Tracking the Red-Eyed, Sluggish, and Ear-Splitting

BROODING OVER DIFFERENCES

There are three groups of Magicicada, comprising seven known species. The three groups – decim, decula, and cassini – differ in behavior, appearance, songs, and genetics. Each group has one species with a 17-year life cycle and at least one that lives 13 years. Periodical cicadas in different regions come out in different years. Each year-class or brood is designated with Roman numerals; this year's is Brood X. Most broods contain members from all three groups. Twelve broods are 17-year cicadas and live mostly in the northern United States and the Great Plains; three broods live 13 years, mostly in the South and Midwest.

In 1998, while still grad students at the University of Michigan, John Cooley and David Marshall (both now at the University of Connecticut at Storrs) built on previous work by Chris Simon (also at Storrs) and discovered a new periodical species. Cooley recalls that they were driving south to continue their study of cicada mating systems. Brood XIX, a 13-year brood, had just emerged, and the students started hearing what were supposed to be M. tredecim choruses at about the Missouri-Arkansas border.

"They didn't sound right to us," Cooley says. They tape-recorded the ruckus and looked at it on a spectrogram, using a computer to analyze the sounds. The typical decim chorus has a dominant pitch at around 1.3 kHz. "These choruses had two tones in them. One was down low at about 1.1 kHz, and one was way up there at about 1.7 kHz. We thought that was odd."

They soon noticed that cicadas making high-pitched sounds never made low-pitched sounds, and they also had dark abdomens. Cicadas making low-pitched sounds had light abdomens. After the previous Brood XIX emergence in 1985, Simon and Andrew Martin had shown that the midwestern and southern cicadas, then both known as M. tredecim, varied in abdomen color and had two different mitochondrial lineages. Cooley and Marshall suspected that those differences, plus the distinctive songs and mating habits (females mated with males having the same abdomen color), suggested two distinct species.¹ The species were not hybridizing in this narrow zone of overlap from Arkansas to southern Indiana, but instead were maintaining their differences with diverse mating calls. The northern darker cicada is now known as M. neotredecim, while the lighter southern form continues as M. tredecim. The new species probably arose no more than 8000 years ago, when glaciers departed.

A WHOLE NEW BUG

How new periodical species arise poses a mystery in light of the insect's survival strategy. Researchers describe them as "predator foolhardy": Some cicadas can flee hungry birds, but the sluggish periodicals are easy snacks. Instead, they survive via predator satiation. They emerge in the billions, so numerous that predators can't eat them all. "They combine the idea of safety in numbers with the idea of not being around for a long period of time," explains Simon.

Tabitha M. Powledge

A Brood-X cicada emerges from its exoskeleton.

Satiation as a strategy makes speciation difficult. Individuals pioneering a new trait are few in number; chances are they would be picked off quickly and fail to pass on the trait. Randel Cox, an earth scientist at the University of Memphis, and Chris Carlton, now director of the Louisiana State Arthropod Museum in Baton Rouge, suggested that neotredecim could have evolved if a dominant gene for the 13-year life cycle moved northward from the southern tredecim to invade septendecim.² In this model, a narrow zone of hybridization between the two populations opened every 221 years, when tredecim and septendecim emerged simultaneously. The model also assumes that only tredecim males carrying the gene mated with septendecim females, because neotredecim possesses the septendecim mitochondrion. The resulting hybrid, a cicada that resembled septendecim in every way except life-cycle length, slowly progressed northward.

Marshall, Cooley, and Simon say that scenario might be possible if 13-year males don't wander far, because in order to survive, their neotredecim offspring must hide from predators among millions of tredecims emerging 13 years later. But in Missouri, the neotredecimrange extends north about 400 km from the overlap with tredecimin the south to a narrow overlap with northern septendecim. To travel 400 km in 8000 years, say the researchers, in every generation the new species must move at least 650 meters north. Their simpler model argues instead that neotredecim arose not from tredecim but from septendecim. The only change it calls for is a life-cycle shift from 17 years to 13.

WATCHING THE CLOCK

Cicadas can indeed modify their periods. Developmental sloppiness is extreme in the periodicals, which frequently drop or add an entire four-year instar. In 2000, a large premature emergence of some of this year's Brood X occurred, which Kritsky says he predicted as early as 1991. And, Kritsky reports, some Brood XIV cicadas, not due out until 2008, instead emerged this year with Brood X. "Brood XIV cicadas emerged this year from a tree that was not planted in 1987, so it had to be an acceleration."

"Periodical cicadas seem to have flexibility in their life-cycle length that is environmentally triggerable, or can be environmentally cued," says Marshall. Researchers speculate that the shift may have something to do with weather or nutrition, but they don't really know. This plasticity might provide a bridge, Marshall says. If many 17-year cicadas are cued to come out in 13 years, and if the cue remains over many generations, natural selection can favor genes that make the new life cycle stick.

TAKING FLIGHT

A key issue is how far cicadas travel, and whether males travel farther. Marshall, Cooley, and Simon argue that their life-cycle shift model of neotredecim evolving from septendecim need account for only the 150 km overlap zone for M. neotredecim and M. tredecim,³ and not the entire 400 km range suggested by the Cox and Carlton model. Neotredecim could have made this 150 km trip in 8000 years by traveling 200 meters per generation, they say.

Enter Wikelski, who has used his microtransmitters mostly to track birds. In his collaboration with Simon, he says he radio-tagged 10 male and 10 female cicadas using 300 mg micropip transmitters, and he and his students checked the location of each one every few hours using handheld scanners. After 11 to 13 days, one transmitter was removed from an apparently sick insect and four simply vanished; Wikelski thinks the unlucky four were either eaten or run over. The researchers recovered 15 transmitters, 10 of them still attached to corpses, within 100 meters of the trees where they had tagged the cicadas. No differences between males and females were observed. Wikelski's conclusion: "Those critters are the most stationary creatures I know of."

So perhaps cicadas aren't enthusiastic travelers, which would support the hypothesis that neotredecim evolved from 17-year decims. But, says Simon, "We can't make any definite conclusions about movement." Sample size was small, and although the tagged cicadas can fly, it's not clear how transmitter weight influenced distance traveled.

Moreover, this year both Kritsky and Cooley observed cicadas that had migrated at least a quarter mile. Kritsky reports, for example, that no cicadas emerged in a Cincinnati housing development built in the '90s. But by mating time, "So many cicadas had flown into the area that cicada choruses were measured at 96 dB. Hearing damage [can occur at] 93 dB."

Wikelski is working on a smaller transmitter, which he says will be ready in time to track dragonfly migration in the fall. "Obviously it's only the first generation of this kind of investigation," he says. Next time, he forecasts, "I'm sure we'll have much better technical help to understand much more about this dispersal." They have to wait only until 2007.

Tabitha M. Powledge tam@nasw.org is a freelance writer in the Washington, DC, area.