A unified theory of animal locomotion
You probably already know that there is a master equation for all life processes based on metabolism. Now, physicists from Duke University have applied the so-called 'constructal theory' to explain how running, flying and swimming modes of locomotion are similar even if they're apparently unrelated. And this single unifying physics theory explains how fast animals get from one place to another and how rapidly and forcefully they step, flap or paddle in relation to their mass." In other words, these scientists argue that the characteristics of animal shape and locomotion are predictable from physics. Read more for additional details, pictures and references about this theory.
Here are the opening paragraphs of the Duke University news release.
A single unifying physics theory can essentially describe how animals of every ilk, from flying insects to fish, get around, researchers at Duke University’s Pratt School of Engineering and Pennsylvania State University have found. The team reports that all animals bear the same stamp of physics in their design.
The researchers show that so-called "constructal theory" can explain basic characteristics of locomotion for every creature -- how fast they get from one place to another and how rapidly and forcefully they step, flap or paddle in relation to their mass. Constructal theory is a powerful analytical approach to describing movement, or flows, in nature.
This theory is the brainchild of Adrian Bejan,Professor of Mechanical Engineering at Duke's Pratt School of Engineering.
"The similarities among animals that are on the surface very different are no coincidence," said Bejan. "In fact, animal locomotion is no different than other flows, animate and inanimate: they all develop in space and in time such that they optimize the flow of material." In the case of animal locomotion, this means that animals move such that they travel the greatest distance while expending the least amount of energy, he said.
"From simple physics, based only on gravity, density and mass, you can explain within an order of magnitude many features of flying, swimming and running," added James Marden, professor of biology at Penn State. "It doesn’t matter whether the animal has eight legs, four legs, two, even if it swims with no legs."
Below is a diagram showing on how birds -- and planes -- are destroying energy for flying (Credit: Adrian Bejan).
[On the figure above,] if the word 'fuel' is replaced by 'food,' then the same drawing is valid for a bird, and reveals how the energy liberated by food is destroyed by all the currents that flow around and through the animal. The useful (liberated) energy is known as exergy in thermodynamics and as energy consumption in biology. The food or fuel exergy is destroyed completely by currents that overcome resistances.
The above figure and its caption have been extracted from a technical paper named "The constructal law of organization in nature: tree-shaped flows and body size", published in 2005 by the 'Journal of Experimental Biology' (Volume 208, Issue 9, Pages 1677-1686, May 1, 2005). Here are two links to the abstract and to the full paper (PDF format, 10 pages, 494 KB).
And for more information about this physics theory, please read this page (which has links to interesting images) or the Constructal Theory Web Portal.
Finally, here is the conclusion from James Marden, who thinks that this theory is not even limited to animals on Earth.
"Our finding that animal locomotion adheres to constructal theory tells us that -- even though you couldn't predict exactly what animals would look like if you started evolution over on earth, or it happened on another planet -- with a given gravity and density of their tissues, the same basic patterns of their design would evolve again," Marden said.
Sources: Duke University news release, December 30, 2005; and various web sites
You'll find related stories by following the links below.