We didn’t necessarily need more direct evidence that brown tree snakes are some of the serpent clan’s champion climbers, but we’ve got it – and with it some fresh insights into snake biomechanics and some new ideas for protecting the beleaguered native birds of Guam.

Brown tree snakes are known for their arboreal abilities. Image © Pavel Kirillov

A study published this month in Current Biology describes a newly defined technique by which snakes – well, brown tree snakes, anyhow – climb a cylindrical object: “lasso locomotion.” The discovery came through a combination of field observations and laboratory experiments related to this colubrid snake’s status as one of the world’s textbook examples of a destructive exotic species.

The lithe, cat-eyed brown tree snake, native to northern Australia, Papua New Guinea, eastern Indonesia, and Melanesia, was inadvertently released onto Guam in the late 1940s or early ‘50s. It flourished in this largest Micronesian island’s tropical dry forests, and then some: Population densities beyond 20 snakes per acre have been recorded.

That flourishing came with a terrible ecological cost, as the night-hunting snake decimated Guam’s birds and lizards. Thirteen of Guam’s 22 native bird species have disappeared from the island, including the endemic Guam flycatcher and Guam rail (which, however, was conserved via captive breeding and has lately been introduced to nearby, snake-free isles). Those few birds that remain, such as the Micronesian starling and the Mariana crow, are still being hammered by the invasive serpents.

Protecting the nests of native birds is paramount, but that’s a tall order given the brown tree snake’s superb arboreal abilities. In 2017, according to Smithsonian, a camera trap on Guam showed a snake raiding a starling nest box perched atop a broad duct pipe thought to be snake-proof. The surprising manner by which the snake ascended this pipe – basically as a shimmying-up loop – jibed with footage captured the year before during a laboratory experiment, and inspired the more in-depth research that produced this new paper.

Until now, scientists had accepted four basic categories of snake locomotion: rectilinear, lateral undulation, sidewinding, and concertina. That last one, concertina locomotion, is the primary climbing movement employed by most arboreal snakes. The concertina technique sees a snake use two anchor points: coiling around a tree trunk or pole, extending its head upward to wrap around a higher point, and relaxing its “downhill” grip only when the “uphill” one is secure to repeat the routine.

Several brown tree snakes in the Current Biology experiment used a completely different method to reach bait (dead mice) set atop cylinders between 15 and 20 centimetres wide. The lasso locomotion observed involves the snake looping nose-to-tail around the cylinder, then nudging upward by propagating a slight bend through this body “lasso.”

A comparative look at the climbing tactics of snakes. © Savidge et al.

Lasso-climbing, which requires just one anchor point, appears to permit brown tree snakes to hoist their way up trees or poles too broad for concertina locomotion. “Using a single large loop to grip,” the study authors write, “lasso locomotion allows snakes to climb cylinders more than twice the diameter than would otherwise be possible.”

But it’s tedious, creeping, and apparently exhausting work. “Slow speeds, slipping, frequent pausing, and heavy breathing during pauses all suggest lasso locomotion is demanding,” the paper notes. Indeed, one of the authors, Colorado State University (CSU) Emeritus Professor Julie Savidge, told Smithsonian that a snake requires roughly two hours to lasso-climb 10 feet, and it may rest for 10 to 15 minutes at a time while doing so.

Another of the study’s authors, Bruce Jayne of the University of Cincinnati, said in a CSU press release, “Even though [snakes] can climb using this mode, it is pushing them to the limits.”

Lasso-climbing may have evolved among brown tree snakes so they could better scale the smooth-barked trees common in their native range, and thus exploit bird nests and other arboreal food sources in canopies off-limits to other snakes because of girthy and slippery tree trunks. (The Current Biology paper notes, however, that smooth-barked boles greater than 70 centimetres in diameter seem to mostly stymie even the formidable lasso locomotion.)

The brown tree snake’s lasso-climbing prowess on smooth poles not only helps explain its tenacious ability to attack bird nests, but also likely the frequency with which it causes electrical outages on Guam.

The findings are prompting a reevaluation of the designs currently used to protect nest boxes on Guam from the invasive reptile. One idea, Smithsonian relates, is a metal nest-box pole armoured with a cone that flares upwards to overwhelm a lasso-climbing snake’s looped grip.

In the CSU press release, Savidge said, “Hopefully what we found will help to restore starlings and other endangered birds, since we can now potentially design baffles that the snakes can’t defeat. It’s still a pretty complex problem.”

Jayne, one of the foremost experts on the biomechanics of how snakes get around, marvelled at the revelation of lasso-climbing. “I’ve been working on snake locomotion for 40 years and here, we’ve found a completely new way of moving,” he said in the press release. “Odds are, there is more out there to discover.”

Header image: Gordon H. Rodda