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Why Brown Tree Snakes Don't Mind the Gaps

The brown tree snake (Boiga irregularis) has ravaged the ecosystem and infrastructure of Guam for nearly 70 years—annihilating 10 of the island's 12 native bird species and costing billions of dollars in damage to electrical power systems. The invasive reptile's destructive habits come from its incredible ability to cross gaps between trees, some as long as 2.2 vertical meters. But new findings on the biomechanics behind the snake's bridging ability may help halt the destructive creatures.

Bruce Jayne, a biomechanist at the University of Cincinnati in Ohio, has been researching brown tree snakes for many years and has investigated how the slithering reptiles can bridge 1.5 meter gaps between horizontal planes, such as two branches at the same height on adjacent trees. But to find a way to stop Guam's nemesis, he and Gregory Byrnes, a biomechanist from Siena College in Loudonville, New York, changed their focus. "All you need to do is look out of a window at a tree to convince yourself that if you've got an animal moving in a tree, it certainly doesn't conform to a horizontal plane," Jayne says. The duo wanted to understand how the snakes move in their complex, three-dimensional environment.

Their research, published today in the Journal of Experimental Biology, provides insight into what lets the snakes cross horizontal and vertical gaps and what causes them to fail. Tree-dwelling snakes need to be good at crossing gaps in order to catch prey in the spread-out canopies of the forest. To test the animals' bridging skills, the team set the snakes on perches in front of various obstacles ranging from vertical ascents and short drops, to horizontal gaps and sharp turns.

Closing the gap. A brown tree snake bridging a gap that is 82% its body length while leaving 90% of its body length unsupported.
Credit: Bruce C. Jayne and Gregory Byrnes

The snakes triumphed over vertical gaps, crossing ones that were 65% wider than horizontal gaps. In fact, one snake surprised the scientists by successfully spanning a vertical opening that was 82% of its body length. However, the snakes had difficulty holding their body parallel to the ground, which is why they could cross only horizontal lengths that were at most 58% of their body length. The snakes could also cross straight horizontal gaps that were 13% longer than horizontal gaps with turns. "It turns out that raising your body is not nearly as hard as supporting a long part of your body out straight," says Byrnes.

The researchers calculated the torques, or turning forces, that each gap exerted on the snakes' bodies. Too much torque could cause the snake to fail when trying to cross a gap. Torques increased as the center of mass of the unsupported region of the snake was farther away from the support. Vertical gaps presented less torque and horizontal gaps presented more.

By gripping small pegs on the perches with their prehensile tails, the snakes could even cross gaps that they couldn't physically reach across by lunging toward them, much the same way that rock climbers do when they swing their bodies toward an out-of-reach ledge. The forward motion enables a snake and a rock climber to get across greater distances than if they tried to extend themselves while staying still.

John "Jake" Socha, a biomechanist from Virginia Polytechnic Institute and State University in Blacksburg who studies the biomechanics of gliding snakes, was surprised that the brown tree snake had the muscular control to pull off vertical feats. He says that the lunging technique used by the reptile is similar to the gliding technique used by the gliding snake. It shows that the snakes independently evolved the ability to cross gaps in creative ways, he notes. "Before you run you have to learn to walk, and for snakes, before you fly you have to learn to cross gaps."

The team's findings could help keep the destructive snake from causing more economic damage in Guam, says Gordon Rodda, a herpetologist with the United States Geological Survey in Fort Collins, Colorado. He says that the findings are critical for improving gap-based barriers that the island uses to keep the snakes off of power lines and away from cargo ships and planes. Snakes are already responsible for causing blackouts every few days by stretching from trees onto power poles and then completing an electrical circuit as they cross insulators between the poles and power wires.

Jayne and Byrnes advise cutting back vegetation around the barriers to create gaps that the snakes couldn't cross. They point out that the Guam snakes can be 3 meters long, and can cross gaps that are 1.5 meters wide horizontally and 2.2 meters wide vertically. Rodda warns that if the brown tree snake got to Hawaii, it would cause billions of dollars a year in damage because of power outages. "The design of the barriers is based off of rather fragmentary information on how the brown tree snakes can climb across barriers," says Rodda, "so the work that Dr. Jayne is doing is very useful for making the design of those barriers rigorous in light of the climbing capabilities of this species."