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Brown tree snakes in Guam have been seen climbing poles by making their bodies into lasso shapes, a type of snake locomotion that hasn’t been documented before.

Five invasive brown tree snakes (Boiga irregularis) were spotted climbing vertical metal cylinders about 15 to 20 centimetres in diameter that were holding up bird boxes. To do so, they wrapped their bodies around the pole and hooked their tail around their midsection, allowing them to wiggle upwards to reach prey.

“The loop of that lasso shape is effectively where all of the action is, because that loop squeezes on the cylinder to prevent the snake from slipping down,” says Bruce Jayne at the University of Cincinnati in Ohio.

Only four other types of locomotion have been observed in snakes until now. Compared with the other forms, this new type is “painfully slow”, says Jayne.

The movement was first spotted when Julie Savidge and Thomas Seibert at Colorado State University were developing safe nesting sites for native birds in Guam. They used a bird box that was perched atop a vertical pole and surrounded by a smooth metal baffle – a protective shield – to defend birds against predators.

“[The research team was] watching video from during the night and, lo and behold, most of the snakes couldn’t defeat the baffle,” says Savidge.

“All of a sudden, [one snake] latched on and started wiggling up,” says Seibert. “The upper part of the snake that’s not in the lasso is constantly searching for the next nubbin or something it can grip onto.”

Lasso locomotion allows these snakes to reach otherwise unobtainable prey, but seems to be energetically demanding for them, as they take several pauses when climbing. “I broadly estimated that it might take around 2 hours to go 10 feet with this sort of movement,” says Savidge.

“I would’ve liked to know how widespread this type of locomotion is and if all snakes are able to exhibit this behaviour,” says Luca Börger at Swansea University in the UK.

Seibert believes that lasso locomotion could be a learned behaviour that arose as smooth metal poles were introduced to their habitat, while Jayne thinks it might be an innate skill.

Gordon Rodda at the US Geological Survey’s Fort Collins Science Center is tasked with managing the invasive brown tree snakes on Guam. He says these animals are good at climbing power poles, and have caused power outages in Guam and Hawaii. “This new work provides actionable intelligence for designing power systems that are impervious to the snake, a big payoff for a comparatively small research project,” he says.