Why Snakes Flick Their Tongues: A Fluid Dynamics Approach

The rapid, oscillatory tongue-flicking of snakes has long fascinated both scientists and the general public, and is arguably one of the most famous aspects of snakes. However, we know very little about how tongue-flicking functions in snakes. The tongue itself serves to collect odor molecules from the environment and transport these molecules to the paired vomeronasal organs that open in the mouth of snakes. Using a combination of high-speed video and digital particle image velocimetry, I describe the behavior of tongue-flicking, and how the rapid oscillations of the tongue provide an advantage for the collection odor molecules. In the garter snake, Thamnophis sirtalis, tongue-flicking consists of multiple oscillations of the tongue moving in a semicircular pattern. The tips of the tongue remain rigid throughout the entire tongue-flick, and rotate about the hinge region. All of the bending occurs in the body of the tongue, posterior to the forked tips. During tongue-flicking, two pair of counter-rotating vortices form at the top and bottom of the tongue-flick. Between the pairs of vortices, regions of high-velocity air flow pull air into the path of the tongue, and quickly jet the air out of the top and bottom of the system. Both the vortices and air flow patterns take several oscillations to form, but persist throughout the entire tongue-flicking sequence. This system of manipulating air movement greatly enhances a snake’s ability to collect odor molecules from the air. To test the importance of this flow pattern, I added four additional species of closely related snakes that encompassed a large range of body sizes (Coluber constrictor, Nerodia sipedon, Diadophis punctatus, Storeria dekayi). Surprisingly, I found that the vortices and flow pattern observed in T. sirtalis was present during tongue-flicking in all snakes. The kinematics of tongue-flicking changed across species, larger snakes had slower tongue-flicks with fewer oscillations. I suggest that the vortex formation and air flow patterns that develop during tongue-flicking are vital to the sensory biology of snakes, and that changes in body size and tongue morphology are compensated for by changes in the tongue-flicking behavior to maintain the unique pattern of air movement