Can leeches leap? New video may help answer that debate

A chance video by a grad student relishing her first big field trip might help resolve an argument that’s raged among biologists for more than a century. The question: Can leeches jump?

Yes, at least one kind of leech can, says Mai Fahmy, a conservation biologist now at the American Museum of Natural History and Fordham University in New York City. She has twice videoed some tiny, plain brown, land-crawler Chtonobdella leeches in Madagascar doing what she sees as jumping as they navigated the rainforest’s low-hanging and fallen leaves.

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The first time, in 2017, “I came back to New York with what I thought was just a neat little video of a leech doing something it always does,” she says. She had, however, stumbled upon evidence related to what Michael Tessler, an AMNH leech specialist, calls “one of the top two or three most contentious ‘facts’ about leeches.”

That phone video, plus a second one shot in 2023, have opened a new chapter in the contradictory natural history lore of leech locomotion going back centuries, Fahmy and Tessler report June 20 in the journal Biotropica.

In the 1300s, Islamic traveling chronicler Ibn Battuta noted that, in Southeast Asia, a “flying leech” lurks on trees and weeds near water “and leaps to the person who happens to pass it.” Yet doubters at least as early as 1886 dismissed this as just mistaking “falling” for “flying,” if indeed leeches really were able to get up into trees and shrubs to begin with.

One hurdle to resolving the debate is that “it’s surprisingly hard to define jump,” says Tessler, also of City University of New York’s Medgar Evers College. “For us, it was about active propulsion.” And yes, he ranks the problem as “something that a leech biologist would sit around in a bar talking about, for sure.”

Leeches belong to the same animal class as earthworms and have similar sex organs. Some leeches sport chocolatey streaks on orange backgrounds or a pair of sky-blue, lightning-bolt zigzag stripes zinging over darkness. Bites can bleed for several minutes, but “I’ll take a leech bite over a mosquito bite any day,” Fahmy says. She’s interested in analyzing blood that leeches collect as a way of detecting hard-to-spot biodiversity such as lemurs (SN: 4/6/22).

Of the 800 or so named leech species now known, many stay in water, but the jumping debate involves land dwellers. Fahmy and Tessler propose that a jump is “an intentional movement that muscularly propels the organism outward and/or upward.” That definition, they say, could apply to vertebrates as well as to squishy legless life.

When Mai Fahmy shot this video on her cell phone in a Madagascar rainforest in 2017, she figured it was just routine leech footage. Turns out it may be some of the best evidence yet that some leeches can jump. Here, she says, the leech appears to first take a tiny hop on the leaf and then coil and leap off the leaf to the forest floor.Mai Fahmy

The leech species they’re applying the definition to, however, is not one of the very few previously proposed leapers such as Southeast Asia’s storied Haemadipsa picta “tiger leech.” Instead, this candidate is one of Madagascar’s ho-hum brown Chtonobdella prowling near the rainforest floor. In her 2017 phone video, an upright leach sways, makes a small move toward the edge of a leaf and then goes into the air over the edge.

The big question about leeches, Tessler explains, is whether they “just kind of would tumble off” after releasing their grip. “What we believe these videos, we hope, pretty obviously show, is that this is a forceful movement.” For instance, before the jumps, the leeches “coil,” he says. It’s not a scrunching down but more a sort of rearing back, as a snake might before launching a strike.

Now other observers will be able to muse over the videos.

Chris Darling, a senior curator of insects at the Royal Ontario Museum in Toronto got an early peek at the 2017 video. Darling, who has described a novel jumping motion in fly larvae, saw the leech motion as “a clear jump.”

But Sheila Patek, whose evolutionary biomechanics lab at Duke University has studied jumping caterpillars, summarizes her reaction to the 2017 video in an email as “Hmmm!!!” She sees the motion as “something in between falling, controlled aerial descent, and directed launch. I honestly cannot tell whether that’s a jump or not.”

So whether this will end the historic back-and-forth on the question remains up in the air, but at least the phone video updates the puzzle for the digital age.