Snake tongue flicking : A fluid mechanics approach

Why do snakes flick their tongues? Although well known as a chemosensory behaviour, the fluid dynamical underpinnings of this fast oscillatory movement remain obscure. Here, we present an experimental three-dimensional fluid mechanics investigation of tongue flicking in snakes. The snake tongue geometry was approximated as a simple y shape : two tapering cylinders emerged from a stalk, with varying angles between them. Each geometry was oscillated in a circular arc at two different peak velocities in a quiescient fluid (water). Two Reynolds numbers (Re 38 and 126) based on peak velocities were investigated. The lower Re corresponds to scaling calculations from previous data on tongue oscillations in air of banded water snake (Nerodia fasciata), and the larger is chosen arbitrarily to explore the change in physics with greater peak velocities. Using Lagrangian flow analysis, we observe complex three-dimensional vortical structures where longitudinal vortices trailing the tongue tines (branches of the Y shape) interact with those parallel to the direction of tongue movement. Vortex patterns indicate a flow field which might promote greater odorant sampling resulting from the y-shaped geometry of the snake’s tongue. To further understand the dynamics, we conducted a modal analysis. This involved decomposing the flow into spatial modes of varying length and time scales, capturing both small-scale and large-scale structures.