Chameleons catch food with sticky spit

Chameleons produce mucus on their tongue that is about 400 times more viscous than human saliva, which might explain how these predators can capture prey up to a third of their body weight, reports a paper published online this week in Nature Physics. The multidisciplinary study combines experimental data with a mechanical model to show that viscous adhesion is sufficient to account for the bond between the chameleon’s tongue and its prey during successful capture.

Chameleons are opportunistic predators that remain motionless and hidden until they sense active prey within striking range. The projectile tongue motion they use to reach prey up to distances twice that of their body length is well understood. The means by which the tongue adheres to their prey, however, remains unclear. Several models have been proposed, including suction, and a crosslinking mechanism that implicates the rough surfaces of both prey and tongue.

Pascal Damman and colleagues show that adhesive properties of the chameleon’s mucus may suffice in the absence of these other factors. The team determined the viscosity of the mucus secreted at the chameleon’s tongue pad. Based on this measurement, the authors built a model to describe how the tongue might exploit this high viscosity to capture prey via a mechanism that compares favourably with observations of different chameleon species. The authors conclude that, due to the high mucus viscosity and the large contact area between the prey and the tongue, viscous adhesion is not a limiting factor on the size of captured prey.