The ‘vine snakes’, a group of superb tree climbers comprising numerous
colubrid lineages, climb using gap-bridging, and can cantilever up to half
their body into open space until their head reaches another branch.
Adaptations for gap-bridging include slender, laterally-compressed bodies
and large vertebral scales that prevent the body from bending dorsoven-
trally. Typically, specialist tree-climbing snakes have slender bodies and
relatively long, prehensile tails for coiling around branches and providing
anchorage as they extend their bodies forwards during concertina
locomotion. Arboreal snakes also have belly scales which span the entire
width of the body. On each side there is a notch, creating a fold where the
belly scales meet the smaller dorsal scales, and forming a ventrolateral
keel. This allows such snakes to modify their tubular shape so that in cross
-section they are flat across the bottom, and curved above. The ventro-
lateral keel together with the overlapping belly scales are highly effective
at grasping irregularities. These snakes are therefore able to scale steep
gradients despite lacking supporting structures other climbing animals
possess. The ventrolateral keel is present but less developed in snakes that
occasionally climb (e.g. Corn Snakes), but is lacking in ground-dwelling
snakes, which are round in cross-section and must expend considerably
greater energy climbing, as they maintain a tight grip whilst slowly inch-
ing their way upward.
Left: a Boa Constrictor; rectilinear locomotion is most common in large
snakes like boids and vipers. Image by Sanne vd Berg Fotografie.
Below left: Eastern Racers (Coluber constrictor) use seven times more
energy when employing concertina locomotion, compared with lateral
undulation. Image by Matthew L. Niemiller.
Right: Brown Tree Snakes (Boiga irregularis) are elite climbers. Image by
Michael Cermak.
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