May 16, 2011 | 1
Red-eyed periodic cicadas emerge every 13 or 17 years, but finding out why could take millennia
In ""Too Hard for Science?" I interview scientists about ideas they would love to explore that they don’t think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as particle accelerators as big as the sun, or they might be completely unethical, such as lethal experiments involving people. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard for Science?" suggests that nothing might be impossible.
The idea: Right now, just outside Chapel Hill, N.C., where Zivkovic lives, "every day I open the door, I hear a deafening and ominous-sounding noise — often described as "horror movie soundtrack — coming from the woods surrounding the neighborhood," he says. "The cicadas have emerged!"
These are not green-eyed annual cicadas, which reappear every year, but red-eyed periodic cicadas that reemerge every 13 or 17 years, with this specific brood, Brood XIX, resurfacing in 13-year cycles. They come out simultaneously to live as adults for a few weeks, climbing up trees, singing, mating, laying eggs and then dying. When the eggs hatch, the larvae fall from the trees to the ground, dig themselves down, latch onto tree roots and wait 13 or 17 years to emerge again.
"I’ve been waiting for this all my life," Zivkovic says. "I was not paying attention ahead of time, so I did not know they were slated to appear this year in my neck of the woods. Fortunately, once they emerge, cicadas are out for a few weeks." As such, despite his busy travel schedule, he managed to capture a few pictures and short videos .
For chronobiologists such as Zivkovic, who investigate the cycles that drive every organism, periodic cicadas such as these present fascinating enigmas. "How do they count to 13 or 17?" he asks. "How do they get to be so exact? Is this just a byproduct of their developmental biology? Is 13 or 17 years just a simple addition of the duration of five larval stages? Or should we consider this cycle to be an output of a ‘clock’ or ‘calendar’ of sorts?" Perhaps these cycles result from interactions between two or more biological clocks, or are sensitive to sounds made by other members of their species as the maturing insects dig their way up to the surface, he adds.
And is there any reason why they choose 13 or 17? "There are a number of hypotheses and speculations why periodic cicadas emerge every 13 or 17 years, including some that home in on the fact that these two numbers are prime numbers (pdf)," he notes. "Perhaps that is a way to fool predators which cannot evolve the same periodicity. But predators are there anyway, and will gladly gorge on these defenseless insects when they appear, whenever that is, even though it may not be so good for them."
Perhaps the precise lengths of the cycles serves to drive species apart, lowering the risk of hybridization between recently split sister species, he adds. There are three species of periodic cicadas that emerge every 17 years — Magicicada septendecim, Magicicada cassini and Magicicada septendecula. Each of these species has a ‘sister species’ that emerges every 13 years — M.tredecim, M. tredecassini and M.tredecula — and a more recent species split produced another 13-year species, M. neotredecim. There are differences between species in morphology and color, but the pairs of sister species are essentially physically indistinguishable from each other — they differ only in the duration of their fifth larval stages. M. tredecim and M. neotredecim do appear at the same time and place, but differ in the pitch of their songs, with M.neotredecim singing with a higher tone.
Perhaps the length of the cycles has no specific adaptive purpose, "and the strangeness of the prime-number cycles is in the eye of the beholder, the humans," Zivkovic says. He would love to unravel the mystery — "This is an event that will get your attention," he says.
The problem: Analyzing a biological rhythm that lasts more than a decade is an extraordinary challenge. "These experiments would last hundreds of years, perhaps thousands," Zivkovic says. "What funding agency would finance them? Why would anyone start such experiments while knowing full well that the results would not be known within one’s lifetime?"
Analyzing even relatively short, daily (circadian) rhythms in organisms often takes a long time. For instance, when investigating how a daily cycle of behavior might get synchronized to an environmental cycle, such as light-dark cycles of day and night, "each data point requires several weeks — measuring the period and phase of the oscillation before and after the pulse or a series of pulses of an environmental cue in order to see how application of that cue at a particular phase of the cycle affects the biological rhythm," Zivkovic says. "It requires many data points, gathered from many individual organisms, and all along the organisms need to be kept in constant conditions — not even the slightest fluctuations in light, temperature, air pressure, etc., are allowed."
"It is not surprising that these kinds of experiments, though sometimes applied to shorter cycles — for example, milliseconds-long brain cycles — are rarely applied to biological rhythms that are longer than a day — for example, rhythms that evolved as adaptations to tidal, lunar and annual environmental cycles," Zivkovic says. "It would take longer to do than a usual five-year period of a grant, and some experiments may last an entire researcher’s career, which is one of the reasons we know so little about these biological rhythms."
In addition, the standard experiments to study biological rhythms would require that scientists bring cicadas into labs, "and that is really difficult to do," Zivkovic says. " When kept in the lab, the only way to feed them is to provide them with the trees so they can drink the sap from the roots. This makes it impossible to keep them in constant conditions — trees require light and will have their own rhythms that the cicadas can potentially pick up, as timing cues, from the sap."
Also, scientists do not know what environmental cues might be relevant to these cycles — perhaps it is light, or temperature, or chemicals in the tree sap they feed on — "we would have to test all of them simultaneously, hoping that at least one of them turns out to be the correct one."
The solution? "One obvious solution is to figure out ways to get to the same answers in shorter time-frames," Zivkovic says. Perhaps researchers can sequence the cicada genome and figure out what each gene does to find out how they regulate timing, such as by inserting their genes into other insects and observing their effects. "This will probably not answer all our questions, but may be good enough," he says.
"Another way is to set aside space and funding for such experiments and place them into an unusual administrative framework — a longitudinal study guided by an organization, not a single researcher getting a grant to do this in his or her lab," he conjectures. "This way the work will probably get done, and the papers will get published somewhere around 2835 A.D."
Photos by Bora Zivkovic
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About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time he has traveled to all seven continents. Follow him on Twitter @cqchoi.
The views expressed are those of the author and are not necessarily those of Scientific American.