Brown tree snakes (Boiga irregularis) appear to have evolved a whole new way of moving that could help explain how they have decimated bird populations in the Mariana Island of Guam, according to a new paper published in the journal Current Biology.
They use a lasso-like motion to propel themselves up smooth vertical cylinders, which the researchers say is distinctively different to the other four known forms of snake locomotion: rectilinear, lateral undulation (or serpentine), sidewinding and concertina.
The nocturnal snakes, which are part of the colubrid family (Colubridae), are native to north-eastern Australia and other humid tropical areas such as Papua New Guinea. They were accidently introduced to Guam, about 2000 kilometres north of PNG, around mid last century, probably as stowaway on a cargo vessel or aircraft.
Like many other invasive species, their impact has been devastating.
“Most of the native forest birds are gone on Guam,” says lead author Julie Savidge, from Colorado State University, US.
“There’s a relatively small population of Micronesian starlings and another cave-nesting bird that have survived in small numbers.”
The snakes have also caused widespread damage and regular power outages across the island.
Savidge and colleagues discovered the legless reptiles’ unique mode of moving when working on a project to protect starling nests. This involved using a metre-long metal baffle (cylinder) to keep them from climbing up to the birds’ boxes – a strategy that has been used successfully with other snakes and raccoons.
At first, the baffles worked. But then the incredulous researchers watched a snake get around it. Co-author Thomas Siebert explains that he and biologist Martin Kastner “had watched about four hours of video when all of a sudden, we saw this snake form what looked like a lasso around the cylinder and wiggle its body up.
“We watched that part of the video about 15 times. It was a shocker. Nothing I’d ever seen compares to it.”
To confirm it was a unique movement, they sent videos to snake locomotion expert Bruce Jayne, from the University of Cincinnati.
Normally, snakes climb steep, smooth branches or pipes using a concertina movement, bending sideways to grip the cylinder in at least two places. Using the lasso locomotion, they use a loop to form a single grip and simultaneously move upwards.
“The snake has these little bends within the loop of the lasso that allow it to advance upwards by shifting the location of each bend,” Jayne explains.
To do this, the researchers write, the snake would need a body length at least as long as the cylinder’s diameter. In Australia, brown tree snakes and other species can’t climb smooth bark trees with diameters larger than 70 centimetres to get to bird nests.
“While they could not climb these very large trees,” they add, “lasso locomotion would facilitate climbing trees within their body length limits and allow access to prey that may not be available to less capable snake species.”
It’s no easy feat, apparently: “Slow speeds, slipping, frequent pausing and heavy breathing during pauses all suggest lasso locomotion is demanding.”
The researchers hope their discovery will help them outsmart the snake and create a barrier that they can’t get past to preserve Guam’s remaining bird species.
Meanwhile, Jayne is taking a fresh look at the anatomy and physiology of snake locomotion – and reconsidering its limits. He thinks lasso locomotion is an entirely new way for snakes to get around. “Lasso locomotion is in outer space,” he told the New York Times. “It’s different enough that it doesn’t fit into any of the four [known] categories.”