A synchronized emergence
In the forests of the eastern United States, species of the periodical cicada genus, Magicicada, spend almost their entire lives underground. As immature nymphs, they feed on xylem fluids from tree roots for 13 or 17 years. Then, in the spring of their final year, when the soil temperature at a depth of about 20 centimeters reaches 18°C (64°F), they emerge in a synchronized wave. The scale of this emergence is immense, with densities reaching up to 1.5 million individuals per acre in some forested areas.
This mass emergence is a survival strategy called predator satiation. Everything from birds and squirrels to snakes and coyotes feasts on the cicadas, which have few defenses. The sheer number of insects overwhelms the capacity of local predators to consume them all. This ensures that enough adults survive the four-to-six-week mating season to reproduce. After mating, females cut slits into tree twigs where they lay hundreds of eggs. Soon after, the adults die, and their decaying bodies contribute a massive pulse of nutrients to the forest floor. The newly hatched nymphs fall to the ground and burrow beneath the surface, starting the long cycle anew.
The prime number puzzle
The fact that Magicicada life cycles are 13 and 17 years—both prime numbers—interests biologists. Two main hypotheses attempt to explain this phenomenon. The first is the predator avoidance hypothesis. A predator with a shorter, cyclical life cycle (for example, 2, 3, or 4 years) would find it very difficult to consistently synchronize with a 13 or 17-year cycle. A hypothetical predator with a 3-year cycle would only encounter a 17-year cicada emergence once every 51 years (3 x 17). If the cicada had a 12-year cycle, that same predator would feast every 12 years, potentially driving the cicada population down.
A second explanation involves the avoidance of hybridization between different broods. There are seven species of periodical cicadas—four with 13-year cycles (Magicicada tredecim, M. neotredecim, M. tredecassini, and M. tredecula) and three with 17-year cycles (Magicicada septendecim, M. cassini, and M. septendecula). Prime-numbered cycles make it extremely rare for different broods to emerge in the same year. A 13-year brood and a 17-year brood will only emerge simultaneously once every 221 years (13 x 17). This temporal isolation prevents interbreeding, which could produce hybrid offspring with non-viable life cycles that would emerge at the wrong time, out of sync with the main brood and vulnerable to predators. Some researchers propose that this pressure to avoid hybridization during periods of climate change and population stress in the Pleistocene was the original driver for the evolution of these long, prime-numbered cycles.