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Unique Reversible Saliva Keeps Frog Tongues Sticky

According to a new study published in the Journal of the Royal Society Interface, a frog’s saliva is thick and sticky during prey capture, then turns thin and watery as prey is removed inside the mouth.

A northern leopard frog (Rana pipiens) catches a cricket. Image credit: Candler Hobbs.

A northern leopard frog (Rana pipiens) catches a cricket. Image credit: Candler Hobbs.

A frog uses its whip-like tongue to snag its prey faster than a human can blink, hitting it with a force about 5 times greater than gravity.

How does it hang onto its meal as the food rockets back into its mouth?

The new study, led by Alexis Noel of the Georgia Institute of Technology, shows that the tongue’s unique stickiness results from a combination of a soft, viscoelastic tongue coupled with a reversible saliva.

The tongue, which was found to be as soft as brain tissue and 10 times softer than a human’s tongue, stretches and stores energy much like a spring.

This combination of spit and softness is so effective that it provides the tongue 50 times greater work of adhesion than synthetic polymer materials such as sticky-hand toys.

The researchers filmed frogs eating crickets in super-slow motion to better understand the physics of the tongue. They also collected saliva samples and poked the tissue to measure softness.

“The tongue acts like a bungee cord once it latches onto its prey. It deforms itself as it pulls back toward the mouth, continually storing the intense applied forces in its stretchy tissue and dissipating them in its internal damping,” Noel said.

“This tissue damping is much like a car’s shock absorbers. The tongue’s softness also allows it to change shape during contact and immediately afterward while retracting.”

The other vital component of the capturing process is the frog’s versatile saliva.

“There are actually 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,” Noel said.

“The saliva turns watery again when the insect is sheared off inside the mouth.”

Unlike water and honey, frog saliva can change its viscosity with shear rate, much like paint. Paint spreads easily when applied, but stays firmly on the wall once the brush is removed.

“For frogs, saliva seeps easily when it hits the insect, then thickens up during retraction,” Noel said.

To identify the shear rate when viscosity drops, the team collected saliva from 18 frogs and placed samples in a rheometer, a highly sensitive device for measuring properties of fluids.

“This study could help engineers design reversible adhesives at high speed,” said senior author Prof. David Hu, also from the Georgia Institute of Technology.

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Alexis C. Noel et al. 2017. Frogs use a viscoelastic tongue and non-Newtonian saliva to catch prey. J. R. Soc. Interface 14 (127): 20160764; doi: 10.1098/rsif.2016.0764

This article is based on a press-release from the Georgia Institute of Technology.