What Were You Doing in the Summer of 1987?
If you lived in Tennessee, you were fending off swarms of cicadas. This year they finally return. There are 3 distinct species of cicada that all emerge together in one area, for only a few weeks, once every 17 years (another cicada brood has a 13-year cycle, mostly in the deeper South; we in Tennessee are fortunate to experience both, though thankfully their timetables coincide only every 191 years...next up, 2106). Reading about their impending presence reminded me of an article I read by Stephen Jay Gould, explaining the evolutionary sense of something as seemingly bizarre as a 17-year cycle. As he asks it, "What does an individual cicada gain by indulging in sex so rarely and at the same time as its compatriots?"

"Humans are slowly growing animals. We invest a great deal of energy in raising a very few, late maturing offspring. Our populations are not controlled by the wholesale death of nearly all juvenile members. Yet many organisms follow a different strategy in the 'struggle for existence:' they produce vast numbers of seeds or eggs, hoping (so to speak) that a few will survive the rigors of early life. These organisms are often controlled by their predators, and their evolutionary defense must be a strategy that minimizes the chance of being eaten. . . . they are eminently and conspicuously available, but so rarely and in such great numbers that predators cannot possibly consume the entire bounty."

If cicadas emerged every year, the fattened summer eaters of 2003 would be pointing their offspring to the well of 2004, and the poor cicada, with few other defenses apart from its patience and synchronization, would stand no chance. So, with a longer cycle, predators might enjoy them, but none can depend on them for their own species' existence. Cicadas maximize this protection by not picking just any large number:

"Why do we have 13 and 17 year cicadas, but no cycles of 12, 14, 15, 16 or 18? 13 and 17 share a common property. They are large enough to exceed the life cycle of any predator, but they are also prime numbers. Many potential predators have 2-5-year life cycles. Such cycles are not set by the availability of periodical cicadas (for they peak too often in years of nonemergence), but cicadas might be eagerly harvested when the cycles coincide. . . . 13 and 17-year cycles cannot be tracked by any smaller number.

I'll try to remember how cool that is when they're driving me nuts. They're just trying to flood an unexpected market. It's the only way they can survive.

FYI, there is a new science-related blog getting some deserved attention, named after another Gould article: The Panda's Thumb