The human race may be caught in a biological catch-22, in which sustainable reproduction rates can only be achieved by consuming more energy.

So hypothesizes Melanie Moses, a University of New Mexico computer scientist who wonders if human societies are bound by size-dependent rules of network efficiency seen elsewhere in the biological world.

If the implications of this seem bleak, take heart: people are born to break the rules.

Moses invokes the Metabolic Theory of Energy, which explains the relationship between mammalian size, lifespans and reproduction rates — the bigger a body, the longer it lives, with fewer offspring — as a function of cardiovascular networks. As the sum length of capillaries and arteries increases, nutrient flow efficiency drops. The less efficient an animal's networks, the more difficult it becomes to acquire the energy needed for raising a child.

Compare the size-lifespan-reproduction curve to the relationship between human economic growth and reproduction rates, and the parallels are eerie.

"Across contemporary nations, the decline in human birth rates with increased energy consumption is quantitatively identical to the decline in fertility rate with increased metabolism in other mammals," writes Moses in an essay published Wednesday in Nature. "Put another way, North Americans consume energy at a rate sufficient to sustain a 30,000-kilogram primate, and have offspring at the very slow rate predicted for a beast of this size."

Perhaps humans obey our own, society-level version of the Metabolic Theory. As social and infrastructure networks grow, they too become less efficient. It gets harder for the average parent to scrape together the energy — the time, the money, the resources — necessary to raise a child.

The implication is that birth rates, consistently shown to drop in wealthy nations, actually fall because life in a wealthy society is so energy-intensive. As a result, writes Moses, energy production needs to be as green and efficient as possible: if keeping the human population at planet-friendly levels requires vast amounts of energy, it might as well be clean.

The connection, however, is not absolute. There are counter-examples: when birth rates fell in post-Soviet Union Russia, so did energy consumption.

Gro Andam, an Arizona State University systems biologist, called Moses' analysis simplistic. An alternative explanation, she said, is our "capacity to take great, personal pleasure in investing in ourselves." The richer people get, the more likely they are to indulge themselves rather than another kid.

Moses acknowledged that the link between social and biological energy patterns is still hypothetical. "It's at the level of a correlation," she said. "That doesn't mean causation, but it's merit for further study."

But unlike animals, said Moses, humans are socially self-aware. With awareness comes the power to change.

"It's obviously a relationship we don't want to maintain. In order to break that relationship, we need to understand its causes," she said. "Just because this has existed in the past, doesn't mean it needs to exist in the future."

Citation: "Engineering: Worldwide ebb." By Melanie Moses. Nature, Vol. 457 No. 7229, Feb. 4, 2009.

*Image: Flickr/Rob Inh00d *

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