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Preys capture by chameleons through viscous adhesion
Several predators use an adhesive tongue to catch preys. Among theseones, chameleons are able to capture large preys by projecting the tongue at high acceleration. Once in contact with a prey, the tongue retracts with a comparable acceleration to bring it to the mouth. A strong adhesion between the tongue tip and the prey is therefore required during the retraction phase to ensure a successful capture. The mechanism responsible for this strong adhesion remains however unclear.Interlocking, where the roughness of both prey and tongue surfaces self-adjust to make physical crosslinks, and suction mechanism, similar to the one at play in rubber suction pads, have been proposed to supplement viscous adhesion. To investigate the mechanism responsible for this strong bond, the viscosity of the mucus produced at the chameleon’s tongue pad was measured by using the viscous drag exerted on rolling beads by a thin layer of mucus. It appears that the viscosity of this secretion is about 400 times larger than the one of human saliva. Knowing this viscosity, a dynamical model for viscous adhesion describing the motion of the compliant tongue and of the prey during the retraction phase was build. The variation of the maximum prey sizewith respect to the chameleon body length is then derived. This dependence is finally successfully compared with in vivo observations forvarious chameleon species and shows that the size of the captured preys is not limited by viscous adhesion thanks to the high mucus viscosity and the large contact area between the prey and the tongue.