Tiny Southeast Asian Snakes Roll Four Times Faster Than They Slither
Dwarf reedsnakes escape danger by rolling across the ground like a wheel, and filming shows the trick is how they keep their heads still.
Most snakes get away from danger by slithering, pushing themselves along in the familiar side-to-side way. Dwarf reedsnakes in Southeast Asia have a second option. They roll across the ground like a wheel, and they do it about four times faster than they can slither.
Four times faster than slithering
Researchers filmed the snakes from several angles and measured how fast they travelled. While rolling, the snakes reached peak forward speeds of between 1.72 and 3.69 metres per second. That is roughly four times their fastest speed when they slither, using the side-to-side motion that most snakes rely on. The videos were then studied closely to see how each part of the body moved during the roll. Speed was not the only surprise. The snakes kept to a narrow racetrack measuring just 20 centimetres across, which shows that the movement is steered rather than an uncontrolled tumble.
A stroke, not a simple spin
The recordings showed that the snakes do not spin their whole bodies at once. Instead they alternate between rolling on one side and swinging a curved section of the body downward beside the head. The researchers called this downward sweep a stroke. On each repeat of the cycle, the side of the body that touches the ground changes, and the stroke switches direction from left to right or right to left. Repeating that carefully timed pattern is what produces continuous forward rolling. The species studied, Pseudorabdion saravacense, belongs to a group of small Southeast Asian snakes known as dwarf reedsnakes. They are among the few animals known to roll deliberately as a way of escaping a possible predator. The behaviour had appeared in field guides and had been seen in a field observation, but the body movements that made it possible had not been worked out before.
A steady head, and no axle
A wheel can spin because it turns around a fixed axle, which separates the rotating part from whatever it carries. An animal has no such part. If an entire body spins, the head and the sense organs inside it spin too, which makes balance, steering and judging direction much harder, and the problem grows as the rolling gets faster. The dwarf reedsnakes avoid this by dividing the work. While the rear portions of the body rotate rapidly, the head stays pointed in the direction of travel instead of turning with the rest. The snakes also keep the head at a fairly constant height above the ground, and at higher speeds they tend to hold it up and reduce upward and downward movement. That steadiness, the researchers suggest, helps the animals stay in control and steer during a quick escape, and it may help them work out which way they are going. There is no axle and no wheel, only a body that behaves as though it had both.