From just 25 leeches, the team found DNA from six mammals, including pigs, cows, the small-toothed ferret-badger, and the threatened goat-like serow. They also found DNA from the Annamite striped rabbit, which was discovered in 1999 and had (at the time) never been seen in over 2,000 hours of camera-trap recordings. And they even detected DNA from the Truong Son muntjac, a small deer that was discovered in 1997 and has never been seen in the flesh. “That suggests that these animals aren’t as rare as we think or that the leeches are very good at finding them,” says Gilbert.

All conservation efforts are predicated upon knowing what lives where, and that knowledge is hard to come by in dense, rugged forests, whose residents are often rare, reclusive, and wary of humans. In such places, direct observation is almost impossible. You could interview villagers, but that’s challenging and often imprecise. You could pepper an area with camera-traps, but you’d have to lug heavy and expensive batteries to and fro. And all of these methods might miss small, tree-dwelling, or burrowing species.

Leeches, however, seem to feed on everything, and so miss little. Collecting them is also cheap, fast, and requires no special skills; as Gilbert says, “The leech collector simply offers his/herself up for bait.” Just by standing around in a rainforest for a day, a lucky collector (or unlucky, depending on how you look at it) can attract hundreds of leeches.

Attracting funding is a bigger problem. “The funny thing about leeches is that I’ve spectacularly failed to get anyone in Denmark to give me any money to study them,” says Gilbert. Fortunately, others have had better luck. Douglas Yu from the University of East Anglia and China’s Kunming Institute of Zoology, has enthusiastically leapt onto the leech-sequencing bandwagon, bringing money and extensive sequencing facilities with him. His involvement allowed the team to scale up.

They are now working with the WWF in Vietnam to find traces of the saola—a recently discovered, rarely seen, and critically endangered antelope that’s also known as the Asian unicorn. Yu has also been working with the Forestry Department of Yunnan Province to train an army of leech collectors. Some two hundred of the departments’ rangers have been walking around, plucking leeches from their bodies, stashing them in rubber pouches, and noting their GPS locations along the way.

Through their efforts, Yu ended up with some 20,000 leeches. Once preserved, the worms turn into dense, rubbery pellets that are too painful to pulverise by hand—the team resorted to small blenders. They then analysed the resulting slurry using a technique called metabarcoding, which amplifies small sections of DNA that vary distinctively between species, and compared these sequences against existing databases. They discovered sequences from martens, bears, tree shrews, mice, mongooses, monkeys, deer, cats, and more.