GO IN DEPTH

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Cicadas spend years underground and come out once in a blue moon for a frenzy of activity. But certain broods, like the one that emerged in 2004, come out like clockwork every 13 or 17 years. In this Science Update, you'll hear what's so special about these numbers.

### Transcript

Why cicadas come out in their prime. I'm Bob Hirshon and this is Science Update.

2004 is the year of Brood X—a group of cicadas that emerge from the soil every 17 years to molt and mate. Other types come out every 13 years. This prompted one listener from Washington, D.C., to wonder why these insects have such odd-numbered life cycles.

We asked biologist Keith Clay of Indiana University. He says 13 and 17 are prime numbers, meaning they can't be divided evenly by any other number except one.

Clay:

The main hypothesis is that it's very difficult for predators to have a similar life cycle, where they could actually specialize on these cicadas 'cause they also would have to have a 17-year life cycle.

If cicadas came out every 16 years, for example, predators with two-, four, and eight-year life cycles would be around that year to eat them.

Clay:

So I said there's no specialist predators, but there's plenty of predators that are opportunistic and just jump on it because they're so abundant, and they're so easy to catch.

Although many do get eaten, enough live to mate and lay eggs, which will become the cicada class of 2021.

If you've got a science question, call us at 1-800-WHY-ISIT. I'm Bob Hirshon for AAAS, the Science Society.

### Making Sense of the Research

This is an area where life science and mathematics intersect. The math comes in the form of a problem that the cicadas need to solve: how do you survive to reproduce before being eaten by predators?

For some kinds of cicadas, the answer lies in adaptations. For example, the dog-day cicadas that are common in the South are fast fliers. Other species use camouflage to hide from predators.

But the 13- and 17-year cicadas don't have any good defenses. They're slow, easy to spot, and apparently very tasty. So how are they going to live long enough to reproduce?

This is where the prime numbers come in. Clay suggests that having a prime numbered life cycle keeps you out of synch with the life cycle of predators, which prevents any particular kind of predator from being too dependent on the cicadas for food. In other words, if you're a predator with a one-, two-, four-, or even eight-year life cycle, you're not going to get too used to catching cicadas that show up only once every 13 or 17 years. So evolution will select predators that target other kinds of prey.

The problem of course, is that lots of cicadas get eaten anyway. But that's okay, as long as some of them survive to reproduce. That's why it's useful for the cicada brood to emerge all at once, instead of in drips and drabs: no matter how easy they are to catch, predators can't possibly eat them all. This is called the "predator satiety" theory—but you might also call it "safety in numbers." According to this theory, conformity is a good thing: cicadas with genes that make them emerge a little earlier or later than the pack would be spotted and eaten right away.

Why such long life-cycles? Why 13 or 17 years, instead of the prime numbers 2, 3, 5, or 7? That may have to do with climate. During the Pleistocene epoch (over a million years ago), when these cicadas evolved, the eastern part of North America occasionally experienced very cold summers. That's bad for cicadas, which can't survive above ground when temperatures drop too low. Because of this, some scientists believe that evolution would have favored cicadas with long life cycles, because the more often you come out of the ground, the more likely you are to get smacked with a cold snap that wipes out your entire population. It's kind of like taking fewer trips on an airline that occasionally crashes.

Another explanation for the prime numbers combines the issues of climate and predation. Mathematically speaking, cicadas with prime-numbered life cycles emerge less often with other cicada species that have shorter life cycles (again, because a prime number is divisible only by itself and 1). When broods emerge at the same time, they interbreed, and produce offspring with some life cycle in between that of their parents. These offspring are more likely to emerge at "off-peak" times, when few other cicadas are around, and therefore are more likely to get picked off. Or perhaps they developed shorter life cycles, which made them more likely to emerge during a cold season.

The 13- and 17- year breeds, on the other hand, had less contact with other broods and stayed "pure." The more of them that emerged at the exact same time, the more of them survived to mate. Stragglers and early birds were eliminated, along with their genes. And over thousands of years, evolution honed these broods to emerge in overwhelming numbers, at precisely the same time, when few other cicadas threaten to dilute their gene pool.

Now try and answer these questions:

1. Why do cicadas have such long life cycles?
2. What are two advantages of prime-numbered life cycles?
3. Why do cicadas that can fly well or hide easily tend to have shorter life cycles?
4. Suppose a predator with a 13-year life cycle were slowly introduced into an area with 13-year cicadas. What would happen? How might the cicada population eventually evolve and adapt?

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