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Guam’s invasive tree snakes loop themselves into lassos to reach their feathered prey

The brown tree snake is an ecological menace on the island of Guam, where it has gobbled up nearly all the native birds since its accidental introduction more than 70 years ago. To save the U.S. territory’s remaining birds, researchers placed nests on top of smooth poles they were sure no snake could climb. But they were in for a nasty surprise: In 2016, video captured the snakes climbing the cylinders, with a never-before-seen lassolike gripping technique.

“To say I was surprised would be an understatement,” says Bruce Jayne, a biologist and snake expert at the University of Cincinnati. “I’ve never seen anything like this.”

The movements of snakes have long been classified into four types: concertina locomotion, lateral undulation, rectilinear locomotion, and sidewinding. Tree-dwelling snakes use the concertina mode to climb: They wrap around a vertical surface with two separate parts of their body while sliding between them. To succeed, snakes must be nearly as long as the circumference of the cylinder they’re ascending, Jayne says, because they must wrap their bodies into two gripping regions and extend or pull as they crawl.

But in the new mode, the nocturnal brown tree snake (Boiga irregularis) hugs its body around the cylinder in one big grip and crosses over itself to make the namesake lasso, Jayne and colleagues report today in Current Biology. It then uses its many vertebrae to make exquisitely fine-tuned propulsions upward, wriggling its way up the pole (see video, above).

After their shock wore off, the scientists tested the snakes in an enclosed concrete arena. They set up an enclosure of 91-centimeter-tall cylinders, each with a tasty rodent treat on top. Then, they let loose 58 recently captured brown tree snakes and recorded their movements using infrared video.

Not all of the snakes—which ranged in length from 99 to 193 centimeters—actually climbed, notes Julie Savidge, an ecologist at Colorado State University, Fort Collins, and a co-author of the study. Some made the lasso, but couldn’t move themselves up the poles, whereas others just hung out on the ground below. And the snakes that did lasso their way up did so slowly, at rates of less than 1 centimeter per second, and they seemed to be huffing and puffing as they rippled upward.

The lasso locomotion is completely unlike any other known mode of movement in snakes, says David Cundall, an expert in snake morphology at Lehigh University who was not involved with the new research. Savidge thinks this adaptation could help the snake climb smooth-trunked trees in the species’ native range throughout Indonesia and northern Australia, and she hopes to someday catch an endemic snake doing it.

Cundall cautions that, so far, this climbing has only been shown in one species—the brown tree snake—but says it suggests snakes have “extraordinary” neurological adaptations yet to be discovered.

“We have no human technology that can come close to what this snake is doing,” agrees Daniel Goldman, a biophysicist at the Georgia Institute of Technology who was not involved with the study. “They’re not just simple slithering machines.”