What Makes Frog Tongues So Sticky?

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March 14, 2017 | 5,371 views

Story at-a-glance

  • Frogs’ tongues are one of the softest biological materials known, which enables them to wrap around prey during impact, leading to a large contact area
  • Frog tongues are very elastic, stretching and deforming upon release and retraction to increase the tongue’s contact with its prey
  • Frog saliva starts out thick and sticky then turns watery when it contacts prey; this allows it to seep into the insect’s crevices before it thickens back up, adhering the prey to the tongue

By Dr. Becker

Frogs feast on a variety of insects ranging from flies and mosquitoes to grasshoppers, but they also eat larger prey like mice and birds. What makes this particularly interesting is that they catch this prey by way of a sticky tongue that’s launched out of their mouth at speeds up to five times faster than you can blink.

Remarkably, frogs use their tongues to catch prey that’s up to 1.4 times their body weight. No commercial mechanisms exist that can grab objects as quickly as a frog’s tongue, researchers at the Georgia Institute of Technology explained, let alone adhere to them.1

The researchers wanted to know what makes frogs’ tongues so sticky, and so they began a series of tests on both frog tongues and their saliva.

Using high-speed videos of frog tongue projection and measurements of the rheological (the ability to flow or be deformed) properties of frog saliva and mechanical properties of frog tongue tissue, they solved a puzzle that’s been a mystery since frog studies first began (circa the 1800s).

As you might suspect, it’s not one factor that gives frogs this remarkable ability, but a combination of factors that makes their tongues so adhesive — 50 times more adhesive than “any known synthetic polymer materials such as the sticky-hand toy.”2

Frogs’ Tongues Are Incredibly Soft and Elastic

The first part of the equation is the softness of frogs’ tongues — 10 times softer than a human tongue, with researchers describing them as one of the softest biological materials known. This enables the tongue to basically wrap around prey during impact, leading to a large contact area.

It’s also very elastic, acting “like a car’s shock absorbers during insect capture.” During the initial outward rotation, the frog’s tongue elongates to allow it to catch prey that’s farther away.

At the same time, Ph.D. student and lead study author Alexis Noel explained, “The frog tongue is like a bungee cord, in that the tongue as it is pulling back actually stretches and deforms,” increasing the tongue’s contact with its prey.3

Frog Saliva Changes From Thick to Liquefied — Then Back Again

The study also revealed that frog saliva is a non-Newtonian fluid, which means it has the unique capability of changing properties. Typically thick and sticky, like honey, force makes frog saliva less viscous by about 100-fold,4 so as soon as an insect hits the tongue, the saliva liquefies.

The watery saliva is then able to quickly coat the prey and seep into every crack and crevice before it thickens back up again, allowing the frog to gobble the insect. Such traits could be useful in one day developing a reversible adhesive that would stick at high speed, the researchers said.

The question then becomes, how does the frog dislodge the insect from its own sticky grip? This is done via its eyeballs, which are retracted inward to help slide the prey off its tongue. The researchers explained:5

“If the tongue is so sticky, how does the frog ultimately remove the insect? X-ray videography has shown that a frog retracts its eyeballs to push food down its throat … This motion provides a shearing force parallel to the tongue …

Much like pushing a hockey puck on ice, objects on the frog tongue are easy to shear, enabling effortless prey removal.”

Why Do Frogs Have Teeth?

Frogs don’t chew their meals; they swallow their prey whole. So why do some frogs have teeth? Surprising to many, frogs have pointy vomerine teeth, located on the vomer facial bone for which they are named. The teeth are located in the front part of the mouth (the roof of the mouth) and are used primarily for holding onto prey.6

Frogs may also have maxillary teeth, which are found on the upper jaw and are also used to help grasp prey. While most frogs have teeth only on their upper jaw, Guenther’s marsupial frog is the only frog species that also has teeth on their bottom jaw.7

At one time, most frogs did have teeth on their upper and lower jaws, but the lower-jaw teeth (mandibular teeth) were mostly lost millions of years ago.8 For creatures that are so efficient at hunting with their tongues, lower teeth would only get in the way.

[+]Sources and References [-]Sources and References

  • 1, 2, 5 Journal of the Royal Society Interface February 1, 2017
  • 3 NPR January 31, 2017
  • 4 The Atlantic February 1, 2017
  • 6 Burke Museum October 22, 2015
  • 7 Ecologica September 9, 2012
  • 8 Evolution January 27, 2011