The frog’s tongue is an incredible adhesive. Many times our artificial adhesives don’t stick firmly to wet and dusty surfaces. But this creature’s tongue can snag wet, dusty, and hairy insects with ease. There are about 4,000 species of frog and toad around the world, and they can grab prey faster than you can blink. Now researchers at the Georgia Institute of Technology have revealed why the frog’s tongue is so sticky.
Its saliva thickens up during retraction
It began when Alexis Noel, a biomechanics PhD student, watched a video of an African bullfrog trying to catch digital bugs with its tongue on a mobile screen. Noel and her colleagues at the Georgia Institute of Technology began to study the stickiness of the frog’s tongue. They discovered that the animal’s saliva can transform from a viscous glue to a thin and watery fluid and back again. It creates an inescapable trap in split seconds.
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Alexis Noel said in a statement that the saliva seeps easily when hitting the target, but quickly thickens up during retraction. The transformation is caused by the shearing forces. Noel likened frog saliva to ketchup. Both of them are shear thinning fluids, she told the Washington Post. When you flip a bottle of Heinz ketchup upside down, the ketchup refuses to flow despite the gravitational force. But it starts flowing when you apply a shearing force, i.e. smack on the bottom of the bottle.
How a frog captures its prey
A globule of frog saliva is thick and viscous on a stationary surface. But when the frog throws out its tongue on the prey at lightning fast speed, the shearing force turns the spit into a thin and watery fluid. The tongue wraps around the target, increasing the contact area. As the creature begins to retract its tongue, the spit returns to its viscous state, gripping the insect in place.
Once the insect is in its mouth, the frog rubs its tongue against the inside of its mouth. It shears the prey free from the tongue. Researchers analyzed saliva samples from as many as 18 frogs using a rheometer to figure out the shear rate when viscosity drops. Findings of the study were published Monday in the Journal of the Royal Society Interface.
What gives its spit a viscous consistency?
Noel said that the viscous consistency comes from a high concentration of long-chain proteins in the frog’s saliva. The same protein is present in human saliva in much lower concentrations. That’s what makes our drool form long strings. While we have salivary glands, the spit in frogs is produced by the tongue itself.
Another factor that plays an equally important role is the softness of the frog’s tongue. Its tongue is as soft as brain tissues, and at least ten times softer than our tongues. Extreme softness of their tongues further enhances the stickiness by coiling around the prey.