Cicadas. They hide underground doing pretty much nothing for a number of years. Not any number of years but number of years that are prime. 3,5, 13, 17.. Aha. Smart. They use prime years to beat predators (that's the theory and I'll go with it). Any other occurence of primes in the natural world that you know about?
I've been smitten by prime numbers lately. If you would like to smite yourself, I recommend Melvyn Bragg's excellent BBC series 'In Our Time' on Primes.
Why would appearing in only prime years "beat the prdators"?
Maybe the composite-number-cycle cicadas went extinct eons ago, and any that emerge anew don't last long.
>Why would appearing in only prime years "beat the prdators"?
It is supposed that the cicadas decrease the chances of coming out in the same year as their predators by using prime years. For instance, as explained in the BBC program, if cicadas appeared every 7 years and the predator appeared every 3, they would appear together only every 21 years, which is better than other numbers (cicadas coming out every 4th year and predators 3rd, for instance, would make them appear together every 12 years).
Thank you Selva!
But why wouldn't the predators evolve to emerge in "prime" years as well?
>But why wouldn't the predators evolve to emerge in "prime" years as well?
Because the math still holds. The chances of the predator evolving the same dormant year as cicadas and then coming out in the same year probably works out to be more improbable than the coinciding years that naturally occur in the cicada-predator cycle.