<P> When push - points are absent, but there is not enough space to use sidewinding because of lateral constraints, such as in tunnels, snakes rely on concertina locomotion . In this mode, the snake braces the posterior portion of its body against the tunnel wall while the front of the snake extends and straightens . The front portion then flexes and forms an anchor point, and the posterior is straightened and pulled forwards . This mode of locomotion is slow and very demanding, up to seven times the cost of laterally undulating over the same distance . This high cost is due to the repeated stops and starts of portions of the body as well as the necessity of using active muscular effort to brace against the tunnel walls . </P> <P> The movement of snakes in arboreal habitats has only recently been studied . While on tree branches, snakes use several modes of locomotion depending on species and bark texture . In general, snakes will use a modified form of concertina locomotion on smooth branches, but will laterally undulate if contact points are available . Snakes move faster on small branches and when contact points are present, in contrast to limbed animals, which do better on large branches with little' clutter' . </P> <P> Gliding snakes (Chrysopelea) of Southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees . These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair . </P> <P> The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning . In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them . Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin . The ribs of the snake do not move in this mode of locomotion and this method is most often used by large pythons, boas, and vipers when stalking prey across open ground as the snake's movements are subtle and harder to detect by their prey in this manner . </P>

Where do most snakes live in the world