Tuesday, June 12, 2007

Cicada cycles and prime numbers

Certain cicadas come back every 17 years. That seems pretty random. Or is it?

With 2,500 known cicada species in the world, North America boasts the Magicicada, which has the longest life cycle of any insect. What’s amazing about them is that every 17 years they all emerge at once, not to be heard from again until another 17 years elapse.

Some scientists believe that 17 being a prime number is no accident: if you're a predator with a one-, two-, four-, or even an eight-year life cycle, you won’t often have the opportunity to feast on cicadas that show up only once every 17 years.

For example, if a parasite has a life cycle of 4 years, the two species will only meet every 68 years, the least common multiple of 4 and 17. The periodical cicada thus escapes normal population control.

But why such a wait? Wouldn’t lower prime numbers like 5 or 7 be enough?

The answer may be climate. Cicadas like it hot. When they evolved over a million years ago, the eastern part of North America occasionally experienced very cold summers. Having a longer period lessens the chance that unusual cold will kill the whole brood.

Cicadas make up for their infrequent visits with high population density, which can reach 1.5 million cicadas per acre. There’s power in numbers. They usually emerge in late spring when soil temperature reaches 64 degrees Fahrenheit.

And do they make a racket! Cicadas can produce sounds up to 106 decibels, louder than a lawnmower and almost as loud as a car horn.

For some reason the cicada strategy has not been adopted by other insects or animals. But scientists have observed a 17 year cycle in the exposure times of the Earth to the interplanetary magnetic field. Maybe cicadas just have a magnetic personality.

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