Most people think they’re safe from leeches as long as they stay out of the water. Most people would be wrong.
In the rainforests of the South Pacific, there are ambitious, blood-slurping leeches that not only can inch their way across the land but can also climb weeds and trees—the better to sink their suckers into unsuspecting passersby.
I know, it’s enough to make you want to reach for your Elon Musk flamethrower. But wait! These land-loving leeches could help us save endangered species!
You see, every time a leech siphons off a bellyful of blood from, say, a dwarf mongoose, what it’s really doing is taking a DNA sample from a wild animal. And scientists can make use of these samples to determine what species inhabit a particular patch of forest.
Indeed, leeches also provide a few advantages that other sampling methods, like camera traps or sight surveys, do not. “Leeches can be collected in a day or two from a given site, and that is much faster than most methods,” says Michael Tessler, an evolutionary biologist and ecologist at the American Museum of Natural History. “Furthermore, they feed on a variety of large and small mammals, while camera traps typically have to be optimized for a given size of mammal.”
Finally, because leeches tend to consume a single large meal that takes a long time to digest, scientists don’t have to worry about untangling a bunch of different species from a single sample. This makes it relatively easy to sequence the DNA found within.
In a study published in February in Systematics and Biodiversity, Tessler and his coauthors collected around 750 leeches from the forests of southern Asia and extracted DNA from the region of the leeches’ guts that digests blood. In all, they found evidence of 31 species from 16 different families of forest dwellers.
According to the DNA, Asian palm civets were the parasites’ most treasured blood buffet across the range, with 19 leeches showing signs of having fed on the tiny viverrids. (You may have heard of civets because there’s an extremely expensive coffee made from their dung.) That’s not so surprising, since palm civets are listed under Least Concern by the International National Union for Conservation of Nature, which means there are plenty of them for the leeches to latch onto.
But what’s more interesting is that one leech scooped up in Cambodia provided DNA from an animal known as a gaur, or Indian bison. The International Union for Conservation of Nature considers the gaur to be vulnerable to extinction, and the entire population could be as low as just 15,000 animals. And one of them apparently got tagged by a leech.
Tessler’s slimy sentinels also picked up traces of DNA from the near-threatened Japanese quail, as well as from species that have been so little studied that we don’t even know whether they’re endangered, like Roosevelt’s barking deer and the rabbit-size Java mouse-deer.
Tracking species by leech isn’t a perfect methodology, unfortunately. Tessler says we’re limited to making matches with genomes scientists have already uploaded to the database. This means that some of the species identified by the leech method were likely not the actual species found in the woods, but genomes that closely resemble them. For instance, the study found evidence of lion-tailed macaques in Bangladesh, China, and Cambodia, even though this primate is known to live only in the Western Ghats of India.
Did the leeches discover some hidden populations of macaques? Probably not. More likely, these samples came from very closely related rhesus macaques or another similar species whose genome hasn’t been sequenced yet. “Our goal with our paper was to show that the method worked generally, not necessarily to provide exact matches,” says Tessler.
Still, the findings are pretty cool.
“I think using leeches for conservation and to sample for wildlife is ingenious,” says Anton Sorokin, a graduate student in biology at East Carolina University who was not involved in the study (but who has experience picking leeches off of some rather unfortunate places). “It seems like a great way to survey for secretive animals and gets the leeches to do the hard job of finding the wildlife for the researchers,” he says.
And there’s more: Not only does the prospect of sampling biodiversity via leech mean cost and time savings for biologists and less-invasive procedures for potentially endangered wild animals, but it could lead to a new appreciation of leeches themselves.
Leeches should be a conservation priority in their own right, says Mark Siddall, study coauthor and curator of segmented worms, or annelids, at the American Museum of Natural History. They are vastly understudied compared with larger, less-loathed creatures like elephants and whales.
In fact, of the leeches the team collected from Cambodia, the most common species was a beige, black-striped squiggly known as Haemadipsa trimaculosa that had never been recorded in Cambodia. Who knows what other species could be out there, and which of them may be endangered?
And that brings us to the best part of leech sampling. You don’t have to chase them down like howler monkeys or polar bears. You just have to take a stroll through the jungle and let the land leeches find you. “It can be overwhelming in a highly populated forest,” says Tessler. “[I]t almost feels like the forest floor is moving toward you.”
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