Non-planar snake gaits: from Stigmatic-starts to Sidewinding

TL;DR

This paper introduces a novel non-planar snake gait called the S-start, models its mechanics, and explores its role in the evolution of sidewinding, highlighting the influence of topology and body size on locomotion modes.

Contribution

It presents a new non-planar escape gait in snakes, develops a mathematical model for active non-planar filaments, and links this gait to the evolution of sidewinding.

Findings

Juvenile snakes can perform S-starts, while adults cannot, due to size constraints.

A propagating pulse of link density drives the S-start locomotion.

Periodic S-starts can naturally evolve into sidewinding gait.

Abstract

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.