Size by Vaclav Smil

Author:Vaclav Smil
Language: eng
Format: epub
Publisher: HarperCollins
Published: 2023-03-14T00:00:00+00:00

Terrestrial mammals exploit their ranges mostly by running, and analyses show a clear dichotomy between the running of small (<1 kilogram) and larger (>1 kilogram) animals. There is no detectable difference in body-mass-dependent scaling efficiency within each of these groups, but small animals are generally less efficient runners than the larger ones.56 Why the small ones are so inefficient is not entirely clear, but the efficiency of large ones clearly owes to the reuse of briefly stored elastic energy. When a leg (be it of a kangaroo, impala, or marathoner) hits the ground, most of its kinetic energy is not lost permanently but is stored fleetingly as elastic strain energy in tendons (they can stretch by as much as 10 percent without breaking) and returned with high (>90 percent) efficiency in their elastic recoil. Just think of those Serengeti antelopes you have seen bouncing high after every recoil as they flee from a pursuing lion or cheetah. This useful capability is obviously size-limited: you cannot imagine the tendons of an elephant-like but bipedal Brobdingnagian recoiling like those of a gemsbok or a white-tailed deer that jumps 6-foot (1.8-meter) fences with ease.

At this point some readers might think that I have already said too much about scaling and size. I called on fascinating unintended scaling lessons from a renowned 18th-century author of fiction; I introduced the work of many famous scientists; I explained the results of numerous human and animal studies; I looked at how sizes of different body organs, ranging from skin and heart to brains and eyes, have changed with evolution and how they relate to the changes of body mass; and I provided basic equations so that readers can make their own calculations. Perhaps the single most important conclusion conveyed by these inquiries is how all organisms operate within predictable mass-dependent limits.

But I am still not done with scaling, and next I will deal with what is arguably the most important allometric story: how does metabolism (the energy consumption required for survival, reproduction, growth, and activity) change with changing body size? Moreover, this metabolic scaling has applications beyond organisms: many machines are subject to it within equally narrow boundaries. Much as carbohydrates, lipids, and proteins energize organisms, fossil fuels and electricity energize combustion engines and electric motors. Does the “metabolism” of these inanimate energy converters scale similarly to the metabolism of mammals, or do those artifacts scale differently? And what about the relationship between energy use and the size of an economy? All of that is for the next chapter.

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