Some spend years underground and then hatch all at once, in their millions. The common cycle lengths are thirteen and seventeen years, and both of those are . To test whether that is a coincidence, models compared prime cycles against cycles that are not prime. What did the prime number buy?
Rarity of overlap. A prime cycle lines up with the life cycle of a predator or a competing species only very occasionally. A predator on a two or three year cycle meets a seventeen-year cicada once in a great many years, and cannot build a population around it. A sixteen-year cycle, by contrast, would run into predators on two, four and eight years again and again.
The fact itself is strange before any explanation. An insect lies in the soil for thirteen or seventeen years, feeding on roots, and then every individual in the area comes up on almost the same night, in millions. The synchrony needs explaining as much as the cycle length does.
The synchrony is the more tractable half, and it is : if everybody emerges together, the local predators are full long before the cicadas run out, and most of the brood survives. A strategy like that collapses if the brood spreads itself over several years.
Hence the question of why thirteen and seventeen in particular. The accepted answer is that a prime number does not line up easily with any other cycle. A predator on a three-year cycle meets a seventeen-year cicada once in fifty-one years, and cannot build its population around it. A sixteen-year cycle, by contrast, would meet predators on two, four and eight over and over.
A second advantage comes with it: two species on different prime cycles almost never overlap, so they rarely interbreed and the synchrony holds. And fairness requires saying this is still an explanation under study. The support it, and no experiment can replay millions of years of evolution to confirm it.
wat. A predator on a three-year cycle meets a seventeen-year cicada once every fifty-one years.
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It is a rare case of an abstract mathematical property showing up directly and plainly in the behavior of an animal.
How do we know?
Field records of the cycle lengths go back more than a century. The prime explanation has been tested in mathematical models that set prime cycles against non-prime ones.
What is still unknown
The explanation is still under study. The models support it, but no experiment can replay millions of years of evolution to confirm it.
Learn something?
Where to next
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