Scientists have discovered a new method that some snakes use to climb trees.
The previously unknown mode of locomotion was observed in the brown treesnake, which forms a loop around a trunk with its body, then wiggles its way up the tree.
Snakes have four main modes of locomotion. The most well-known are ‘lateral undulation’ — which creates the signature serpentine S shape — and superficially similar ‘sidewinding’ motion used by rattlesnakes. There’s also the ‘rectilinear’ wave from muscle contraction (resembling an earthworm’s movement) and ‘concertina’.
Biologists previously thought that snakes could only climb smooth, vertical cylinders such as tree trunks using concertina locomotion, where the body is stretched out and grips the cylinder in two or more places, sticking to the surface through friction as the tail end is pulled up toward the head.
A new study has now revealed that snakes can also climb using ‘lasso’ locomotion.
After analysing videos of the brown treesnake (Boiga irregularis), researchers at Colorado State University and the University of Cincinnati observed that the animal adopts a posture that produces a large loop and shifts small bends along its lasso-like body to move upward — a bit like wiggling a wedding ring off your finger.
The behavior enables the treesnake to climb wider cylinders: whereas concertina locomotion uses at least two gripping regions, each as long as the circumference of a tree trunk, the single gripping region in a lasso only needs to be a little longer than the snake’s body length.
Despite its benefits, lasso locomotion is slow and requires a lot of energy. In the study, a snake would sometimes slip back down and often pause to catch its breath. It could only climb an average of four centimetres per minute.
The study is the byproduct of a conservation project that aims to protect native species on Guam, such as the Micronesia starling — one of only two forest birds that survived after bird populations were decimated when the brown treesnake was accidentally introduced to the Pacific island in the late 1940s or early 1950s.
Lasso locomotion would explain why the invasive species was so destructive to the island’s ecosystem, as its ability to climb trees might have allowed it to access eggs in nests — to exploit resources that were once out of reach from predators. (The snakes also climb electrical poles to find food, causing short circuits and power cuts.)
One upside of the discovery is that it could help conservationists protect the native species. A metre-long (three-foot) metal cylinder around a pole or trunk is normally big enough to prevent pests from climbing a tree, but the study showed that a cylindrical ‘baffle’ isn’t much of an obstacle for a brown treesnake.
Now that researchers know that lasso locomotion occurs, they can design better baffles — ones that will actually stop the invasive species from reaching endangered birds.