Why Do Cicadas Spend So Much Time Underground?
In 1996, the cicadas of Brood II (the “East Coast Brood”) swarmed the northeastern United States and then disappeared almost as quickly as they came, leaving only their eggs and molted exoskeletons behind. Once the eggs hatched, the new generation of cicada nymphs crawled underground, where they’ve spent the last 17 years biding their time and living off of tree roots.
This year, when the time is right and the soil is warm, they’ll emerge again to molt, enter their adult stage, mate, make a lot of noise, and lay their own eggs. Seventeen years is a long time to hang out underground. It’s long enough for Bill Pullman to go from an awesome fictional president to an awful one. Why do cicadas spend so much time out of sight and out of mind?
Not all cicadas play this long game of hide and seek. Most North American species are “annual cicadas” that have unsynchronized, 2 to 5 year life cycles and emerge every summer. It’s only a handful of species, grouped into broods based on the calendar year they emerge, that have longer, synchronized life cycles and burst forth, all at once, from the ground every 13 or 17 years.
These species, known as “periodical” cicadas, have had scientists scratching their heads since at least the 1600s, when a visitor to the American colonies made mention of the mass emergence in first volume of Philosophical Transactions, the oldest scientific journal. For a while, one leading hypothesis was that the long cycles kept broods from emerging at the same time and competing for limited resources.
Another idea, argued by Stephen Jay Gould in his 1977 book Ever Since Darwin: Reflections in Natural History, is that the cycles protect the cicadas from predators and parasites with shorter life cycles. He wrote:
“They are large enough to exceed the life cycle of any predator, but they are also prime numbers (divisible by no integer smaller than themselves). Many potential predators have 2-5 year life cycles. Such cycles are not set by the availability of periodical cicadas (for they peak too often in years of nonemergence), but cicadas might eagerly be harvested when the cycles coincide. Consider a predator with a cycle of five years; if cicadas emerged every 15 years, each bloom would be hit by the predator. By cycling at a large prime number, cicadas minimize the number of coincidences (every 5X17, or 85 years, in this case).”
In other words, staying out of sync with predators’ life cycles keeps the cicadas from becoming a reliable, annual source of food, and keeps the predators from adapting to specialize in hunting them or feeding on them. The hitch is that because cicada emergences are so far apart, Gould’s hypothesis is difficult to test, but mathematical models, like those created by Glenn Webb and Mario Markus, lend his argument some support.
Even though there are no predators that feed exclusively on periodic cicadas, there are plenty of animals—birds, rodents, snakes, lizards and fish, for example—that will feed on them when they emerge just because they’re there, they’re abundant, they’re easy to catch (cicadas don’t really have any defenses to speak of) and they look about the right size for a meal.
Fortunately for the cicadas, their abundance works in their favor, at least for some of them. Since so many of them emerge at the same time, the number of cicadas in any one place is far higher than the predators in that place would be able to eat. A few do get eaten, but once the predators are full, the rest can go about the business of mating and egg laying without being harassed.
If you live in cicada territory (Brood II will emerge in parts of Connecticut, Maryland, New Jersey, New York, North Carolina, Pennsylvania and Virginia) and want to help scientists predict and track their emergence, consider building or buying a soil thermometer and taking part in WNYC’s “Cicada Tracker” citizen science project.