Chameleons are ambush predators that stay motionless and hidden until an ant or perhaps a smaller lizard comes within striking distance. They can capture prey weighing a third of their own body mass – and from a distance of twice their body length. According to a new Nature Physics study, chameleons have their spit to thank for that as the mucus on their ballistic projectile tongues is 400 times more viscous than our saliva.
The highly specialized feeding system of chameleons allows them to whip their tongue out at high accelerations ranging from 300 to 1,500 meters (nearly 1,000 to 5,000 feet) per second. Once the tongue is in contact with the prey, it retracts with a comparable acceleration, bringing the tasty meal directly into the chameleon’s mouth. This retraction phase requires an incredibly strong bond between the tip of the tongue and the prey item. Exactly how the tongue sticks to prey remains unknown.
To investigate, a team led by Pascal Damman of Université de Mons studied the mucus secreted by glands on the tongue pad of five veiled chameleons (Chamaeleo calyptratus). A thin layer of this mucus was collected from the chameleons onto a microscope slide by placing them in front of prey. The slide was then tilted at an angle, and a steel ball was allowed to roll freely down the slide while the team recorded the motion using a high-speed camera. This allowed them to measure the viscous drag exerted on the rolling bead.
Turns out, the mucus slowed the ball down dramatically. The team found that the viscosity of the secretion was about 0.4 Pascal-seconds. For comparison, the viscosity of human saliva is 0.001 Pascal-seconds. “This unexpectedly large mucus viscosity strongly suggests that the prey sticks to the chameleon’s tongue through viscous adhesion,” the team wrote. Previous studies have proposed that other mechanisms – such as suction – must be supplementing viscous adhesion.
Using this measurement, the team then built a model that includes the motion of the tongue and the captured prey during the retraction phase. They found that prey size isn’t limited by viscous adhesion because of that high mucus viscosity as well as the large contact area between the prey and the tongue.