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Jul 27th 2021

Brown Tree Snake Invasion Powered by Lasso Locomotion

Sometime during the waning days of World War II or the years just after, humans accidentally introduced a brown tree snake population to the tiny Pacific island of Guam. The snakes, native to New Guinea and parts of Australia, probably had concealed themselves in military cargo delivered to the newly liberated island.

The impact of the invasive snakes on the ecosystem of Guam was devastating. Lacking natural enemies, the snake population of the island has swelled to some 2 million, as Science Alert reports. Local bird populations had no natural defenses against the intruding snakes and were decimated — 10 of 12 species have vanished. Loss of the birds, which helped to scatter seeds, has in turn drastically reduced new forest growth.

And just in case a plague of snakes was not enough, Reptiles magazine notes that the absence of birds has allowed spiders to proliferate unchecked on Guam.

Along with environmental devastation have come other ill effects, such as frequent electricity outages due to snakes climbing power poles. But Guam’s ongoing struggle against the brown tree snake recently led to an unexpected discovery: a previously unknown way that snakes can climb trees or other cylindrical objects.

A Different Way to Slither

Scientists thought they understood snake locomotion pretty well. According to Science magazine, in the last hundred years, observers had seen and defined four basic ways that snakes get around — not counting accidental transport by humans.

These four well-known methods of slithering are lateral undulation, rectilinear locomotion, sidewinding and concertina (so named, per Smithsonian Magazine, for a cousin of the accordion).

Wildlife biologists looking for ways to provide birds with safe places to nest tried mounting nesting boxes atop lengths of exhaust-duct piping turned upright. A brown tree snake, they reasoned, would not be able to climb the large, smooth-sided cylinders.

But the snakes kept getting to the birds. As National Geographic notes, the research team captured an attack on time-lapse video. The video showed a brown tree snake wrapping its body around a cylinder like a lasso, then using the loop of the “lasso” to scooch its way up the cylinder.

“We just kind of looked at each other in shock,” research team member Tom Seibert told National Geographic. “I mean, this wasn’t something a snake was supposed to be able to do.”

What a Hungry Snake Can Do

For a brown tree snake, as SciTech Daily reports, lasso locomotion is no easy task. Progress is slow. The snakes often slip back in their upward progress; they breathe hard and must frequently pause to rest. “Even though they can climb using this mode, it is pushing them to the limits,” says Bruce Jayne, another member of the research team.

But while it may push them to their limits, the snakes succeed in using lasso locomotion to climb poles or other cylinders, including trees, which they couldn’t climb using their four previously known means of movement.

This demonstrates two fundamental truths. One is that a meal is a powerful motivator. The other is that we know less about the evolution of snakes than we thought we did.

We now know that snakes, or at least brown tree snakes, have five means of locomotion — not just the four we previously knew about. But, as National Geographic notes, we don’t yet know whether other snakes share this capability or if the brown tree snake uses this method in its natural habitat.

It will take many years of research to answer the new questions that have now been raised about the evolution of snakes. And, as usually happens with science, every answer we find is likely to raise more questions.

For now, the good news for Guam’s remaining birds is that the discovery of lasso locomotion will help us design more effective barriers to keep snakes from climbing up to attack their nests. It may even help to prevent snakes from causing electricity outages after lassoing their way up power poles.

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