As Brood II emerges, questions remain about cicada cycles
The how and why of years lived underground are among the unsolved mysteries about the loud, obvious insects
Periodical cicadas may be marvels of synchrony, but they’re also marvelously out of step, usually years off, with much of their own species.
This year researchers published the most detailed look yet at the evolutionary history of the seven species of Magicicada that burst out of the earth after 13 or 17 years of sucking tree sap underground. The cicada class of 2013, known as Brood II, mixes three of these periodical species, often in the same neighborhood.
The three species look different. They can sound different, like B-movie spaceships, blender pulses or water hitting hot fat. Their last common ancestor lived nearly 4 million years ago, Chris Simon of the University of Connecticut in Storrs and her colleagues reported in March in the Proceedings of the National Academy of Sciences. Yet every 17 years they reemerge together. And all three species in Brood II have members that are likely never to meet, cycling with other broods that emerge in different years.
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Cicadas locked in with other broods do resemble the three 2013 species, Simon says. The time-displaced species members look and buzz the same way. If they ever met on the same twig, they should be able to make viable baby cicadas. Maybe eons from now they will fragment into different species, but for now they’re just on a different clock.
The planet has several thousand cicada species, but only a swath of eastern and central North America has the dramatic periodical ones. Cicada specialists are still working out how these mass emergences came to be.
The ridiculous number of cicadas that emerge at once probably has something to do with surviving predators, says Rick Karban of the University of California, Davis. Outliers emerging just a few at a time are doomed. These are big, noisy insects without many escape skills. “You can pick them off a tree,” he says. “They’re just seemingly … dumb.”
So the evolutionary forces wiping out strays that emerge a few at a time favor the development of tight synchrony. With thousands, millions or billions living conspicuously at the same brief time, each individual has a better chance of surviving. Predators can’t eat the whole generation.
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Even synchronizing with a different species beats coming out in small numbers. So as segments of different species overlap in their reproductive timing, they “get sucked into a brood,” Simon says.
Surging in numbers that outdo predator gluttony is not some special cicada thing, Karban notes. Mayflies “hatching” from their aquatic to aerial forms synchronize, as fly fishing enthusiasts know. Oak trees drop occasional bumper crops of acorns. But cicadas, much louder than acorns, get the headlines.
How the cicadas synchronize may be a trickier question than why. Karban has dug up samples of periodical cicadas during their underground years and found all kinds of out-of-sync mixes of development. “They’re growing very idiosyncratically,” he says. Those that race through the five stages of their underground life just wait for the final signal to emerge, giving the laggards time to catch up.
What those emergence signals are is still a question. Soil temperature probably cues the right spring day for the neighborhood mass emergence, but figuring out the right year is a puzzle. They could “count” the years with seasonal changes in the tree sap they feed on, Karban speculates, and he has tested this idea by trying to fool them. He dug up cicadas with two years yet to go underground and moved them onto the roots in a colleague’s research plot of peach trees. The colleague coaxed the trees to fruit twice in one year, and cicadas emerged as if two years had passed instead of one.
But why so many years underground? Karban’s answer is basically, why not? A long immature period may have fewer disadvantages than advantages. Again he has gone digging. His samples of cicadas underground don’t show much evidence of premature death by predator attack. And spending more time growing may mean bigger bodies with the power to have more offspring. The 17-year cicadas he unearthed in the Midwest were in the process of forming more eggs than 13-year ones living nearby.
A long development time could have been a big boon for surviving the ice ages, says geologist Randy Cox of the University of Memphis in Tennessee, who has analyzed how climate affects the pattern of cicada emergences. ”It’s so convoluted,” he says. “That’s why I like it.”
During ice ages, he points out, even southern refuges had chilly years, and a really cold spell could wipe out a population. The longer a cicada’s cycle, the fewer times populations would have to play climate roulette.
If big numbers are good for cicada life cycles, he and other researchers suspect that big, odd numbers are even better. Predator populations can rise and fall in cycles too. If cicadas had a 12-year cycle instead of a 13-year one, for example, they’d coincide more frequently with big years of any predators on two-, three- or four-year cycles.
Cicadas may even somehow influence predator cycles, suggests ornithologist Walt Koenig of Cornell University in Ithaca, N.Y. Decades of nationwide citizen-science breeding bird surveys show that cicadas tend to emerge during dips in numbers of seven cicada-eating birds including American crows and blue jays, he says. In addition, a surge of easy insect food may somehow set bird populations on trajectories of rises and falls that favor the cicadas, he and Andrew Liebhold of the USDA Forest Service Research Station in Morgantown, W.Va., proposed in the January American Naturalist. “Even we think this is kind of weird,” he says, “but it fits the data.”
However the brood emergences came about, they’re worth seeking out, Karban says. Even in the year of a big brood like II, patchy distribution means there will be cicada haves and have-nots in the brood’s range. For those who missed the show this year, Karban says one of the 15 broods of periodical cicadas will break out loud and dumb somewhere almost any year.
“Cicadas are one of the big natural spectacles of North America,” Karban says. “It’s a lot like seeing the bison, like seeing the passenger pigeons.”