How a Hydrogen Isotope Reveals a Dragonfly’s Long-Distance Journey
By Ed Ricciuti
North America’s monarch butterflies (Danaus plexippus) get all the ink when it comes to insect migrations, with their annual 3,000-mile trek, but a little dragonfly’s travels throughout many parts of the world are just as epic, and likely much more so. Evidence collected a few years ago by Maldives-based biologist Charles Anderson, Ph.D., suggests the dragonfly Pantala flavescens, sometimes known as the globe skimmer dragonfly, completes an 8,000-mile round trip between India and Africa over four generations or so, hitchhiking on high-altitude winds that prevail each way.
Anderson is part of a team of scientists—the others from Japan and Canada—who now have documented that many P. flavescens found in Japan in summer originate from as far as Tibet and northern India, about 3,000 miles away. Others, especially early in the season, travel less, from places such as southern China and the Korean Peninsula, while some are home-grown, from southern Japan.
Their research, published in November in the journal Environmental Entomology, not only describes the migratory patterns of P. flavescens that summer in Japan but also supports the efficacy of tracking migratory insects by analyzing natural isotopes naturally present in their body, in this case the hydrogen isotope deuterium. The research answered long-standing questions about the dragonflies in Japan while demonstrating the worth of stable isotopes for mapping insect movements.
“Our laboratory pioneered the use of naturally occurring stable isotope ratios as a means of inferring origins of migratory insects and small birds,” says lead author Keith A. Hobson, Ph. D., of the Department of Biology at the University of Western Ontario in Canada. “Our work is significant because it reveals the movement patterns of this dragonfly reaching Japan but also demonstrates clearly how this isotope-tracking technique can provide robust and important information for vast numbers of other migratory insects.”
Every chemical element has isotopes, with almost identical chemical properties and the same number of protons but a different number of neutrons and atomic weight. Both the more abundant radioactive isotopes as well as stable isotopes are used by scientists as tracers and markers. The former are detected by radiation while stable isotopes, safer to use, are detected and measured with techniques such a mass spectrometry, an old standby for assaying the kind and ratios of chemical components of a sample material. It works by separating compounds into gaseous ions and measuring their mass-to-charge ratio, which is then plotted on a mass spectrum.
Isotopes of elements in the environment are transferred to animal tissues through local food webs. Organisms thus imprinted bear an isotopic signature, derived from distinctive isotope patterns of geographic regions, called “isoscapes,” which in turn are influenced by factors such as precipitation and temperature. Deuterium in the environment, for example, decreases with latitude and altitude due to patterns of rainfall.
“Our work has shown that deuterium is especially useful because we know the pattern of this isotope in precipitation shows dramatic and repeatable patterns across continents,” says Hobson.
If the genesis of tracking techniques using stable isotopes did not cause entomologists worldwide to crack open the champagne, perhaps it should have. The development means that even the tiniest insect comes equipped with its own internal tracking device, implanted by nature and not artificially by researchers. Using isotopes already part of the insect is a lot easier than, for example, trying to attach a transmitter or implant a radioactive tag on a dragonfly—or a mosquito. Making techniques with isotopes work, however, depends on building an algorithm that relates the measurements of isotope values in tissue with isoscapes.
Pantala flavescens is a natural for migration studies because it probably is the most widespread dragonfly on Earth, found on every continent except Antarctica. The skimmer’s is a world-wide gene pool, with individuals sometimes crossing oceans to find mates. Technically speaking, is likely that all globe skimmers, according to sex, are potentially able to mate with one another. So tough and adaptable is P. flavescens that it was the first dragonfly to colonize Bikini Atoll after nuclear testing there. Under two inches long, it travels not only far but high as well, to more than 20,000 feet, feeding in flight on other aerial insects encountered.
Pantala flavescens migration seems timed to both prevailing winds, to boost its flight, and periodic rains, critical to its reproduction. Unlike most dragonflies, which spend 10 months as aquatic larvae, P. flavescens goes from egg to adult in six weeks, so it can use temporary pools from seasonal rains for reproduction.
Specimens of P. flavescens collected in Japan by authors of the paper provided evidence for the association of their migration and reproduction with wind and rain patterns. Dragonflies collected in early summer showed an isotope pattern consistent with areas far to the west, including parts of China and the Tibetan Plateau. The timing coincides with the East Asian summer monsoon and the annual Baiu Rains, which are associated with a stationary front stretching from the Tibetan Plateau to Japan, and high-altitude southwest winds known to carry insects. Body mass of migrant dragonflies was greater than Japanese natives, likely due to greater fat loads stored for energy required by migration.
“Our investigation underlines the power of combining stable isotope data with other information such as wind speed and direction, arrival dates, and body mass to estimate origins and to understand the life history of this and other insects,” write the authors.
Looking to the future, the authors suggest that isotopes of strontium, carbon, and nitrogen merit consideration for use in filling in the blank spots in the picture of P. flavescens origins.
Dragonflies enriched in the nitrogen isotope 15N for example, would suggest they came from a locale with substantial agriculture, helping target their origins.
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.