Chameleons have the seemingly impossible ability to capture prey simply with the flick of their tongue, all while remaining motionless. This sensationalized predatory ability depends in part on a sophisticated ballistic projection of the chameleon’s tongue. The chameleon is able to extend its tongue as far as two body lengths away during a predatory attack, sending it toward its victim using accelerations that range from 300 to 1500 m/s2.

Given the forces involved, what happens next is a bit surprising: the victim sticks to the tongue, even in cases where the prey is up to 30 percent of the chameleon's own body weight. Recently, a team of scientists investigated how this works.

It all depends on extremely viscous spit. The team characterized the viscosity of the mucus that's present on the chameleon's tongue by rolling small steel beads over a thin mucus film. During the rolling, the viscous forces of the mucus produce a drag force on the beads, which can be used to indirectly measure the viscosity. The scientists determined that the mucus viscosity (0.4 ± 0.1 Pa-s) is roughly 400 times larger than that of human saliva (~10-3 Pa-s).

The scientists suggested this unexpectedly high value may indicate that prey stick to the tip of the chameleon's tongue through a viscous adhesion. Viscous adhesion is characterized by the internal resistance of a fluid to deform under an exerted force, almost like a “fluid friction.”

Modeling the forces of the attack

In order to develop a model to characterize the adhesion strength, the scientists evaluated kinematics data of a prey capture recorded on a high-speed camera, including both the tongue projection and retraction. They determined that the tongue moves out of the mouth at a high acceleration. Once out of the mouth, it continues at a roughly constant velocity (with no acceleration) until it reaches the retraction point, where it begins to decelerate. The tongue behaves as a stretched elastic material only over a small region near the capture and retraction point.

The sequence they documented is consistent with previous observations that the chameleon’s tongue utilizes nested sheaths that slide along one another during an attack, in a fashion similar to the tubes of an extending telescope. Only once the tongue is fully extended will elastic stretching occur. This information was used to determine specific parameters that the researchers plugged into their modeling equations.

Through analysis of their theoretical model, the scientists found that viscous mucus exhibits a time-dependent force as it deforms during an attack. At the onset of the tongue retraction, the mucus fluid thickness rapidly decreases, resulting in a sharp increase in the adhesive force, which reaches a maximum value comparable to the retraction force. As the retraction continues, the mucus fluid thickness increases, and the adhesive force vanishes.

This analysis revealed that the speed at which the mucus thickness changes is dependent on the mass of the prey. To retract heavier prey, a larger adhesive force is required, which requires a larger mucus fluid thickness. This suggests there is a maximum prey mass that this system will work for.

Validating the model with real-life data

Previous investigations figured out the maximum prey size through analysis of chameleons' stomach contents. When this was calculated using the adhesion model, it yielded a size that is always very close but larger than the experimental data.

Their modeling revealed that both the mucus viscosity and the contact area of the tongue influence the success of the adhesive trap. If the mucus viscosity was similar to that of human saliva, the maximum prey size would be dramatically reduced—by a factor of 50. Chameleons maximize the effectiveness of their saliva by forming a cup with the tip of the tongue on contact, resulting in a drastic increase in the tongue-prey contact area.

The scientists concluded that the viscous adhesion alone is high enough to enable capture of large prey, including birds, lizards, and mammals, when the opportunity arises. Still, most of the stomach contents found in these animals indicate that the opportunities don't arise that often.

Nature Physics, 2016. DOI 10.1038/NPHYS3795 (About DOIs).