depending on the species, every 7, 13, or 17 years these periodical cicadas simultaneously emerge en masse, transform into noisy, flying creatures, find a mate, and die not long after.
While this is a rather rock’n’roll ending for our nerdy cicada, it raises an obvious question: Is it just by chance that they adopted 7, 11, or 13-year life cycles, or are those numbers somehow special?
As it turns out, these numbers have something in common. They’re all prime numbers—numbers that can only be divided by themselves and 1 (that is,1, 2, 3, 5, 7, 11, 13, 17, 19, 23, and so on).
But why does that matter?
Research has shown that the population of creatures that eat cicadas — typically birds, spiders, wasps, fish and snakes — often have shorter 2 – 6 year cycles of boom and bust.
So, if our cicadas were to emerge, say, every 12 years, any predator that works in either 2, 3, 4 or 6 year cycles would be able to synchronize their boom years with this regular cicada feast. In fact, they’d probably name a public holiday after it called Cicada Day.
That’s not much fun if you’re a cicada.
On the other hand, if a brood of 17-year cicadas was unlucky enough to emerge during a bumper 3-year wasp season, it will be 51 years before that event occurs again. In the intervening years, our cicadas can happily emerge in their tens of thousands, completely overwhelm the local predator population, and be mostly left in peace.