Species of cicadas in the Eastern and Southern United States have life cycles of seventeen and thirteen years, respectively. Being relatively new at the science of group think and collective behavior, we have no idea how vast broods of these insects synchronize their schedules and emerge simultaneously. They must have extremely accurate biological clocks to keep perfect time for 13 or 17 years!
And now, the point of it all: Because they emerge so infrequently, no predators evolve specifically to live off of eating cicadas. And because they emerge en masse, predators cannot possibly devour the lot of them when they do. But what rhyme or reason is there to cycles of 13 or 17 years? What evolutionary advantage do those particular cycle lengths give?
Well, you may have noticed that both numbers are prime: you can't divide them evenly by any smaller numbers (except one). Prime numbers have enormous significance in mathematics, but is it merely a coincidence that cicadas have evolved indivisible life cycles? The entomologist Stephen Jay Gould thinks not. Prime cycles, he says, have a major evolutionary advantage over cycles that are multiples of smaller numbers of years, for the simple reason that they're elusive. Here is his crystal-clear explanation:
"Many potential predators have 2-5 year life cycles. Such cycles are not set by the availability of cicadas (for they peak too often in years of nonemergence), but cicadas might be eagerly harvested when the cycles coincide. Consider a predator with a life-cycle of five years: if cicadas emerged every 15 years, each bloom would be hit by the predator. By cycling at a large prime number, cicadas minimize the number of coincidences (every 5×17, or 85 years, in this case). Thirteen- and 17-year cycles cannot be tracked by any smaller number."
So they spend their lives ticking away underground, waiting to emerge in mathematically elusive intervals so as not to be tracked by predators. All that for a few short weeks of fresh air on their wings, and a genetic desire to carry on.