Lifecycles Of Cicada Species
In 15 states, from Georgia to Michigan, people are bracing themselves for an invasion. But the new arrivals will be more of a spectacle than a threat, they'll only stay for a few weeks, and they won't be back for 17 years.
Beginning any day now, millions of periodical cicadas will emerge from the ground for the brief four- to six-week span of their adult lives. They belong to a particular year-class, or 'brood,' that last emerged in 1987. There are 12 broods that emerge, mostly in the northern states, on a 17-year cycle, and three broods in the South on a 13-year cycle.
How and why these insects evolved such lengthy but precise lifecycles is not yet well understood. These and other questions, such as the relationships between 13-year and 17-year cicadas and the mechanisms by which new species are formed, are the focus of research by Chris Simon, a professor of ecology and evolutionary biology.
Simon says these insects have fascinated scientists and amateurs for centuries. Their name, Magicicada, refers to their magical qualities.
Although their lifespan is long, periodical cicadas are short-lived in their adult form. After emerging above ground, their primary task is to reproduce. The males attract a mate with a shrill noise that is among the loudest sounds produced by any insect.
After mating, the female lays her eggs in tree branches. When they hatch, the ant-sized nymphs fall to the ground, burrow down, feed on tree roots, and slowly mature through five nymphal stages over a period of either 13 or 17 years.
Although it is generally believed that the day they emerge is determined by ground temperature, the mechanism that triggers which year the cicadas emerge is not well understood. Simon calls it an 'inborn,
biological clock.' The cicadas mature at variable rates, with some reaching the final nymphal stage several years ahead of others. Yet they still emerge together and on schedule.
"All the adults come out within two or three days of each other," says Simon. "You'd think with that kind of precision, the nymphs would all grow at the same rate."
Simon says periodicity evolved millions of years ago. As an evolutionary strategy, it gives the species plenty of time to mature, allows nutritional cycles of dearth and plenty to even out, and has led to a safety-in-numbers strategy for survival. The adults are prey to many different species, including birds, turtles, squirrels, dogs, mice, spiders, and even humans. At densities of hundreds of thousands per acre, even though many are eaten, the population is able to survive.
But the long, 13- or 17-year lifecycle also helps protect them: In answer to the question 'Why 13 and 17?' Simon points out that both are prime numbers. "Predators with shorter lifecycles can't synchronize with them by coming out on one-third or one-half of the cycle," she says. In addition, cicada species on different lifecycles are less likely to hybridize if the cycle is a prime number. The emergence of specific 17-year and 13-year broods will coincide only once in 221 years.
Simon says there may be another connection between the 13-year and 17-year lifecycles. She posits that, perhaps as recently - in evolutionary terms - as 10,000 years ago, some 13-year cicadas needed more time to develop during periods when the climate was uncertain. If they emerged just one year later, they would be decimated by predators, who would be thriving after feeding on the cicadas that emerged on the regular 13-year cycle. Even two or three years' delay could be risky, but emerging after an extra four-year interval would likely assure the survival of the species.
She adds that, where more than one brood of periodical cicadas is found in a given area, the different broods emerge four years, or a multiple of four years, apart.
Simon says there are three morphologically distinct 17-year species and three parallel species of 13-year cicadas. For each 17-year species, there is one similar species with a 13-year cycle. A fourth 13-year species, Magicicada neotredecim, was discovered by Simon and her colleagues in the late 1990's.
In 1998, when the emergence of one brood of 17-year cicadas coincided with that of a 13-year brood, two doctoral students at the University of Michigan, now postdocs at UConn in Simon's lab, noticed that in one particular area, the cicadas' sounds were unusual. Previously, Simon and her students, by sequencing mitochondrial DNA, had demonstrated that in this area there were two genetically different forms of 13-year periodical cicadas.
She suggested that one form was recently derived by life-cycle switching from nearby 17-year cicadas, and that following the switch the two formerly isolated evolutionary lineages would hybridize. "We predicted random mating, but it wasn't. There was almost perfect assortative mating," she says.
John Cooley and David Marshall found that the cicada population they were studying had cicadas with two very different mating-song pitches - a strategy, says Simon, that helped assure that males of each genotype mated only with females of the same genotype. Previously, Simon and her group had found that the mitochondrial DNA genotypes were highly correlated with an easily visible abdominal- color polymorphism; Cooley and Marshall found that they were correlated with the song pitches as well.
They collected specimens in alcohol, labeling them according to the pitch of their song. Back in the lab at UConn, they discovered through DNA analysis that the two pitches, as predicted, were associated with the two different mitochondrial DNA genotypes discovered by Simon and her students.
Simon's research has been funded by the National Science Foundation since 1978. She has also received National Geographic and Fulbright grants. Her work on periodical cicadas has been published in journals such as Nature, Evolution, Trends in Ecology and Evolution, and Ecology.