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Bugging the bugs: Students collect data on cicadas
By Steven Schultz
Princeton NJ -- For many people, they are a curiosity, or perhaps a mild annoyance. But for several students and faculty members, the red-eyed insects that have begun crawling out of the ground in great numbers are a rare opportunity for scientific research.
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Junior Marian Bihrle and a member of "Brood X." |
During the next few weeks -- as these "Brood X" cicadas emerge from 17 years of subterranean growth and play out their brief but noisy above-ground mating ritual -- Princeton students will investigate a range of questions about how the insects behave and interact with the environment. One will collect data on how much damage cicadas inflict on trees as they deposit their eggs into thin branches, while another will attach tiny radio transmitters to the insects and track their movements from birth to death.
"Since this occurs only once every 17 years, it's a puzzle for evolutionary biologists -- why do they take so long to develop? And why do they all come out at the same time?" said Peter Grant, professor of ecology and evolutionary biology. Grant normally does not study cicadas himself, but is advising undergraduate Uta Oberdörster, who is planning a senior thesis for next year based on the cicada research she is conducting now.
Junior Marian Bihrle, working with assistant professor of ecology and evolutionary biology Martin Wikelski, is leading the radio tracking experiments and also is planning a senior thesis based on the results.
The cicadas, which began to emerge on campus around May 17, are expected to remain active for four to six weeks (see related story).
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Junior Marian Bihrle is working with a radio transmitter to track cicadas' movements from birth until death. She plans to use the research she collects as the basis for her senior thesis next year. |
Both students became interested in the subject during their department's junior tutorial, in which they read a number of scientific papers, including one on cicadas. They went on to write junior papers that reviewed the scientific literature on cicadas and outlined the major open questions, of which there are many.
"To witness the natural wonder of millions of periodical cicadas emerging in perfect synchrony during a few days of spring once every decade or so," Bihrle wrote in her second junior paper, "it is easy to overlook how their dormancy is really part of an elaborate development and life cycle which has been evolving over thousands of years -- resulting in a meticulously timed development period that has achieved nearly perfect periodicity in order to culminate in mass emergence."
Probing biology's deepest questions
There are more than 1,500 cicada species, most of which emerge annually. For reasons that are still unclear, only a handful of the 1,500 species have developed "periodical" behavior. The insects that make up Brood X include three of these periodical species, each with its own mating sound, which have evolved to behave as a single group with a 17-year life cycle. Other members of the same three species have evolved together to form broods with a 13-year life cycle.
According to the Princeton researchers and others, these unusual evolutionary patterns could offer unique insights into one of the deepest questions in all of biology: How do new species form? Bihrle and Oberdörster, however, had to narrow their focus to questions that could be tackled for senior theses.
For her research, Oberdörster plans to collect data on what kinds of trees female cicadas choose for laying eggs. After the cicadas emerge from their 17 years of underground development (during which they suck on tree roots), the insects remain sluggish for a couple days before flying into trees to mate. The males congregate in "chorusing centers" where their cacophonous songs attract females. The females then drill into thin branches and deposit their eggs, causing the ends of the branches to die and fall to the ground. Six to eight weeks after they are deposited, the eggs hatch and newborn nymphs burrow down to begin a new cycle.
A better understanding of the cicadas' preferences and behaviors could help explain how the insects maintain their strict coordination on a local level, according to Oberdörster. As a second avenue of research, Oberdörster will investigate whether birds and other predators are harder on the male or female cicadas. The ability to synchronize their emergence and above-ground activities is critical for these insects, which have no defense against predators other than their overwhelming numbers.
"In my junior paper I mention that there is a theory that maybe females are preyed upon more than males. I will be examining that to see if it actually happens," Oberdörster said.
Although she and Grant have prepared for her research, Oberdörster expects to make in-the-field decisions about what to measure and where to look. "It is very similar to the research my wife (lecturer and senior research biologist Rosemary Grant) and I do in the Galapagos," said Grant, noting that they work out the details of their research ahead of time so they can gather good data in a short time. On the other hand, he said, "you need to keep your eyes open and be opportunistic."
Noticing something unusual about the cicada songs in one location, for example, could open a new avenue of investigation, Grant said. "If there is anything puzzling -- or even just interesting -- that comes up she can fasten on it and make it part of her research," he said. "That is why it is very exciting. There are so many questions that I am sure she will come up with something interesting in the end."
Documenting a bug's life
Bihrle also faces many unknowns in her field work. With funding from her department and the University as well as support from Wikelski's research grants, she and Wikelski have purchased 20 radio transmitters that weigh just one-third of a gram, or about 15 percent of a cicada's body weight. Initial tests showed that the tagged cicadas fly just as well (or poorly -- cicadas are not good flyers) as untagged ones. Bihrle planned to choose cicadas (10 male and 10 female) for tracking and work with sophomores Steven Bellan and Aaron Dawes to follow them during the daylight hours, the only time they are very active.
The students have to choose a representative field location and then gauge the timing so they catch cicadas that are not early or late emergers, which would make them more vulnerable to predators. Earlier studies suggest that the cicadas stay within 50 meters of the spot from which they emerge, but no one has done a radio tracking experiment before so the students do not know how far they may have to go. Wikelski has arranged to hunt down any lost wanderers by airplane if necessary.
The effort is worth it, because understanding the insects' range of movement is critical to understanding how the cicadas have maintained or shifted their timing, locations and breeding patterns. "The dispersal questions that Marian Bihrle is working on are very central to our work," said Chris Simon, a professor of ecology and evolutionary biology at the University of Connecticut. Simon, a leading investigator of cicada biology, collaborated with Wikelski and Bihrle to design the research. "We need to know if males migrate more than females or vice versa or neither migrate."
For Bihrle, the work is both nerve-wracking and fun. "I just hope the cicadas we choose aren't eaten by birds and spit on the ground," she said. But, with good preparation, she is confident she will learn from the experience. "I think if we could touch upon something new that hasn't been looked at that would be really interesting," she said. "The implications would be very exciting."
[p] This cicada carries a radio transmitter that weighs just one-third of a gram or about 15 percent of the insect's body weight. Initial tests showed that the tagged cicadas fly just as well (or poorly -- cicadas are not good flyers) as untagged ones. The experiment is the first effort to track cicadas by radio.