17-Year Cicadas: Math, Mystery, and Climate Change
We have much to learn from these extraordinary beings from a past generation.
It has begun — the emergence out into the warm spring air after being in the dark for so long, with pale skin and a craving for sunlight.
No, I am not talking about humans venturing out again after a year of pandemic hibernation but rather, millions of time-traveling beings that have sequestered for far longer. Famous for their six-week above-ground debutante party that quickly becomes their swan song, 17-year cicadas are tunneling their way up from their underground habitats where they have been developing for, you guessed it, seventeen years.
The periodical cicadas — three species of seventeen-year cicadas, along with four species of their thirteen-year counterparts — belong to the genus Magicicada, an appropriate name for these intriguing and sometimes mystifying creatures.
Brood X, the most widespread of the 15 periodical cicada broods, will soon migrate above ground in the millions and perhaps billions. Also known as the Great Eastern Brood, their numbers will swell in fifteen states up and down the east coast, stretching as far west as Illinois and as far south of Georgia.
In some areas, these other-worldly beings with their bulging red eyes and ear-deafening mating calls are already crawling above ground to live out the final chapter of their lives in a glorious finale of song and mating.
And if their pseudo-alien looks are not enough, visitors from U.S. cicada-free zones might mistake the loud, raucous sound of their mating calls (males only) for an alien spaceship.
Can anybody find me somebody to love?
When the soil reaches 64 degrees Fahrenheit, nearly adult nymphs will head upward from their underground habitats and metamorph into full adults within about a week, shedding their exoskeleton and inflating their wings. The males will woo mates for the next few weeks by flexing their tymbals, drum-like organs found in their abdomens, creating a cacophony that rivals that of a lawnmower (around 90 decibels) but can reach an ear-splitting 120 decibels at close range. (Imagine being at a rock concert with cicadas blasting out their mating calls instead of Queen.)
Once impregnated, the females lay their eggs in the branches of small or young trees. When the eggs hatch, tiny nymphs drop into the ground to feed on nutrients from plant and tree roots, and the cycle starts again.
A Prime Example
But it is not just their extended life cycle that makes the periodical cicadas so unique. They are math geniuses. No, really. The Magicicada species possess an uncanny ability to emerge en-masse precisely in 13- or 17-year cycles.
Entomologist Lou Sorkin at the American Museum of Natural History told ABC News that there are approximately 15 broods of periodic cicadas, all synchronized on 13- and 17-year life cycles.
“No other species has as synchronized of an emergence,” Sorkin explained.
But why 13 and 17? Some mathematicians speculate that the answer may lie with the prime-cicada connection. Thirteen and seventeen are both prime numbers, meaning they only have two dividers: one and themselves.
After encountering multitudes of 17-year cicadas in his backyard, mathematician Glenn Webb at Nashville’s Vanderbilt Library created a theoretical model in which the cicadas emerged during non-prime years. Sure enough, Webb’s model indicated that predators would quickly drive non-prime cicadas to extinction or near extinction.
Based on his results, Webb proposed that 13- and 17-year cicadas’ emerge en-masse in prime-number years as a strategy for out-numbering and out-surviving predators.
“To me, it’s a little puzzle from evolution,” Webb stated.
Periodical cicadas do not have any physical defense mechanisms; they do not bite or sting and are not poisonous. If the cicadas emerged in non-prime life cycles, say five or ten years, their appearance would coincide with the peak cycles of predators such as birds, who would eventually destroy them.
“No other species has as synchronized of an emergence.”
By coming out in ‘off’ years, periodical cicadas outsmart and overwhelm predators by the millions, or even billions — an evolutionary tactic known as predator satiation, or in terms of economics, the supply of cicadas eclipses the demands of their predators.
But why 13 and 17 and not other prime numbers such as 11 or 23? And why the four-year difference between species? These and other questions continue to mystify entomologists and mathematicians alike.
Climate change alarmists
Periodical cicadas have another trick of their proverbial sleeves. Because they are sensitive to soil temperatures and changes in the tree roots they feed on, they may be acting as alarm clocks warning against climate change.
In 2017, Gene Kritsky, an entomologist at Mount St. Joseph University in Cincinnati, noted that the cicadas’ arrival had shifted earlier by a month over the last century.
“In the first half of the 20th century, it was rare to hear cicadas chirping before late May, but this year, reports started coming in late April,” Kritsky commented.
Kritsky also observed that ten percent of the 17-year species appeared in 2000 rather than in 2004 — timing consistent with their four-year cycles.
Likewise, in 2017, large swaths of 17-year cicadas emerged early in Cincinnati and around the Baltimore-Washington metro area; motorists reported seeing (or hearing) significant numbers of cicadas along the East Coast I-95 corridor.
“Because periodical cicadas are sensitive to climate, the patterns of their broods and species reflect climatic shifts.”
It would not be the first time a climate event precipitated an early appearance of 17-year cicadas. Chris Simon and John Cooley, professors of ecology and evolutionary biology at the University of Connecticut, wrote an article for The Conversation that discussed the potential for periodical cicadas to be harbingers for climate change.
“Because periodical cicadas are sensitive to climate, the patterns of their broods and species reflect climatic shifts,” Simon and Cooley pointed out.
Simon and Cooley had collected data that indicated that the warming climate after the last North American glacial melt caused 17-year cicadas in the Mississippi Valley to make a transition.
“As the environment warmed, 17-year cicadas in the area emerged successively, generation after generation, after 13 years underground until they were permanently shifted to a 13-year cycle,” they wrote.
Scientists wonder if portions of Brood X might once again permanently shift to a 13-year cycle if the trend of early emergence continues. Simon hypothesizes that longer growing seasons could be causing some 17-year cicadas to surface four years early.
“It’s possible that climate change is helping more of them to grow faster,” said Simon. “If the conditions are really good, then a lot of them will come out.”
Social distancing is for humans
Whatever the answers to these and other questions about 17-year cicadas may be, there is no denying that they are a wonder of nature. Cooley stressed the magical nature of this event.
“There aren’t really any other places in the world that you can go and see something like that,” he said. “So sit back and enjoy it, because it’s unique.”
So, if you are lucky enough to be living in the eastern United States this spring, turn down your rock music and celebrate these noisy time-traveling creatures with curiosity and respect. But be advised, cicadas are not into social distancing.
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If you live in the Eastern half of the U.S. and would like to help scientists document and track the spring 2021 cicada emergence, download the Cicada Safari mobile phone app to take photos of cicada appearances near you and follow real-time research at cicadas.uconn.edu.
