If they aren’t using prime numbers to avoid predators, then how did they get to 13 & 17? What was wrong about 12, 14, 15 & 16?
What I’ve read is that that minimizes how often the 17 year and 13 year cicadas come out at the same time. Only once every 13*17 years do they come out at the same time. If they were only 12 and 16 year cicadas, they could overlap every four years.
That would only mean the two types of broods would only need to be relatively prime with each other, but if mutants appear with other number of years, each brood would itself need to be prime to minimize running into the mutants often.
(“Broods” probably isn’t the right term, since that usually refers to groups with the same period, coming out in different years.)
Ah okay. Thanks for the explanation!