Every 13 or 17 years, some parts of the country are on cicada watch. They're waiting for billions of the thumb-sized insects to crawl out of the ground, create an incredible racket with their calls, and then die after successfully mating. One Science Update listener asked how the bugs know when to finally come out.
Can cicadas tell time? I'm Bob Hirshon and this is Science Update.
This month, noisy insects called cicadas are making their debut across the Northeastern and Midwestern United States. One listener from Washington, D.C., asked us how cicadas know when to come out after being in the soil for 17 long years.
Well, we consulted Keith Clay, a biologist at Indiana University in Bloomington, who's studying this year's brood. He describes an experiment done where fruit trees were grown in large pots in a greenhouse. By turning the lights on and off, the trees were accelerated through their seasonal cycles.
These large pots had cicadas in the soil on the root system, and the cicadas emerged after the 17th cycle—not 17 years.
Clay says cicadas drink sap from tree roots, so they could be keeping track of how it flows or could be detecting something in the sap.
They still have to keep count, and how they do that we don't know. But then, in the 17th year, their emergence from the ground appears to be triggered by soil temperature—around 64 degrees, they know it's time to come out.
If you've got a science question, don't let it sit for seventeen years. Call us today at 1-800-WHY-ISIT. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
If you live in cicada country, you might have already met Brood X, the 17-year cicadas. If not, you will soon enough. Whether you think they're fascinating, disgusting, or just plain noisy, you have to admit that they have an incredible built-in alarm clock. They spend the first seventeen years of their life underground—basically hibernating—but without fail, they emerge in the seventeenth summer to mate and die.
So how do cicadas keep track of the time? Obviously, you can't ask them. But you can conduct experiments where you play around with the cycles of nature, like light, temperature, and plant life cycles, to see if that changes the cicada clock.
As you heard, the clock still isn't fully understood. But it looks like at least two different steps are involved. First, the cicadas' bodies have to count from one year to seventeen years. They seem to do this by keeping track of the sap cycles in the roots of the trees. We can guess this because when you plant trees in a lab, and use shorter daylight cycles to trick them into speeding up their life cycle, the cicadas emerge after seventeen "tree-years," not seventeen actual years. In other words, if the trees go through spring, summer, fall, and winter every six months instead of every twelve, the cicadas will come out in eight and a half calendar years instead of seventeen.
Why? Well, since the cicadas are underground, they probably aren't following the lights. But they are constantly sucking sap from the trees' roots. Since sap flow varies across the course of a year, it would make sense that the cicadas' bodies were somehow keeping track of the seasonal changes. How they do that remains a mystery.
Once you get to year 17, however, the cicadas usually seem to come out when the soil temperature reaches 64 degrees Fahrenheit about a foot below the surface. Clay suspects that because cicadas are cold-blooded—in other words, they can't generate their own body heat—it may take at least 64 degrees of warmth to make it possible for the cicadas to become more active.
So there may be some kind of switch in the cicada's body that's sensitive to temperature. When its body temperature reaches 64 degrees, the switch goes on, and the cicada becomes more active and crawls out of the ground. However, this switch, in turn, may be controlled by a master switch, which doesn't turn on until seventeen sap cycles have passed. Until the master switch is turned on, the temperature switch won't turn on either, no matter how warm it gets.
But why 17 years, instead of 10 or 5 or 1? You'll hear more about that in the next lesson.
Now try and answer these questions:
- What two factors appear to control the life cycle of cicadas? How do we know this?
- Scientists suspect that cicadas keep track of the years by following seasonal changes in the sap flow from the roots of trees. Suggest an experiment that would test this hypothesis further.
- Suppose cicadas were raised in a greenhouse for seventeen years. During the seventeenth year, the soil was kept below 64 degrees Fahrenheit. What would you expect to happen?
- Suppose the experiment just described were extended into the eighteenth year. If the soil temperature was raised above 64 degrees and no cicadas emerged, what would that tell us? What would it tell us if the cicadas came out?