Lasso locomotion expands the climbing repertoire of snakes

1. What they did

Brown treesnakes are a highly invasive species. The authors filmed brown treesnakes, using infrared video, as they climbed smooth vertical cylinders 15 to 20 cm across, far wider than snakes were known to be able to climb. They described the body posture and movement the snakes used, measured how fast they climbed, and compared this with the concertina climbing of brown treesnakes, Corallus hortulanus and Python reticulatus on smaller cylinders.

2. What they found

A new way of climbing

• The snakes formed a lasso: the head and neck pointed uphill, above a single large loop of the body that wrapped around the cylinder and gripped it.
• The body always crossed over itself at least once in the loop, though the exact arrangement varied a lot. No snake climbed the large cylinders without forming this crossover.
• Small bends to the left and right travelled backwards along the loop. These moved part of the body uphill while shifting where the snake gripped.
• The loop often slid down a little, even though it held a grip for a moment at a time.

Slow and hard work

• Climbing was extremely slow: on average 0.40 cm per second during each movement cycle (5 snakes).
• The snakes paused often, so over longer periods they climbed even more slowly. Slow speeds, slipping, frequent pauses and heavy breathing during pauses all suggest that lasso locomotion is demanding.

How it differs from concertina climbing

• In concertina climbing, a snake sets up a new grip above before letting go of the grip below, and each grip needs a length of body about as long as the cylinder’s circumference. This limits the size of cylinder a snake can climb.
• Lasso locomotion lacks the large stretching and bunching of the whole body, lacks the build-up and release of a separate upper and lower grip, and moves by bends travelling along the gripping loop.
• With one large loop, snakes can climb cylinders more than twice the diameter that would otherwise be possible.

3. What they concluded

• Lasso locomotion is a new and distinct mode of snake locomotion, not a type of concertina locomotion. For nearly 100 years, snake movement has been placed in four modes: rectilinear, lateral undulation, sidewinding and concertina.
• The bends within the loop may let snakes keep muscle tension in the loop while shifting their grip, and may let them use their large side-bending muscles to tighten the loop. The crossover appears to interlock and improve the grip.
• Brown treesnakes still cannot climb very large smooth trees (over 70 cm across), but lasso locomotion would help them climb trees within their body length limits and reach prey that less capable snakes cannot.
• Together with their skill at bridging gaps, swimming, climbing steep surfaces and crawling along thin branches and wires, this ability may add to brown treesnakes’ damaging impacts, such as wiping out native vertebrates on Guam and short-circuiting electrical lines.
• The study shows how a change in behaviour can greatly expand what an animal is able to do.

6. Other technologies, skills or innovations

• Infrared video of the climbing snakes, used to map body posture and measure vertical movement over time.
• A video of the climbing and a video abstract are available with the paper online.

7. Implications for conservation (from the paper)

• Versatile movement may widen the range of places an invasive species can use and make its ecological impacts worse.
• Knowing that brown treesnakes can climb large smooth cylinders can be applied to protecting native species and vulnerable infrastructure.