Younger sidewinders weave identically to their elders

Abstract

The verb ‘snaking’ could have been invented for sidewinder rattlesnakes (Crotalus cerastes). Keeping two points of the body in contact with the ground at all times, each rippling serpent lurches its head forward as it initiates the next wave, while the weaving back end is still completing its previous wriggle. ‘It's nearly impossible for human eyes to make sense of how sidewinders move’, says Jessica Tingle from the University of Akron, USA, and it was unclear whether the animals modify their writhing manoeuvres as they grow. ‘As we grow, legged animals have to support more weight’, says Tingle, explaining that adults tend to move in quite different ways from their offspring as a result. However, snakes require little additional support as they grow, remaining largely in contact with the ground, so Tingle, engineering graduate Brian Sherman and Theodore Garland Jr. (University of California, Riverside, USA) decided to find out whether the serpents adjust their manoeuvres as they grow.‘We spent hours walking around in the desert at night with a flashlight searching for the snakes’, says Tingle, who often followed distinctive sidewinder tracks for 1 km in the hope of finding an animal. Fortunately, when Tingle and colleagues from Rulon Clark's lab (San Diego State University, USA) found a snake, the animals were usually pretty docile. But the scientists still took no chances, using snake tongs to pick them up and gently anaesthetising each animal to record their vital statistics – ranging from head size to body length and girth – before painting 10 dots along the snakes’ bodies to keep track of their manoeuvres. Then Tingle filmed the animals moving in 3D. ‘It's way too hot to have sidewinders outside during the daytime, so we stayed in shacks that had previously been used by the U.S. Air Force and I set up a sandbox and cameras in mine’, she recalls. After filming 66 serpents writhing across the sand, Tingle selected movies of 14 females, ranging in length from 253 to 688 mm, and 12 males, from 226 mm up to 553 mm. Then, Sherman wrote a MATLAB program to untangle the fine detail of the snakes’ sinuous movements. ‘As the program processed each trial, it displayed an animation of digitized points’, says Tingle.Surprisingly, when the trio compared the manoeuvres of the young and adult rattlesnakes, the youngsters were moving almost identically to the older animals; the movements of young sidewinder rattlesnakes are ‘petite versions of the adults, making exactly the same shapes with their bodies at a smaller scale’, says Tingle. And the youngsters achieved similar average speeds (0.18 m s−1) to their elders (0.21 m s−1), despite being several times smaller, ‘contrary to many results from limbed terrestrial animals, where large individuals move appreciably faster than small ones’, she adds. And when the trio compared the animals’ speeds as they travelled across the sand with how many ripples they performed per second – the frequency – and the distance they lurched forward at the beginning of each ripple – their effective stride length – the scientists found that the snakes barely extended each ‘step’ when moving at higher speeds. Instead, they rippled faster and the slenderest snakes formed tighter curves than the stouter animals.Comparing the snakes’ dimensions, the team also found that the smaller males had larger heads in proportion to their bodies than the females, which could be related to the snake's diet. ‘Because they swallow prey whole, a snake's range of potential prey is limited by its gape’, says Tingle, explaining that smaller males may require larger heads to gobble down the same-sized morsels that female sidewinders swallow with ease.