and even blind, but when our eyes are healthy, and most of the time they are, we can see quite well with them. In any case, evolution couldn't have come up with an organ impervious to failure.

But, if we were to rate the individual functions of a multi-function organ like the beaver's tail, would we be able to give each function a rating of 4 or higher? In other words, could a multi-function organ be as good at each of its functions as a single-function organ, like the eye, is at its one function?

The problem is that when an organ has evolved to perform more than one function, the morphological and physiological requirements of all of those functions may interfere with each other. Such conflicts may then result in what are known as evolutionary trade-offs: the organ evolves to become less good at one particular function so that it can also become mediocre at one or more additional functions. The resulting multi-function organ evolves to be good at performing multiple functions but it may not be as good at performing any one of those functions as it would be if it had evolved to perform only that function.

One way evolution can overcome this problem is to have a given function performed by multiple organs. The net result is that the overall performance of a given function then obtains a high rating, say a 5. This mechanism also provides insurance: if one of the organs is disabled, its functions can still be performed, albeit now slightly less efficiently, by other organs.

So, going back to the beaver's tail, how good is it in performing each of its functions and does the beaver have other organs that function at least some of the tail's functions?


Joel VanDerMeulen said...

Unless of course, you look at the eyes of a Mantis Shrimp. In comparison our eyes are probably a 0.5~0.7.

And yes, I believe a beaver can do all those other things without a tail. Maybe not one so efficiently, and then again, I'm not really sure if beavers have some sort of vocal warning call or not.


Oh, but I am not comparing the eyes of different species. That would be meaningless in this context. Of course, the eyes of a shrimp or a predatory bird are much better for the things they evolved to do; but human eyes have not evolved to work under water or to spot tiny prey from hundreds of meters away.