This Spring, the Cicadas Are Gathering Like It’s 1803

Their life cycles also notably occur in prime number years. This has proven an evolutionary advantage against predators, as applied mathematicians Frank Hoppensteadt and Joseph Keller argued in 1976. If their cycle was a composite number, such as 16, then their emergence would coincide with the maturation of predator species that develop in 2, 4, 8, and 16-year cycles. By emerging every 13 or 17 years, cicadas reduce the likelihood that they mature in synchrony with predator species, lessening their chances of being eaten.

Credit: Gene Kritsky

The 13-year and 17-year cicadas may overlap in some areas.

“We have a pretty good idea of how they count the years,” says Kritsky. They keep the tally by tracking the seasonal warming of xylem, the nutritious fluid in trees that cicada nymphs feed on. But this tally is imperfect. In the winter between 2006 and 2007, trees in southwestern Ohio began growing leaves and flowers early in an unseasonably warm December and January, before freezing temperatures killed off the budding foliage in February. When the weather warmed again, it was clear that the cicadas miscounted the double warming and freezing cycles that winter as the passage of two years. “The cicadas came out a year early,” says Kritsky.

But mysteries about cicada math remain. “What we don't know is how they remember what year it is,” says Kritsky. “How do they remember the count?” Clues may lie in the cicada nymphs’ early development underground (13-year nymphs molt faster underground than 17-year ones).

It’s also unclear how cicadas behave collectively. Previous research had found that the cicadas emerge at a critical ground temperature of 18 degrees Celsius. But in the real world, the ground temperature is far from uniform and depends on factors such as topography, depth, and the presence of shade. Yet the cicadas knew to emerge together even with these large temperature fluctuations. How?

In recent research, published in February in Physical Review E, physicists at the University of Cambridge repurposed a simple condensed matter model to answer this question. Originally, physicists devised the model, known as a random-field Ising model, as electron spins arranged in a grid that can flip up or down in a randomly fluctuating magnetic field. To study cicadas, the physicists changed the magnetic field to a map of fluctuating temperatures underground, and the spin direction corresponded to whether the cicadas would emerge.

Using their model, the researchers made a mathematical argument for why the cicadas have to communicate with each other. “Were they not communicating, it would be hard to understand how coherent swarms come out,” says Raymond Goldstein, one of the researchers.

But the model doesn’t say how the cicadas communicate — perhaps by releasing chemicals or making noise, proposes Adriana Pesci, a co-collaborator. Goldstein thinks of their research as “advancing hypotheses,” he says. Their model can help researchers determine what types of data to collect. He and Pesci think it would be useful to collect ground temperature data in more detail, for example, or to use drones to capture the patterns of the cicadas emerging.

For Kritsky, who has studied cicadas for five decades, the emergence of each cicada brood marks a timestamp in his memory of the eras of his life. He recalls the last two times he saw Brood XIII — 17 years ago, and then 34 years ago. In 1990, he remembers mapping the brood while staying with his aunt and uncle in Illinois. In 2007, he saw the cicadas crawling up a beehive after a rain, following a lecture he gave in Illinois.

This year, Kritsky recommends going out at night to watch the cicadas emerge from the ground and molt, a process that takes about 90 minutes. “If you’ve got kids, get them outside,” he says. “They’ll never forget this.”

Sophia Chen is a writer based in Columbus, Ohio.