Friday, February 25, 2005

Metabolic Energy Clock

Live fast and die young. Although there are exceptions, most big animals live a long time, while small ones live fast and die young. Metabolic rates, the speed at which an organism gets energy from food, seem to be proportional to body mass raised to the 3/4 power.
"It appears as if we've been gifted with just so much life, you can spend it all at once or slowly dribble it out over a long time."
-Brian Enquist, ecologist, University of Arizona, Tucson
An article published last summer in Ecology describes a "Metabolic Theory of Ecology" which argues that metabolic rates depend on the efficiency with which resources can be transferred into cells. That efficiency, the authors contend, is related to the "animal's effective surface area." Specifically they're taking about the area of all surfaces (in the digestive and circulatory systems) that nutrients have to pass through before they reach the cells. This equation (originally put forth in 1883, corrected in 1932, justified by the same authors in 1997) holds true for organisms ranging in size from microbes to whales.

The new breakthrough here is the inclusion of temperature as a second variable in their metabolic equations. Up until this point, the equations really only worked well for mammals and birds. Since most chemical reactions slow down when the temperature is decreased, animals that have cooler body temperatures should have slower metabolisms, which is exactly what the authors find.

The figure shown is a plot of the metabolic equation (including body temperature) versus mass. There a few important things to notice here: One, it's a great line that includes plants, mammals, and bats - organisms that have little in common and probably shouldn't fall on the same line (but do!). Two, the axes are both ln, or natural log. Be skeptical of log-log plots; engineers joke that any data set plotted on log-log paper will give a line.

Although it's easy to be skeptical of this theory because it attempts to explain so much (see the full paper or the link below for the rest of their claims), such amazingly large patterns can only be seen if you step back far enough and look.

Via: ScienceNews
Also covered at NuSapiens and Roland Piquepaille's Technology Trends.

1 Comments:

Anonymous Bill said...

That's interesting.

12:54 PM  

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