Discerning how snakes climb trees

When climbing a tree, a snake thinks “safety first,” according to new research published in Biology Letters. Instead of gripping the tree with just enough force to keep from sliding back down, snakes overcompensate and grip with a force that far exceeds what’s necessary.

Researchers think that in doing this, snakes are choosing between getting up the tree more easily and decreasing their risk of falling.

Greg Byrnes, a herpetologist at Siena College in New York and lead author of the paper, realized that although scientists knew that snakes climbed trees and had a rough idea how they did it, no one knew exactly how much force the snakes generated to climb or how they determined how much force to use.
Byrnes set out to solve that mystery and was surprised by what he found.

“Climbing vertically isn’t easy, as anyone who has climbed a rope in gym class can attest. Reaching new heights requires a lot of energy. Still, for plenty of animals, climbing is well worth the cost. For a snake, being safe is way, way more important than being cost-effective,” he said. For snakes, some of which are arboreal species and spend most of their time in trees, it can be a way both to escape predators and to catch their prey.

Factors

How an animal climbs a tree depends on its physical features. Cats, for example, can grip a tree with their claws, which helps make climbing easier.

Humans have to rely on muscle strength to exert enough force to keep from falling.Although snakes don’t have limbs, they also use muscular force to climb trees, which they create by firmly wrapping their bodies around the trunk of a tree.

Byrnes and colleague Bruce Jayne at the University of Cincinnati measured grip strength in the lab by building a 94.4” tall (240 cm) cylinder to act as a tree trunk. They placed pressure sensors on various parts of the “trunk” and wrapped it with a textured tape.

They filmed the process as five different species of snake climbed the “tree.” Some of the species, such as Morelia nauta, live almost their entire lives in trees. Others, such as Boa constrictor, spend a lot of time in trees as juveniles to hide from predators, but then come down as adults.

Because snake bodies are long and thin, the animals could wrap themselves around the artificial tree trunk in a variety of orientations, from wrapping evenly around the trunk to bunching most of their body at one height.

However the snakes did it, Byrnes and Jayne found that all five species used far more force than was strictly necessary to keep their bodies from sliding back down - sometimes almost three times as much, the researchers report.

Byrnes believes that this extra-strong grip, which requires extra energy, benefits the snake by decreasing its chances of falling. The dangers of a fall, Byrnes says, are less about direct physical harm and more about exposure.

“A 10-metre fall is unlikely to really hurt a snake, but being back on the ground could expose them to predators. Then the snake will have to climb the tree again, and it might be more energy efficient to be more careful the first time,” Byrnes said.

Byrnes believes that strategies to prevent falling are the rule in the animal kingdom, rather than the exception. So the next time your arms start shaking as you climb that rope in gym class, remember that your muscles are fighting the age-old battle to stay safe at great heights.