Of the vast variety of animal gaits, one of the most striking is the non-planar undulating helical motion of a sidewinder. But non-planar helical gaits are not limited to sidewinders. Here we report a new non-planar gait used as an escape strategy in juvenile anacondas (Eunectes notaeus). In the S(tigmatic)-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. We present a mathematical model for an active non-planar filament that interacts anisotropically with a frictional substrate to quantify our observations and show that locomotion is due to a propagating localized pulse of a topological quantity, the link density. A phase diagram as a function of scaled body size and muscular torques shows that relatively light juveniles are capable of S-starts but heavy adults are not, consistent with our experimental observations. We further show theoretically that a periodic sequence of S-starts naturally leads to sidewinding. All together, our characterization of a novel escape strategy in snakes using non-planar gaits highlights the role of topology in locomotion, provides a phase diagram for gait feasibility as a function of body size, and shows that the S-start forms the fundamental kernel underlying sidewinding.

翻译：在种类繁多的动物步态中，最引人注目的莫过于侧向蜿蜒蛇类（Sidewinder）所展现的非平面螺旋运动。然而，非平面螺旋步态并非侧向蜿蜒蛇类的专利。本文报道了幼年黄水蚺（Eunectes notaeus）在逃逸策略中采用的一种新型非平面步态。这种以独特形状命名的“S形启动”（S(tigmatic)-start）表现为：蛇身在不打滑的情况下，围绕其身体中段向前滚动，同时进行平面外的扭动与弯曲，从而产生瞬态运动。我们构建了一个主动非平面细丝的数学模型，该模型与摩擦基底呈各向异性相互作用，用以量化观测结果，并证明这种运动源于拓扑量——连接密度（link density）的局域化传播脉冲。以缩放后的体型和肌肉力矩为参量的相图表明，体型较轻的幼蛇能够完成S形启动，而成年重蛇则不能——这一结果与实验观测一致。我们进一步从理论上证明，S形启动的周期性序列会自然演化为侧向蜿蜒运动。总而言之，本研究对蛇类利用非平面步态的新型逃逸策略的特征描述，凸显了拓扑结构在运动中的作用，提供了步态可行性随体型变化的相图，并揭示出S形启动构成了侧向蜿蜒运动的基础核心。