Brown tree snakes have driven five-sixths of Guam’s bird species to local extinction since they arrived by stowing away on planes 70 years ago. If they reach other Pacific Islands, particularly Hawaii, it is feared they could trigger a similar birdpocalypse. Part of these snakes’ deadly effectiveness lies in a never-seen-before form of motion that allows them to climb where other snakes cannot.
Snakes make up for their leglessness with four forms of locomotion (besides gliding and jumping), known as rectilinear, lateral undulation, sidewinding, and concertina modes documented for almost a hundred years. Professor Julie Savidge of Colorado State University has now observed another, which she and co-authors have called “lasso locomotion,” described in Current Biology.
Savidge is working to preserve Micronesia starlings, one of only two species of birds that have so far survived the havoc the brown tree snake has wrecked on Guam. Even the starlings are barely holding on by their claw tips, and their loss would have serious implications. “The starling serves an important ecological function by dispersing fruit and seeds which can help maintain Guam's forests,” Professor Savidge said in a statement.
Savidge and others have been building snake-resistant bird boxes but found the snakes getting into unexpected places. Night-vision videos of the snakes on the climb revealed the back half of the snake looped around a tree or pole while the front stretches upwards, like a reverse lasso.
Other snakes deploy concertina motion to tackle steep climbs, but even this has limits. By making small bends within the lasso and shifting the bends’ location the brown tree snake (Boiga irregularis) can climb cylinders too wide, relative to the snake’s length, for other techniques.
"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," said co-author Professor Bruce Jayne of the University of Cincinnati in a statement.
Jayne notes, however, even brown tree snakes find this new mode a struggle. "Even though they can climb using this mode, it is pushing them to the limits," he said. The snakes were seen to stop frequently and breathe heavily on lasso-climbs, sometimes slipping back.
The researchers are keen to use their new-found knowledge to design a baffle beyond even this snake’s capacity. This might not only save the starling but allow the reintroduction of some of Guam’s lost bird species from snake-free islands.
Another important topic will be whether brown tree snakes are unique in their lasso-climbing, or if other members of the cat-eyed snake family can do the same. It was once thought brown tree snakes might have done so much damage on Guam thanks to their distinctive venom. More recent research has shown that although their venom is a hundred times more lethal to birds than mammals, the same is true of many of their relatives, suggesting these pose a similar risk to island populations. However, it remains to be seen if other snakes have a similar capacity to reach nesting sites.