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Video: Scientists discover new mode of ‘snake locomotion’

The brown tree snake has a fascinating trick hidden in its coils.

"Snakes. Why did it have to be snakes?"

Indiana Jones' fear of snakes — a contrast to his general fearlessness otherwise — has become a modern-day Achilles' heel, and the clips of Dr Jones flinching away at the asps in the Raiders of the Lost Ark has been turned into countless memes.

But if Jones had encountered the brown tree snake, he'd have a darn good reason to be petrified.

In new research published in the journal Current Biology, scientists reveal how the brown tree snake uses a seemingly distinct form of movement, known as "lasso locomotion," to climb larger cylindrical surfaces — like trees — and ensnare prey which would otherwise be out of reach.

A video created by researchers to explain lasso locomotion. Courtesy of the researchers

How they did it — Like many great scientific feats, the researchers made this unsettling discovery by chance.

While working on a conservation project focused on a local bird species — the Micronesian starlings — in Guam, the researchers tried to use a metal cylindrical device known as a baffle to keep brown tree snakes away from the birds.

But the brown tree snakes were able to get around the baffle by scaling the cylinder used in the study — a remarkable feat of athleticism which the researchers captured on video.

"We didn't expect that the brown tree snake would be able to find a way around the baffle," Thomas Seibert, study co-author and professor at Colorado State University, says in a press statement. "I had watched about four hours of video and then all of a sudden, we saw this snake form what looked like a lasso around the cylinder and wiggle its body up."

"It was a shocker. Nothing I'd ever seen compares to it."

How it works — Lasso locomotion involves the snake essentially looping itself around a cylindrical surface, and then coiling or crossing over parts of its own body to create a knot — a lasso — to tie off the loop.

According to researchers describing the motion in the video, the snake's head points upward, while it uses "small sideways bends" to create tension or friction which ultimately propels the snake upward. This lasso method creates a significant observable gap between the head of the snake and its tail.

This figure from the paper shows the snake as it progresses up the cylinder — in this case, a 15.2-cm-diameter tube.J. Savidge et al, Current Biology (2021)

What's new — Prior to this discovery, researchers had only known of four modes of snake locomotion: rectilinear, lateral undulation, sidewinding, and concertina.

The concertina method uses a similar friction grip to the lasso locomotion, but it lacks the unique sideways bends of the lasso style. Lasso locomotion enables the snake to grip the cylinder in only one location, whereas other locomotion methods require the snake to grip a surface in two locations.

"Our most important finding is a new mode of snake locomotion," co-senior author Julie Savidge and professor at Colorado State University, says in the statement. "Only four major types have been recognized for nearly 100 years, and we have discovered a fifth mode."

Although the snakes were able to evade the baffle that researchers used, the lasso method clearly takes a toll on the snakes, which were seen taking frequent pauses, slowing down, and heavily breathing — not unlike humans during a tough workout.

According to co-senior author Bruce Jayne, a professor at the University of Cincinnati, this mode of motion "is pushing them to the limits."

This photo shows a brown treesnake in a tree.Bruce Jayne

Why it matters — Understanding the brown tree snake's unique mode of locomotion can help us figure out why it's so good at killing local bird species in Guam. Following the introduction of the invasive snake in the 1940-50s, local bird populations in Guam began to plummet.

"Most of the native forest birds are gone on Guam," Savidge says. "There's a relatively small population of Micronesian starlings and another cave-nesting bird that have survived in small numbers."

The lasso mechanism likely aided the invasive snake in wiping out local bird species. The brown tree snake may be able to readily climb trees and access prey which would be inaccessible to other snake species that don't use lasso locomotion.

"Understanding what brown tree snakes can and cannot climb has direct implications for designing barriers to reduce the dispersal and some of the deleterious effects of this highly invasive species," Jayne says.

Furthermore, lasso locomotion allows us to see evolution in action, showing how innovative adaptations can allow predators to overcome the natural challenges of their habitat.

The study states: "Lasso locomotion illustrates how a behavioral innovation can potentially permit exploitation of resources that would otherwise be unavailable."

This photo shows a brown treesnake on a pole Bruce Jayne

What's next — The scientists would like to use this new understanding of lasso locomotion to save local bird species from the brown tree snake, and, ultimately, extinction.

"Given that brown tree snakes can use lasso locomotion to defeat poles or cylinders of a certain size, we can design baffles to better protect bird houses used for restoring some of Guam's birds," Seibert says.

Summary: The diverse ways and environments in which animals move are correlated with morphology, but morphology is not sufficient to predict how animals move because behavioral innovations can create new capacities. We document a new mode of snake locomotion —'lasso locomotion’ — that allows the brown tree snake (Boiga irregularis) to ascend much larger smooth cylinders than any previously known behavior. This lasso locomotion may facilitate exploiting resources that might otherwise be unobtainable and contribute to the success and impact of this highly invasive species.
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