Q&A: Using Leeches to Track Rare Animals

Working with leeches doesn’t always suck—just ask Tom Gilbert.

A biologist at the Natural History Museum of Denmark, Gilbert has a newfound appreciation for the blood-thirsty creatures, the subjects of his latest research. By grinding up leeches that had been living in the Vietnamese rain forest, Gilbert and colleagues extracted DNA of the animals the leeches had feasted upon—including rare and shy mammal species.

The study—published in April in the journal Current Biology—may introduce a new tool for conservationists to monitor certain species of mammals, a quarter of which are threatened. Weird & Wild recently chatted with Gilbert about his work.

Q. Can you explain the purpose of your research?

A. The purpose is very simple—to contribute a new tool to the monitoring of threatened mammals. It’s a surprisingly big challenge to know what mammals live where, especially in tropical jungles where it’s so thick. Currently scientists do some tracking, or some use camera traps—infrared cameras that take photos when animals go past. My colleague Mads Bertelsen of the Copenhagen Zoo came up with the idea while in the forest monitoring tapirs—he got attacked by a leech.

A medicinal leech magnified eight times under a microscope. Photograph by Darlyne A. Murawski, National Geographic

Q. What did you do in your research, and what did you discover?

A. After my colleague had the idea, we wanted to see whether the idea was going to work, or if the problem was it would work only after the leeches ate. So we bought medical leeches and fed them goat blood—we basically left them in a little leech room where they sat and waited. Over time we killed them and extracted DNA [from them]. Even four months later we could still find goat DNA. This is what got us excited—the leech doesn’t have to have just fed to extract the DNA. Once we knew there were grounds for it to work, we asked another colleague, Nicholas Wilkinson, in [the Central Annamite region of] Vietnam to collect the leeches that try to eat him, then intercept them and put them in a liquid. He shipped us 25 [leeches]—to our great surprise, 21 of the 25 had mammal DNA, and some of them were interesting mammals.

Q. What do you mean by interesting?

A. We could have hit the everyday standard animals that run around the forest. We did hit pigs and deer, but we also found three other species that were actually interesting. The first is the small-toothed ferret badger. There are three ferret badger species [in Vietnam], but you can’t tell which is which without catching them and measuring their teeth. We also found two other species, one is called the Annamite striped rabbit, other called the Truong Son muntjac—both were discovered in the past decade or so. Extremely little is known about them, and despite the fact people had been monitoring [this region of Vietnam], they’ve never seen them. That’s the cool thing—what it suggests is the leeches are fairly generic in their mammal diet, and secondly it suggests you can get a nice picture of what mammals are present without setting up surveys, camera traps, or ranger patrols. [This new leech method] can detect things the conventional methods don’t pick up at all.


Leeches and goat blood used in the experiment. Photograph courtesy Mads Bertelsen

Q. Can you tell me some stories of working with leeches?

A. One of the real problems in this project was feeding the leeches. I’ve worked in the rain forest, I’ve been eaten to death by leeches—you never get used to it when you find a bloody, squishy leech on you. We read somewhere that if you fill condoms with blood and keep them in a warm place, leeches will find it. [But as it turns out], persuading the buggers to drink the blood is really hard—my poor student spent days trying to feed leeches. Actually I used to be quite disdainful of leeches, [but] nowadays I’m getting quite attached to them. They’re quite remarkable—some are meant to live years or decades. I feel guilty grinding up this thing that was meant to live for decades.

Q. Do you know why they didn’t want to eat?

A. [Not exactly, but it’s possible] they didn’t appreciate being forced up next to a rubbery condom.

Q. What is the next step?

A. What we’ve done now is shown that the basic concept works. … It would be nice [if the method could reveal] different numbers of individuals, for example if we could see there are five rhinos in the area, or three tapirs in the area. We also want to apply it to different regions—we want to start sampling in Southeast Asia and Madagascar. [The idea is to] simply use it for conservation, [for example gathering population data on a particular species]. There’s a lot that can be done from a conservation point of view. The nice thing about this method is you don’t have to pick [a species that] you’re looking for. What you say is, I’m going to see if there’s mammal DNA, and then it tells you what mammals [are there]. It’s very nice and generic in that way.

Q. Anything else you think our readers would be interested in?

A. Another thing is we like to make the point that this [method] is actually quite cheap and efficient. [For example], camera traps are expensive equipment, but we can collect many leeches for the cost of a camera trap.

The “tyrant king” leech’s anterior sucker, with opening from which large teeth (not shown) emerge during feeding. Photograph courtesy PLoS ONE

As a side note, I’m guessing Gilbert’s glad he doesn’t have to work with the “tyrant king” leech (pictured above), which was found in Peru in 2010. The critter uses its teeth to saw into the tissues of mammals’ orifices, including eyes, urethras, rectums, and vaginas.

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Meet the Author
Christine Dell'Amore, environment writer/editor for National Geographic News, has reported from six continents, including Antarctica. She has also written for Smithsonian magazine and the Washington Post. Christine holds a masters degree in journalism with a specialty in environmental reporting from the University of Colorado at Boulder. Her book, South Pole, was published in 2012.