The most basic function of a living organism is to take in energy and spend it on survival and reproduction. If humans are unique, then so must our energy use be. What we know today is that our energy use has both deep similarities to, and differences from, that of other animals.



Part of our energy budget is dedicated to biological metabolism, which is the set of processes through which our bodies produce and spend energy derived from the food we eat. This we share in common with all other animals. Unlike all other mammals, however, humans also use extra-biological energy, and a lot of it. The rate of biological energy use (or the metabolic rate) of a human is about 100 watts, equivalent to 2,000 kilocalories per day. But in the most developed countries, our average per capita energy use is on the order of 10,000 watts, or 100 times greater than what we need biologically. The total energy usage of an average person in the U.S., Canada, the Eurozone or Japan is equivalent to the biological metabolic rate of a hypothetical 30-ton primate. About 80 percent of this extra-biological energy consumption comes from burning fossil fuels.

Tim Hetherington/Magnum Photos, Figure from doi: 10.1111/j.1469- 185X.2011.00192.

In addition, unlike all other mammals, different communities of humans have vastly different energy consumption patterns. The New Mexico Human Macroecology Group has documented how energy use scales with GDP, a standard measure of economic activity. Across countries today, as per capita GDP increases by about 700 times, per capita energy use increases by about 200 times. This means that in the poorest developing nations of sub-Saharan Africa, the average per capita energy use is barely more than the 100 watts required for basic biological metabolism. Socially derived variations in energy consumption among other species do exist: for example, the alpha male in a wolf pack or a band of gorillas will often have a better diet and spend more energy. But these variations are rarely greater than a factor of two.

On the other hand, there are ways in which our energy usage is completely in character with the rest of the animal kingdom. First, across animals, biological metabolic rates scale with body size, and human rates are almost exactly where one would expect them to be, given our body size. Second, and more surprising, is the relationship between our energy usage and birth rate. In general, reproductive rates in mammals decrease with increasing body size and metabolic rate. Smaller mammals have shorter lifespans, and consequently must produce babies more frequently in order to maintain their numbers. Figure 1 shows that this trend produces a straight-line plot (when logarithmic scales are used).

Reprinted with the permission of John Wiley and Sons.

The surprising bit is that birth rates across countries vary in almost exactly the same fashion, and with about the same slope. The higher the level of economic development and total energy usage, the fewer babies are produced per year (this is also shown in Figure 1). Our non-biological energy use appears to affect our birth rate the same way that biological energy use affects the birth rate of other primates.

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Subsistence hunter-gatherer societies have reproductive rates similar to what we’d predict for a primate of human size, and similar to that of chimpanzees and gorillas. Reproductive rates fall from there, as energy use increases. The fecundity of a woman in the first world is similar to what one would predict for a 30-ton primate: about one birth every 15 years on average.

Sociologists and demographers have long known that increasing economic development tends to be accompanied by decreasing birth rates. They call this the demographic transition. Many factors are undoubtedly involved. Our Human Macroecology Group believes that parents in developed countries rear a smaller number of children so that each is able to receive a greater resource investment, making them more likely to succeed in a high-tech, competitive economy.

Perhaps the most important similarity between our energy use patterns and those of other animals is that energy scarcity threatens our survival. Current projections suggest a global population of 9-10 billion people by 2050. Since the amount of arable land is severely limited, the requirements for increased food production will almost certainly require the increased use of machinery, energy, water, and fertilizers. Some of these resources are already scarce.

A basis for optimism is the demographic transition. The birth rate, and hence the rate of population growth, should decrease as poor countries develop. But the challenge is enormous. Our group estimates that if optimistic scenarios for population and economic growth are accurate, we will require about a fivefold increase in global energy use by 2050 (Brown et al. 2011). This will be impossible without major technological, behavioral, political, and economic changes, because the vast majority of the current energy supply comes from fossil fuels which are rapidly being depleted. We set ourselves apart from other species with our unique energy use patterns, but like them, our future depends on energy.





James H. Brown, Ph.D., is Distinguished Professor Emeritus at the University of New Mexico. He is a founding member of the informal New Mexico Human Macroecology Group. Currently active members are J.R. Burger, W. Burnside, M. Chang, A. Davidson, T. Fristoe, M. Hamilton, S. Hammond, A. Kodric-Brown, N. Mercado-Silva, J. Nekola, and J. Okie.





Additional Reading:

Brown, J.H., et al. Energetic limits to economic growth. BioScience 61, 19 (2011).

Burnside, W.R., et al. Human macroecology: linking pattern and process in big picture human ecology. Biol. Rev. 87, 194. (2011).

Ehrlich, P.R., Ehrlich, A.H. Can a collapse of global civilization be avoided? Proceedings of the Royal Society B 280, 1754. (2013).

Hall, C.A.S., Klitgaard, K.A. Energy and the Wealth of Nations: Understanding the Biophysical Economy. Springer Publishing, New York, N.Y. 2011.

Moses, M. E., & Brown, J. H. Allometry of human fertility and energy use. Ecology Letters, 6, 295 (2003).

People and the Planet. The Royal Society Science Policy Centre report 01/12. DES2470. (2012).