Evolution selects for Energy Conservation

Kim Kastens
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published Jul 30, 2009

On the national scene, the powers that be are once again squabbling over the question of should the United States meet its future energy needs by finding new sources of energy or by energy conservation. Against this backdrop has been growing in my mind the realization that evolution selects for energy conservation.

You may think this is obvious. But it isn't how I was taught in school, and it isn't how natural selection is being presented to the public today.

Three disparate tidbits of information merged in my mind to form this insight.

  1. Through my research on spatial thinking in geosciences, I became familiar with the work of Dennis Proffitt on how humans estimate the steepness of landscape slopes. Through an ingenious series of experiments, Proffitt has shown that humans tend to overestimate the steepness of landscape slopes, especially if they are tired or carrying a heavy burden. He makes the case that this was advantageous for our evolutionary ancestors because it enabled them to perceive subtle differences in steepness in the terrains they had to walk across and then traverse the route that would require the least expenditure of metabolic energy (Proffitt, 2006).
  2. Through teaching a case study on Peak Oil for aspiring science and environmental journalists, I came across an essay by David Price, called "Energy & Human Evolution." Price describes the history of life as the exploitation of ever "deeper reservoirs" of energy, culminating in human's exploitation of fossil fuels. Price got me thinking about the relationship between energy and evolution.
  3. Photos of shade and sun leaves from red oak
    sun (upper) and shade leaves from same tree
    Drinking wine at a fund-raising reception, an ecologist happened to mention that the leaves of oak and maple trees vary systematically in shape depending on where they are in the tree canopy. Leaves at the exterior of the tree have deeper indentations and narrower lobes. This means that they don't hog all the light energy arriving at their layer of the tree; they allow a fraction of the incident light to penetrate deeper into the tree's interior. The interior leaves have broader lobes and less well developed indentations, which enables them to capture a higher percentage of the diminished energy arriving at their level. The narrow-lobed leaves in the sunny canopy exterior, and the broad-lobed leaves in the shady interior each capture enough energy to support the life functions of their respective branches, with minimum wasted energy. (A student field lab based on this idea is here.)

Once I began to look at the world through this lens, I found evidence all around me:

  • Why are fish and marine mammals streamlined? To conserve energy while swimming.
  • Why do birds have hollow bones? To conserve energy while flying.
  • Why do mother birds sit on their eggs? To minimize heat loss from the eggs, and thus conserve energy.
  • Why do mammals have fur? To protect against heat loss from their warm-blooded bodies, and thus conserve energy.
  • Why do birds have feathers? Ditto.
  • Why are baby birds and many baby mammals so fuzzy? Fuzziness traps air, providing improved insulation, thus conserving energy: babies' tiny bodies, with a large surface area to volume ratio, tend to lose heat quickly.

In contrast, the version of natural selection that I learned in school could be characterized as the "Drill, baby, drill" approach: the fittest organisms, the organisms favored by natural selection, are those that are good at obtaining energy, that is to say food (and also good at avoiding predators, resisting disease, and reproducing). Over time different species became expert at exploiting different energy sources, with the archetypical example being Darwin's Galapagos finches.

The notion that evolution selects for energy conservation was not in evidence in my own education. I took a quick tour of popular websites about evolution, and found the same lack of attention to energy conservation as an important element of evolutionary fitness.

I think our national conversation about energy would be on a firmer footing it were broadly understood that energy conservation is an essential function of living organisms--rather than merely a "sign of personal virtue."


  • Proffitt, D.R. (2006). Embodied perception and the economy of action. Perspectives on Psychological Science, 1(2), 110-122.
  • Westmoreland, D. (1989). Leaf morphology & light measurement: A field exercise. The American Biology Teacher, 51(5), 303-306.

    (Revision Note: Photo of leaves added Sept 16, 2009. KK)

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