When biologists found three locally endangered Micronesian starlings dead in their nest box on Guam in 2017, the culprit was obvious. The birds are a frequent target of invasive brown tree snakes. The confounding part was how a snake managed to get into the nest in the first place. The nest box sat at the top of a steel duct pipe that scientists thought was too large for the brown tree snake’s usual climbing tactics.
But an infrared trap camera pointed at the nest provided CCTV-like time lapse footage of the break-in: a snake had looped its body around the duct pipe and squirmed to the top in just over 15 minutes. It was the first time that wildlife biologists Thomas Seibert and Martin Kastner had seen the behavior in the wild.
But the year before, the scientists had witnessed the behavior in a lab. While trying to find strategies to stop snakes from reaching nest boxes, the scientists had placed a three-foot-tall, eight-inch-wide stovepipe over the top half of a six-foot-tall metal pole. They fastened a wide platform with two live mice in a cage at the top and placed the contraption in an enclosure with 58 snakes. When reviewing time-lapse footage of the set up taken at night, they saw a snake wrap its tail around the pole, grab the other end of its body to form a secure loop and shimmy up to the top.
“We looked at each other in total shock because this was not anything that we expected or had ever seen,” says Seibert. “We had to watch this over and over just to be sure that we were seeing what we thought we were seeing.”
The unexpected climbing strategy is a unique form of snake locomotion that has never been seen before. The scientists describe the snakes’ movement, which they dub “lasso locomotion,” in a study published today in the journal Current Biology. “I never would have thought in my wildest dreams that a snake would move in this fashion,” says co-author and University of Cincinnati biomechanics specialist Bruce Jayne, who has studied snake locomotion for over 40 years.
The finding provides new insights into why brown tree snakes have been so devastating to birds on Guam, and will help conservationists devise new tools to protect the birds, like Micronesian starlings, that remain.
In order to better understand lasso locomotion, the researchers set up a new experiment in 2019 at the United States Geological Survey’s brown tree snake laboratory in Guam that would encourage the behavior. They swapped the large stovepipe for a smaller, six-inch-diameter stovepipe from Home Depot, and topped the pipe with a cage containing a dead mouse as bait. They placed the pipe in an enclosure housing 15 brown tree snakes.
Five of them latched on and made the climb using lasso locomotion.
The discovery had immediate consequences. Biologists quickly relocated nest boxes that had been placed on poles that were the same size or smaller than the stovepipe used when the behavior was first observed, says co-senior author and Colorado State University wildlife biologist Julie Savidge. The team also presented the first lasso-climbing video at an annual meeting of brown tree snake researchers in 2016.
“I think everyone was amazed by the video,” says Iowa State University biologist Haldre Rogers, who studies the impact of bird losses in Guam and wasn’t involved in the new study. “This is the first time that people had spent a lot of time looking at how [the snakes] climb cylinders.”
Brown tree snakes are nocturnal and spend most of their time balancing on branches in the treetops. They’re well-adapted for spanning the open space between branches in order to get around the forests in both their native habitat in northern and eastern Australia and New Guinea, and Guam, where humans accidentally introduced them in the 1940s.
Snakes normally climb trees using concertina locomotion, named for the accordion’s hexagonal cousin. In concertina climbing, snakes grip a tree trunk or pole at two anchor points. Like rock climbers, snakes use their upper bodies to pull themselves upward, and then establish a grip with their lower bodies. When their lower grip is stable, snakes repeat the process to scale the structure.
Unlike snakes using concertina locomotion, lasso-climbing snakes have only one anchor point, the loop around the cylinder. A slight bend in the lasso moves along the snake’s body, from its head toward its tail, slowly shifting the snake upward and creating its steady ascent.
Lasso locomotion joins four other types of snake locomotion that have been recognized for more than 100 years. The method is the slowest and most strenuous way for snakes to move. On average, it takes a snake about two hours to climb just ten feet, says Savidge. The snakes take frequent breaks during the strenuous tactic to catch their breath. Sometimes those breaks last for 10 to 15 minutes.
Now that scientists know that brown tree snakes can climb this way, they can create better protections for the birds living in Guam.
“It opens up more potential for us to be able to develop snake proof things, whatever it is we want to keep snakes out of,” says Rogers. “We know that they can get access to all sorts of things, but the larger telephone poles are relatively impervious to snakes.”
Because large utility poles are not present across all of Guam, Savidge and Seibert are also testing new nest box structures on thin metal poles that are protected by a cone that is smallest at the bottom and flares at the top. The idea is that if a snake tries to lasso-climb the cone, it would need to loosen its grip as it climbed, which would make it fall.
The new devices would be distributed across the island to help rebuild the Micronesian starling population. By the 1990s, brown tree snakes had driven ten species of Micronesian birds locally extinct on Guam. Micronesian starlings are the only remaining tree-dwelling birds on the island, and they are limited to two areas: Andersen Air Force Base, and a small island off the southwestern tip of Guam that the snakes invaded in 2019.
The loss of Guam’s local bird species has caused havoc to its ecosystem because birds spread seeds and eat insects. Rogers’ research showed that new tree growth has declined by up to 92 percent after the loss of seed-dispersing birds.
The lasso locomotion shows just how creative snakes can be when they are faced with a new challenge.
“Sometimes I think we focus on other characteristics of invasives, such as their diet or habitat,” says Savidge. “And we need to remember that the locomotor abilities are also very important in success of the species.”
The discovery opens up several paths for future research. Scientists may want to find out whether brown tree snakes in their native range also show lasso-climbing abilities, whether other snake species have the same climbing skills and whether lasso locomotion may lend itself to engineering.
“This definitely could have interesting applications in robotics,” writes Emory University physicist Jennifer Rieser, who studies snake-inspired soft robotics, in an email. Once scientists figure out more details, including how the invasive predator moves up without slipping, Rieser writes, “this newly discovered mode of locomotion could definitely expand snake robot maneuverability by enabling robots to climb big objects, which could provide access to previously inaccessible locations.”