Of the vast variety of animal gaits, one of the most striking is the non-planar undulating motion of a sidewinder. But non-planar gaits are not limited to sidewinders. Here we report a new non-planar mode used as an escape strategy in juvenile anacondas (Eunectes notaeus). In the S-start, named for its eponymous shape, transient locomotion arises when the snake writhes and bends out of the plane while rolling forward about its midsection without slippage. To quantify our observations, we present a mathematical model for an active non-planar filament that interacts anisotropically with a frictional substrate and show that locomotion is due to a propagating localized pulse of a topological quantity, the link density. A two-dimensional phase space characterized by scaled body weight and muscular torque shows that relatively light juveniles are capable of S-starts but heavy adults are not, consistent with our experiments. Finally, we show that a periodic sequence of S-starts naturally leads to a sidewinding gait. All together, our characterization of a novel escape strategy in snakes highlights the role of topology in locomotion, provides a phase diagram for mode feasibility as a function of body size, and suggests a role for the S-start in the evolution of sidewinding.

翻译：在众多动物的步态中，非平面蛇型振动运动是最引人注目的之一。但非平面步态不仅限于侧行蛇。在这里，我们报道了一种新的非平面模式，它被水蟒的幼体(Eunectes notaeus)用作逃避策略。称为 S-起动，在其名义形状下，当蛇扭动并弯曲时，它会在不滑动的情况下绕其中部滚动而超出平面。为了量化我们的观察结果，我们提出了一个用一个与摩擦性基质异向相互作用的主动非平面丝状物的数学模型，并展示了运动是由一个拓扑量，即链密度的传播局限脉冲所致。一个由身体重量和肌肉扭矩组成的二维相空间表明，相对较轻的幼蛇能够用 S-起动，但重的成年蛇则不能，与我们的实验结果一致。最后，我们展示了 S-起动的周期序列自然地导致了侧滑步态。总之，我们对蛇的新的逃避策略的表征凸显了拓扑在运动中的作用，为模式的可行性提供了一个作为身体大小的函数的相图，并暗示了 S-起动在侧滑演化中的角色。