Leeches and their blood meals offer a fast, cheap method for surveying biodiversity. Such basic data can be surprisingly hard to come by, yet is often critical for making conservation decisions.

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Michael Tessler realized his life had taken an odd turn. His days were spent not in an office, not out with friends — but alone in the woods, attracting leeches.

Sometimes, they were so bountiful that “it was like the forest floor was moving toward me,” he recalled. “Even for someone who’s used to having swarms of leeches coming at me, it could be intimidating to see that many of them.”

Tessler, a postdoctoral fellow at the American Museum of Natural History’s Sackler Institute of Comparative Genomics, subjected himself to this horror-movie scenario for the good of science. He collected hundreds of leeches and analyzed their last blood meals, hoping to identify their animal victims — and thus to reveal the range of species living in the forest.

Copious leech bites later, Tessler’s sacrifice paid off. He was a co-author on two papers confirming that leeches and their blood meals offer a fast, cheap method for surveying biodiversity. Such basic data can be surprisingly hard to come by, yet is often critical for making conservation decisions.

“That this bloodsucking worm might suddenly advance conservation efforts is something few would have predicted,” he said.

Terrestrial leeches are found in humid regions stretching from Madagascar to southern Asia to a number of Pacific islands. Some 70 species have been described, with many more most likely awaiting discovery.

They’re a diverse bunch: Some are drab brown, others strikingly patterned in greens, reds and blues. Some crawl across the forest floor in search of a meal, while others occupy leafy perches and leap onto unsuspecting hosts.

They all share a taste for blood. The tiny vampires may swell to 10 times their body weight after feeding, transforming from agile, threadlike worms into engorged blood sausages. Remnants of a leech’s last gluttonous meal may remain in its body for months — a boon for researchers curious to see what it previously fed on.

The idea of using leech blood meals as an identification tool may have been inspired by a criminal case in Tasmania in 2009. Investigators recovered DNA from a blood-filled leech to link a suspect to a robbery.

Several years later, researchers published the first field study showing that the method worked to identify wildlife, too. While encouraging, the initial study was based on a sample of just 25 leeches caught in Vietnam.

Eager to see if the method might be applicable on a broader scale, Tessler and his colleagues set out to conduct an investigation in Bangladesh, China and Cambodia.

The first step — collection — was simple, said Sarah Weiskopf, a biologist at the U.S. Geological Survey and co-author of the new papers: “You just get to your spot in the forest and look around for things crawling toward you.”

Of the thousands of leeches Weiskopf, Tessler and their colleagues captured, 750 were selected for genetic analysis. The researchers cut out the parasites’ digestive tracts and filtered them to extract DNA.

They used primers — short, known sequences of genetic material — to separate mammal from leech DNA, and then they sequenced the results and compared them with a genetic database of known species.

The leeches, the researchers reported in the journal Systematics and Biodiversity had fed on 26 different mammal species, and three birds.

Nguyen Quang Hoa Anh, a project manager for the World Wildlife Fund in Vietnam who was not involved in the research, has been using leeches for several years to survey wildlife in remote jungles near the Lao border. He confirmed their utility as a monitoring tool, especially when paired with other methods. “We need as much information as we can possibly get if we are going to identify endangered species and head off the extinction crisis,” he said.

Researchers have traditionally made such identifications by catching animals, collecting hair or dung samples, or setting up camera traps. Capture stresses out and sometimes injures animal subjects, however, and hair and dung can be difficult to find.

Camera traps are the current gold standard in tropical rain forests, but they tend to require significant time and expense.

In the second study, published in The Journal of Applied Ecology and led by Weiskopf, then at the University of Delaware, the researchers set up 30 camera trap sites in four forest reserves in Bangladesh and captured 200 leeches in the same spots. While the leeches produced evidence of 12 species of mammals (including one that the camera traps missed), the cameras documented 26 species. But the cameras were rolling for nearly nine months and their work cost $24,800; work on the leeches, collected in just four days, came in at just $3,770.

Future studies, Tessler said, could be made even more efficient by blending hundreds of leeches into a slurry and genetically sequencing all of their blood meals in one go.

“I don’t know how big this will become, but I think leeches have quite a bit of potential,” he said. “This is just a fascinating method.”