As the first day of spring approaches a scientific mystery will soon return with a roar— the 2013 return of the east coast b rood of cicadas, or Brood II. Now a team of scientists hint they may have a solution as to why this brood and its fellows bizarrely emerge only after lulls more than a decade long—to control their surroundings in ways that may lead to crashes in numbers of predatory birds.
Periodical cicadas are the longest-lived insects known. After childhoods spent underground living off the juices of tree roots, broods of red-eyed adults surface in precise cycles— 13 years long in the southeastern U.S. and 17 years long in the northeastern part of the country. Fifteen broods are known to exist today on Earth, all native to North America. Brood II is set to emerge this spring in New York State, Connecticut, Maryland, North Carolina, New Jersey, Pennsylvania and Virginia with choruses of males bent on wooing mates with their din. It remains an enigma why these cicadas only emerge together in the adult stage every 13 or 17 years, as opposed to some other duration — other cicada species are not so synchronized.
"The 'periodical cicada' problem is one that's been kicking around for nearly 350 years at this point," says behavioral ecologist Walter Koenig at the Cornell Lab of Ornithology. "The first known mention of periodical cicadas was in Volume 1 of Philosophical Transactions, the first real scientific journal ever published, in 1665 . The fact that these insects still pose some of the most challenging problems there are in evolutionary biology is impressive."
A number of possible solutions to this mystery have been suggested over the years. For instance, researchers have speculated that the lengths of these strange cycles may reduce unwelcome interbreeding and competition for resources among broods — even if there was any geographic overlap of 13- and 17-year cycle broods, the lengths of the cycles would ensure that they only emerged simultaneously once every 221 years. Little such overlap exists, however, weakening this argument. Others have suggested these cycles may help periodical cicadas avoid predators or parasites that have shorter, even-numbered life cycles, but no strong evidence exists in support of these ideas either.
Now Koenig and his colleagues suggest the lengths of the cycles may somehow cause the numbers of cicadas’ predators—insectivorous birds—to drop. "Our hypothesis is, frankly, hard to believe, even by us, if for no other reason because we know of no mechanism to explain how cicada emergences could be driving a cycle in the birds that is so long—13 or 17 years,” Koenig says. “Because of this, I don't blame anyone for being skeptical. Unfortunately, it's not easy to study animals that only show up two or three times during one's scientific career."