If carrion flies have one enviable talent, it's finding animal carcasses in the wilderness, something they surpass even the most systematic and intrepid field biologists at doing. Now, researchers may be able to capitalize on the insects' gruesome gift to survey biodiversity. Capture the flies, a new study shows, and DNA from their last meals will tell you which animals live in the area.
Even today, the distribution and abundance of many animal species remains poorly documented, and figuring out a habitat's who's who is no easy task. The terrain can be vast and difficult to traverse, and many creatures are secretive by nature. Traditionally, biologists have searched for the animals themselves, or for burrows, nests, footprints, droppings, and other traces—and all that searching can be time-consuming and costly. In recent years, they've been turning to labor-saving methods, such as setting out microphones, cameras, and traps that snag hairs, or studying animal DNA left behind in water or soil.
But why not just let someone else do the searching? Carrion flies—which include blowflies (family Calliphoridae) and flesh flies (family Sarcophagidae)—live around the world in virtually every terrestrial habitat occupied by vertebrates. Best of all, they're abundant and much easier to capture than vertebrates—even dead ones.
"In the rainforest, many animals die each and every day, but it's really rare to find a carcass," says Sébastien Calvignac-Spencer, an evolutionary biologist at the Robert Koch Institute in Berlin and lead author of the new study.
Calvignac-Spencer and colleagues collected carrion flies in two tropical habitats: Taï National Park rainforest in Côte d'Ivoire and dry, deciduous Kirindy forest in Madagascar. They began by analyzing flies they captured under mosquito nets shrouding dissected mammal carcasses of known species, showing that DNA from the carcasses could be retrieved from the flies.
They then trapped 115 flies at random in the two forests and found that 40% contained identifiable DNA fragments from a total of 20 mammal taxa, two bird species, and an amphibian. In Kirindy, the catch represented 13% of the documented mammal community. In Taï, the mammals aren't fully cataloged, but the scientists turned up DNA from six out of nine known primate species and one very rare antelope, they report this week in Molecular Ecology. Those results are "remarkable" for a modest sample, according to a commentary in the same issue.
In addition to scanning an area's biodiversity, the technique has the potential to reveal species that are new to science, Calvignac-Spencer says. Biologists may also be able to use it to monitor wildlife mortality rates and to identify disease outbreaks. If the DNA of known individuals were on file, the carrion-fly technique could also reveal deaths of particular animals or provide estimates of population sizes. The idea is "remarkably obvious with hindsight. It could have been done 15 years ago—maybe less efficiently, but it could have been done," he says.
Researchers widely use DNA to identify invertebrate diets, and previous studies have detected vertebrate DNA inside bloodsucking tsetse flies, mosquitoes, and ticks. Last year, scientists in Denmark showed that terrestrial leeches can be used similarly to sample an area's fauna, though they feast on fewer species than do carrion flies, don't travel as far in search of prey, and live in only certain habitats. Nevertheless, Calvignac-Spencer and colleagues write that they envision a "toolbox" of invertebrate surveyors that researchers could choose from depending on the habitat that they're studying and the kinds of animals that they're looking for.
The technique is likely to be very powerful and has numerous potential applications, including the study of endangered species, which is often difficult because of regulations and permitting requirements, says Natalia Ivanova, lead DNA scientist with the Canadian Centre for DNA Barcoding in Guelph. To sequence DNA, the approach relies in part on an advanced technique known as next-generation sequencing, which is becoming increasingly widespread and affordable. "The ability to use invertebrate DNA for detection of host DNA will become more and more popular," she predicts.
Equally impressed with the approach's potential is Alfried Vogler, a molecular systematist at Imperial College London. If it can be modified to rely entirely on next-generation sequencing technology, he says, researchers should theoretically be able to identify not just which species are dying, but also which pathogens killed them. Additionally, although carrion flies mainly home in on large animal carcasses, other invertebrate surveyors could help scientists study different ecosystem layers—for instance, carrion beetles, which feast on smaller carcasses, including those of birds. And forensic scientists could potentially use the technique to uncover human corpses. "It's just amazing what you could do with it in principle," Vogler says.