Turning without fins: Quantifying the distinct kinematics and vortex dynamics of maneuvering swimming snakes

The agile turning maneuvers of anguilliform swimmers remain one of the least studied aspects of aquatic locomotion, despite being crucial for survival in underwater environments. To change direction, animals must find a balance between minimizing their moment of inertia and maximizing torque. Anguilliform swimmers, like snakes, are highly maneuverable and accomplish this goal without the aid of limbs or pectoral fins. In this study, we quantify the differences between turning and forward swimming kinematics and hydrodynamics. We present flow field and body deformation measurements taken simultaneously by combining a volumetric three-component defocusing digital particle-tracking velocimetry system with a camera mounted above the tank for swimming kinematics. We determine how snakes manipulate their body position to change swimming direction by analyzing the swimming kinematics of six turns by six individuals as 2D projections (five Natrix maura and one Nerodia rhombifer). Analyses of the body kinematics show that snakes curl their body inwards to shift their center of mass and reduce their moment of inertia to perform faster turns. The volumetric wake measurements allow us to quantify the timing between the turning kinematics and vortex shedding and to compare it with forward swimming measurements. Our results indicate that snakes can generate substantially stronger vortices when turning; for example, the vortex force can be as much as 2.5-8.6× larger when compared to forward swimming. We also use volumetric wake measurements to show that the vortices shed during a turn have the same tubular vortex pair composition as forward swimming. These results help explain how elongated anguilliform swimmers change swimming direction. They widen our understanding of how animals use transient maneuvers to navigate aquatic environments, with implications for both natural swimmers and underwater robots.