Is viscous adhesion strong enough to allow prey capture by chameleons

Chameleons are able to capture very large preys by projecting the tongue and retracting it once it is in contact with preys. A strong adhesion between the tongue tip and the prey is therefore required during the retraction phase to ensure a successful capture. To determine the mechanism responsible for this strong bond, the viscosity of the mucus produced at the chameleon's tongue pad is measured by using the viscous drag exerted on rolling beads by a thin layer of mucus. The viscosity of this secretion is found to be about 400 times larger than the one of human saliva. With a dynamical model for viscous adhesion describing the motion of the compliant tongue and of the prey during the retraction phase, the evolution of the maximum prey size with respect to the chameleon body length is derived. This evolution is successfully compared with in vivo observations for various 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.