The 17 year cycle is to avoid predators and the mating cycle of other circadas.
Predators might time their life cycles to match the cicados. 17 years, being a prime number, means that predators can’t match or even partially match the circada’s cycle with a 2 year, 3 year, 4 year, 5 year etc. cycle of their own.
13 years sounds like a better way to go to avoid predators by the same principle. But another circada species already had that 13 year cycle tied up. A 17 year cycle avoids predators and the other circadas which might make it difficult to find food, maybe find your own circada species for mating.
Definitely an extreme version of timing of a mating cycle to avoid the maximum number of predators and maximize resources available and avoid competition with similar species.
I wonder how bad it is when the 13 year and the 17 year broods come out together, it will be every few hundred years but I imagine THAT would be a racket....
Boy,it makes so much sense if you think about them having a council meeting and discussing the finer points of prime factors and such before “programming” their mating seasons.
“But I don’t WANT to wait 17 years for sex!”
“Consider yourself lucky we didn’t choose 29.”