In the X-Men series, Toad is a mutant with a nasty secret behind his alias: an elastic tongue that can stretch further than 13 feet. With that elongated, sticky licker, he can do the dastardly things expected of a member of the Brotherhood of Evil Mutants: slamming doors, disarming enemies, and flipping people. His amplified skills are consistent with the very real weapon that frogs and toads have IRL — a whip-like tongue that can hit with a force that kills.
In a paper published Wednesday in the Journal of the Royal Society, researchers from the Georgia Institute of Technology explain why a frog’s tongue is such a successful, sticky tool. Their research showed that the tongue can unfurl with a force five times greater than gravity and pull in prey up to 1.4 times the amphibians’ body weight. Its power is rooted in the unique qualities of frog saliva, which can reverse between thick and sticky to thin and watery as it extends and retracts. Reversible saliva slathered on a tongue as soft as brain tissue is the perfect medium for catching prey.
While over 4,000 species of frog and toads use sticky tongues to catch their prey, this study focused on a type of common leopard frog called Rana pipiens. The researchers collected saliva samples of 18 frogs, measured the softness of their tongues, and filmed the frogs eating crickets in slow motion to examine the physics of the tongue’s movement. The saliva samples were placed in a rheometer, which is a device that analyzes the properties of fluids.
They found that the tongue works much like a car’s shock absorbers. Its softness allows it to change its shape before and after it makes contact with the cricket. The unfurling tongue pushes out like a bungee cord, and the force it creates is absorbed into the stretchy tissue. Meanwhile, the consistency of the saliva changes.
In a statement, the paper’s lead author, Alexi Noel, a Ph.D. student, explained that the saliva goes through three phases. “When the tongue first hits the insect, the saliva is almost like water and fills all the bug’s crevices. Then, when the tongue snaps back, the saliva changes and becomes more viscous — thicker than honey, actually — gripping the insect for the ride back. The saliva turns watery again when the insect is sheared off inside the mouth.”
Noel and her team hope that this biological revelation can be helpful to humankind as a source of inspiration for new reversible adhesives (like Band-Aids) and new materials for soft manufacturing. As of now, frog tongues are 50 times more adhesive than any known synthetic polymer.