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：在动物步态的丰富多样性中，最引人注目之一便是侧向蜿蜒蛇的非平面起伏运动。但非平面步态并不仅限于侧向蜿蜒蛇。本文报道了幼年南美水蚺（Eunectes notaeus）使用的一种作为逃逸策略的新型非平面模式。在以其同形形状命名的S形启动中，当蛇在无滑动情况下绕其躯干中部向前滚动、同时扭动并弯曲出平面时，便会产生瞬态运动。为量化我们的观察结果，我们提出了一个主动非平面细丝的数学模型，该细丝与摩擦基底各向异性相互作用，并表明运动是由拓扑量（即连接密度）的传播局部脉冲驱动的。一个以缩放体重和肌肉扭矩为特征的二维相空间表明，相对较轻的幼体能够执行S形启动，而沉重的成体则不能，这与我们的实验一致。最后，我们证明S形启动的周期性序列自然会导致侧向蜿蜒步态。综合来看，我们对蛇类新型逃逸策略的表征，凸显了拓扑学在运动中的作用，提供了模式可行性与身体尺寸关系的相图，并暗示了S形启动在侧向蜿蜒演化中的角色。