primed to survive

Cicadas, those tubby brown bugs that spend most of their lives underground, emerge from dormancy every 7, 13, or 17 years-all prime numbers. Physicists Eric Goles of the University of Chile and Mario Markus of the Max Planck Institute in Germany sensed an evolutionary logic behind this pattern. To prove it, the researchers designed a computer model of how cicadas interact with their predators. In the simulation, predator and prey follow randomly assigned life-cycle durations. If the cicadas appear when many predators are waiting, their population drops; if the cicadas arise when few predators are around, they flourish. Eventually the cycles settled into the most beneficial pattern, and sure enough, the periods were prime numbers. Because of their metabolic demands, predators cannot remain dormant for many years. It's better for the predator's life cycle to be a small, even number because then they most often have the same peak year as their prey,* says a physicist on the German team. That explains why there are no even-cycle cicadas. For the cicada, 12 years is bad because predators on 2-, 3-, 6-, and 12-year cycles would eat them,* he says. But 13 (as with 7 and 17) offers safety in numbers because it is divisible only by itself and 1.