The 17-Year Itch: Why Cicadas Love Prime Numbers

The invisible hand at work -- in bug shells and Gap storefronts

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The mystery of cicada reproduction is two-fold. Such strange timing and such large portions.

What would the bugs gain by having sex in large prime-number intervals -- 13 or 17 years -- and all at the same time, Stephen Jay Gould asks in his famous essay "Of Bamboos, Cicadas, and the Economy of Adam Smith." He had a theory. It's called predator satiation.

I'm going to tell you about predator satiation in just a second. But first, I have to tell you about Gap's Memorial Day Sale.

On my street in Manhattan this weekend, the storefronts brimmed with limited-time-only sale signs. I happened to have plenty of time to shop and no shorts for summer. It's as if Retail America, in particular the Gap around the corner, almost knew I'd be expecting a sale on a three-day weekend before the summer. Actually, not almost knew. They totally knew.

John Wanamaker is credited with the invention of the department store and the price tag, but without the third leg of retail -- the bargain blowout -- he had a perennial problem. After Christmas at his 19th century stores, sales plummeted and inventory languished on shelves. He needed a solution. So he invented the holiday sale to clear the shelves. Holiday sales are great cues for both shoppers and suppliers. If shoppers expect discounts, and suppliers correctly expect that shoppers expect discounts, then stores can stock up before holidays -- Thanksgiving, Memorial Day Day, and so on -- to make the best use of their merchandise and cut down on wasted inventory.

Cicadas play the expectations game to perfection -- but in reverse. They aren't trying to give their inventory (seeds) away. They're trying to protect their young from predators that want to consume them. So over thousands of years, they've evolved to reproduce in massive quantities in prime years to separate themselves from the life-cycles of animals that are partial to the taste of cicada larvae. It is nature's version of the anti-sale: Stock up when the consumers least expect it. Here's Gould:

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The invisible hand (wing? whatever.) was always at work in Darwin's theory. "The ideal economy, [Adam] Smith argued, might appear orderly and well balanced, but it would emerge 'naturally' from the interplay of individuals who follow no path beyond the pursuit of their own best interests," Gould writes. Evolution being a higher-stakes game of supply, demand, and consumption, Darwin "grafted" Smith onto nature in his theory of natural selection from the beginning. In musing on the peculiarity of cicada sex, Gould does a beautiful job of teasing out the influence.