Chameleons have the seemingly impossible ability to capture prey with a simple flick of their tongue while remaining immobile. This sensational predatory ability depends in part on an advanced ballistic projection of the chameleon’s tongue. The chameleon can extend its tongue up to two body lengths during a predatory attack and steer it towards its victim at accelerations ranging 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 present on the chameleon’s tongue by rolling small steel balls over a thin film of mucus. During rolling, the viscous forces of the slime produce a drag force on the granules, which can be used to indirectly measure viscosity. The scientists found that the viscosity of mucus (0.4 ± 0.1 Pa-s) is about 400 times greater than that of human saliva (~10-3 pass).
The scientists suggested that this unexpectedly high value may indicate that prey sticks to the tip of the chameleon’s tongue through a viscous attachment. Viscous adhesion is characterized by the internal resistance of a fluid to deform under an applied force, almost like a “liquid friction”.
Modeling the forces of attack
To develop a model to characterize the adhesion force, the scientists evaluated kinematic data from a prey capture captured on a high-speed camera, including both tongue projection and retraction. They found that the tongue moves out of the mouth with a high acceleration. Once out of the mouth, it continues at a roughly constant speed (with no acceleration) until it reaches the pull-in point, where it begins to slow down. The tongue only behaves like a stretched elastic material over a small area near the point of capture and retraction.
The sequence they documented is consistent with previous observations that the chameleon’s tongue uses nested sheaths that slide past each other during an attack, in a manner similar to the tubes of an extending telescope. It is not until the tongue is fully extended that elastic stretching occurs. This information was used to determine specific parameters that the researchers put into their modeling equations.
Through analysis of their theoretical model, the scientists found that viscous slime exhibits time-dependent strength when it deforms during an attack. At the onset of tongue retraction, the thickness of the mucous fluid decreases rapidly, resulting in a sharp increase in the adhesive force, which reaches a maximum value comparable to the retraction force. As the retraction continues, the thickness of the mucous fluid increases and the adhesiveness disappears.
This analysis showed that the rate at which slime thickness changes depends on the mass of the prey. Retracting heavier prey requires greater holding power, which requires greater mucus fluid thickness. This suggests that there is a maximum prey mass for which this system will work.
Validate the model with real-life data
Previous studies have determined maximum prey size by analyzing the stomach contents of chameleons. 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 mucus viscosity and tongue contact area influence the success of the adhesive trap. If the mucus viscosity were comparable to that of human saliva, the maximum prey size would be drastically reduced – by a factor of 50. Chameleons maximize the effectiveness of their saliva by cupping the tip of the tongue on contact, resulting in a drastic increase in the contact area between tongue and prey.
The scientists concluded that only the viscous adhesion is high enough to capture 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 present themselves as often.
nature physics2016. DOI 10.1038/NPHYS3795 (About DOIs).