What’s so special about frog tongues?
If you have ever tried swatting a fly, you know just how quickly these pests escape our attack. What happens when you rely on these pesky bugs for your dinner? Frogs, a group of amphibian, grab this fast, elusive prey using only their sticky tongues.
The versatile frog tongue can grab wet, hairy and slippery surfaces with ease. Not even household tapes can firmly stick to wet or dusty surfaces. What makes this tongue even more impressive is its speed: over 4,000 species of frog and toad snag prey faster than a human can blink. You may think that the frog tongue succeeds in capturing only lightweight prey, and yet studies have shown that the frog tongue can pull up to 1.4 times its own body weight. In this study, we elucidate the mechanism of how frog tongue stick using high-speed videography, materials testing and an overarching theoretical model.
The history of frog tongues
These wet, slimy amphibians have been studied as far back as 1849 AD, when Augustus Waller published the first documented frog tongue study on nerves and papillae. Waller was fascinated with the soft, sticky nature of the frog tongue: “The attention of physiologists was first directed by me to the peculiar advantages possessed by the tongue of the living frog…the extreme elasticity and transparency of this organ induced me to submit it to the microscope.”
High speed video of prey capture
We filmed a variety of frog and toad at Zoo Atlanta and the Atlanta Botanical Gardens. The frog’s tongue is able to capture an insect in under 0.07 s, five times faster than a human eye blink. In addition, insect acceleration during capture can reach 12 times the acceleration of gravity (in comparison, astronauts normally experience around 3 times the acceleration of gravity during a rocket launch). Unlike humans, frog tongues are connected at the front of the lower jaw, rather than at the back of the throat. The frog tongue projects out of the mouth using an inertial projection mechanism: the jaw rapidly opens, the tongue rotates and inertia of the tissue causes the tongue to project toward the prey. We filmed a variety of frog and toad species, all which use a soft and sticky tongue to catch insects.
The frog tongue is 10 times softer than a human tongue
In order to understand how the frog tongue sticks to insects, we had to understand the tissue and saliva properties. We measured the softness of the tongue tissue with an indentation machine, which pokes biological materials and measures forces. Softness describes tongue deformation when a stretching force is applied. The force-displacement relationship can then describe softness based on the indentation head shape, such as a cylinder or sphere. We found frog tongues are about as soft as brain tissue and 10 times softer than the human tongue (The Northern Leopard frog tongue can have Young’s modulus values as low as 1.5 kPa!). We also measured softness values for 7 other species of frog and toad, all which have comparable tongue softness.
Frog saliva is a shear-thinning fluid
To understand how the saliva aids adhesion, we looked primarily at saliva viscosity. Viscosity is a common metric for comparing fluids, and describes saliva’s resistance to movement. The rheometer (the machine that measures viscosity) requires about one-fifth of a teaspoon of fluid to run the test. Amphibians are unique in that they secrete saliva through glands located on their tongue. So, one night we spent a few hours scraping 15 dead frog tongues to get a saliva sample large enough for testing. After testing, we were surprised to find that the saliva is a shear-thinning viscoelastic fluid. The saliva has two distinct regimes: high viscosity (thick like honey) and low viscosity (50 times less viscous). The two regimes are dependent on how quickly the saliva is sheared, when resting between parallel plates. Shear rate can be compared to rubbing fluid between your fingertips at different speeds. A common shear-thinning fluid is ketchup, which sticks to the insides of bottles but can flow when the bottle is tapped.
How a saliva-infused soft tongue is the ultimate adhesive
The soft tongue and the shear-thinning, viscoelastic saliva working in unison to adhere to insects. The extreme softness of the tongue allows it to deform and engulf the insect during prey impact. During insect retraction, the tongue acts like a spring, storing energy in the soft tissue and reducing separation forces between saliva and insect. You can imagine the tongue acts like a bungee cord for the insect; if the tongue were stiffer, it would be like a human jumping off a bridge with a stiff rope wrapped around the ankle. The saliva has unique properties that maintain adhesion. We found that the saliva is shear-thinning; in a low shear rate regime, the saliva is incredibly viscous (more viscous than honey). In a high shear rate regime, the saliva becomes more than 50 times less viscous. This two-phase saliva regime is helpful in adhering to prey during impact and removing prey once in the mouth. During prey impact, the saliva experiences high shear rates, resulting in the saliva becoming thin and liquidy, penetrating insect cracks. During insect retraction, the saliva experiences low shear rates, firming up and maintaining grip on the insect. Once the insect is inside the mouth, the frog eyeballs drop into the mouth cavity and push the insect down the throat. This shearing motion again results in the saliva becoming thin and liquidy, allowing the insect to slide off the tongue.