Sidewinding Locomotion in Snakes

Abstract

THERE is a widespread belief that the curious horizontal broadside movement of Crotalus cerastes, known as is a specific adaptation to locomotion on the sandy surfaces of the desert. The persistence of this theory results in part at least from acceptance of the assumption that there is actual advantage in this rolling type of movement as opposed to the conventional and supposedly less efficient sinuous means of progression. In the rolling, locomotion that simulates the motions of the flattened coils of a helix, (Mosauer, 1932; Van Riper, 1955) action and reaction forces usually are spread over two segments of the body rather than several separate, smaller body loops. Since to move any body of given mass will require the same thrust against the ground, it might be argued that many separate thrust points would prove more efficient on unstable slopes with shifting ground than would the same amount of thrust when combined and applied by only two segments as in sidewinding. If there is any real advantage to sidewinding it might lie in the difference between amount of friction during movement of the snake's belly against sand and reduction of this by the rolling movement of the sidewinder. Because of the glossy surface of the gastrosteges and the light weight of the body as contrasted with the mass of the sand required to withstand the thrust, it is highly dubious if sidewinding is actually any more efficient on loose sand than the standard sinuous movement. In fact it might be demonstrably, but minutely less efficient. I have made no exhaustive search of the literature to discover the origin of the term sidewinding nor the first instance of ascribing to it more effective locomotion on sand. However, it seems probable that Cope may have been the first to mention the term in technical literature. He reports the discovery by Dr. J. L. Wortman of the identity of the snakes referred to as sidewinders by Arizonans, but he says nothing regarding this type of locomotion as being adaptation to loose sand. Ditmars (1907) says Here we see Nature's 21 admirable provision to enable a heavy-bodied snake to progress at some speed over desert sands. That this is the only fashion in which a thick-bodied snake could display agility on soft soil is well illustrated by the fact that their singular movements have been adopted by the vipers of the African deserts and again far removed from them, by our one species of rattlesnake that inhabits the deserts proper. (The italics are his.) Later workers have shifted the emphasis from speed to the soft soil aspect, but Ditmars emphasis on speed may be nearer the truth. Pope (1937: 19, 111) possibly following Mosauer (1932), merely alludes to this trait as adaptation to travel on loose sand. Gloyd (1940) mentions the desert adaptations of our sidewinding rattlesnake, but he does not fall into the positive error of accepting shifting sand as the prime cause for the success of this peculiar locomotion. Stebbins (1954) carefully notes that this snake is not restricted to wholly sandy situations, for it has been found on bare hardpan and in areas where there are scattered rocks on firm soil. Usually, however, some sand is present nearby. He also mentions its presence on sand hills and rocky hillsides as at Lovejoy Buttes on the Mojave Desert, and he circumspectly states that an advantage of this type of locomotion may be to minimize slippage on a shifting surface, and calls attention to the fact that the locomotory forces are applied vertically to the surface on which the snake is traveling. Cowles (1920), quoted in extenso by Van Denburgh (1922), plunged headlong into the pitfall of what may prove to be the seemingly obvious, and ascribed to this type of locomotion a specific adaptation to movements over a sandy, hence a yielding, and for conventional ophidian motion, a nearly tractionless surface. Since 1919 and 1920, years of growing familiarity with the sidewinder, that is, throughout the last 35 years since publication of the 1920 paper from which Van Denburgh quoted, there have been many occasions