Saturday, February 23, 2013

Why Larger Species Live Longer

Adrian Bejan, the J.A. Jones Professor of mechanical engineering at Duke and father of the Constructal Law, is the author of a paper (1) in which he tries to explain the longer lifespan of larger creatures. What Constructal Law says is that anything that flows, be it blood, a river, a jet stream, a highway system, will evolve toward a similar mathematical configuration to maximize efficiency.

It's all about Math.
He then proposes to look at an animal simply as a flowing mass in motion. Mathematically calculating the size and age of both animate and inanimate objects, he found the same correlation: larger rivers are older, larger jet streams last longer, larger animals live longer. They all also cover larger territories of travel.

Breaths of Life.
Even more interesting, however, is the finding that if body size and lifespan of animals are plotted on a curve, they fall on a slope of about ¼. And then, if you plot the frequency of breathing to body size, it forms a slope of - ¼. Combining the two lines of inquiry he found that an animal has a constant number of breaths per lifetime. (2) The larger animals breathe more slowly in order to cover a longer lifespan.



Beating to Death.
Which brings me to a joke which may turn out to be true in the end. When I asked a colleague why he doesn’t exercise, he said that he believed we all have a set number of heartbeats per lifetime and that he didn’t want to use them up too quickly.

If you simply substitute breaths for heartbeats, does it mean that if I exercise and increase my breathing it will decrease my lifespan?
 

Which brings me to a quote (3) that may govern my lifestyle in the future: “Whenever I feel the urge to exercise I lie down until the feeling goes away.”

1) Bejan, Adrian, SCIENTIFIC REPORTS | 2 : 594 | DOI: 10.1038/srep00594
2) Duke University (2012, August 24). Bigger creatures live longer, travel farther for a reason. ScienceDaily. Retrieved February 23, 2013.
3) The quote is variously attributed to Paul Terry, the founder of the Terrytoons animation studio, Mark Twain, the film and stage actress Edna Mae Olive, humorist J. P. McEvoy, University President Robert Maynard Hutchins, politician Chauncey Depew, comedian Jimmy Durante, and others.