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Brown tree snakes use their tails as loops to climb wide trees

Snakes do much more than slip. Some swim, while others make a side wind on the sand (SN: 10/9/14). Some snakes even fly (SN: 29/06/20). But no one has seen a snake move the way brown tree snakes do when they climb certain trees. By wrapping its tail around a tree or pole in a loop-like grip and twisting to propel itself, a brown tree snake can make structures that would otherwise be too wide to climb.

A better understanding of how brown tree snakes (Boiga irregularis) move could serve as strategies to control their population in Guam, where snakes are an invasive species. Reptiles are famous for destroying almost all birds in the native forest of Guam and often cause power outages as they climb public service poles.

The discovery of the brown tree snake loop climbing method, reported online Jan. 11 in Current Biology, was something serendipitous. Julie Savidge, an ecologist at Colorado State University in Ft. Collins and colleagues were investigating ways to keep these tree-climbing snakes away from the Micronesian starlings of Guam, one of the two remaining native forest birds on the island.

One such form consisted of testing to see if a wide tube or baffle around a pole could prevent predators from reaching a starling nest box at the top. By reviewing the hours of deflector footage to control how well it deterred brown tree snakes, the team saw a snake do something totally unexpected: the snake pulled itself around the baffle and began firing upwards.

“We were in total shock,” says study co-author Thomas Seibert, also a Colorado state ecologist. "This is not supposed to make a snake."

Images of a brown tree snake, captured with an infrared camera at night, reveal the unique way this species climbs: the snake laces itself around a pole and twists the tail loop to propel itself upwards. The technique allows the reptile to move structures that would otherwise be too wide to climb.

Brown tree snakes and other snakes usually climb trees too smooth to slip by curling around a trunk several times. A snake wraps the front of its body around the trunk and then wraps its back around the tree in another loop to get a second grip. The snake extends its neck upwards and repeats the process up to inches upwards. But wrapping a tree several times limits the width of a tree that a snake can climb. Using a single, large, loop-like grip allows the brown snake to climb wider – or baffling – trees, explains study co-author Bruce Jayne, a biologist at the University of Cincinnati in Ohio.

In later laboratory experiments, researchers observed several brown tree snakes using this loop posture when placed inside an enclosure with a wide pole topped with a dead mouse as bait. But the loop climbing method is not very efficient. Five brown tree snakes, between 1.1 and 1.7 meters long, rose less than a millimeter per second, on average.

“It was extremely exhausting (for snakes). You could see them breathing heavily and taking frequent breaks, "says Seibert. As a result, snakes are likely to use the bow movement only on rare occasions when they find trees or poles too wide and smooth to be scaled otherwise.

Using infrared cameras to see brown tree night snakes at night, the researchers observed reptiles using a strange technique to climb wide poles: snakes (a photo) used their tails as lassos to grab a pole and move upwards.J. Savidge et al / Current Biology 2020

It’s “a little bold” to see how a snake moves like that, says Gregory Byrnes, a biologist at Siena College in Loudonville, New York, who didn’t participate in the work. But Byrnes is not entirely surprised that brown tree snakes design a way to deal with broad trees or baffles. These snakes are notoriously agile climbers who can bridge large gaps, glide over steep surfaces, and traverse fine wires. “They have so much control over their bodies that if they are presented with a challenge … they discover a way to (overcome it),” he says.

Testing the limits of snake agility in brown trees could help design new baffles or other tools to protect endangered birds in Guam, Savidge says. Already, after researchers placed several boxes of birds on utility poles on the island that were too wide for brown tree snakes to launch upward, "the birds adopted these bird houses and did it very, very well." she said.

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A better understanding of the repertoire of snake climbing techniques could also be better for robots, says Henry Astley, a biologist at Akron University in Ohio who did not participate in the work.

Astley and others are interested in building snake-like robots to navigate terrain too difficult for robots or paws or wheels to traverse (SN: 16/11/12). It predicts snake bots sliding through the rubble of the earthquake to search for survivors or fighting inside large machines to conduct inspections. Discovering new and clever ways that real snakes exploit their incredible flexibility could help engineers make better use of serpentine machines.

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