Nav: Home

Reversible saliva allows frogs to hang on to next meal

January 31, 2017

A frog uses its whip-like tongue to snag its prey faster than a human can blink, hitting it with a force five times greater than gravity. How does it hang onto its meal as the food rockets back into its mouth?

A new Georgia Institute of Technology study says the tongue's stickiness is caused by a unique reversible saliva in combination with a super soft tongue. A frog's saliva is thick and sticky during prey capture, then turns thin and watery as prey is removed inside the mouth. 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 Georgia Tech 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," said Alexis Noel, a Georgia Tech mechanical engineering Ph.D. student who led the study. "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."

This tissue damping, Noel said, 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," Noel said. "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."

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," she said.

This spit switch can't be seen in the slow-mo videos. To identify the shear rate when viscosity drops, Noel collected saliva from 18 frogs and placed samples in a rheometer, a highly sensitive device for measuring properties of fluids.

David Hu, a professor in the George W. Woodruff School of Mechanical Engineering, is Noel's advisor who has also studied how mosquitos fly in the rain, how dogs shake off water and why eyelashes need to be an ideal length. He says the frog study could help engineers design reversible adhesives at high speed.

"Most adhesives that have been created are stiff, especially tape," said Hu, who is also a faculty member in the School of Biological Sciences. "Frog tongues can attach and reattach with soft, special properties that are extremely stickier than typical materials. Perhaps this technology could be used for new Band-Aids. Or it could be used to create new materials in soft manufacturing."

The Georgia Tech team worked with Mark Mandica, the leading herpetologist at the Amphibian Foundation in Atlanta. The foundation brings together top researchers in the field of amphibian biology, conservation, and applied science to address the causes of global amphibian declines. Current research estimates that 38 percent of the world's amphibian populations are in decline or already extinct.

The study, "Frogs use a viscoelastic tongue and non-Newtonian saliva to catch prey," is published in the Journal of the Royal Society Interface.
-end-
The project is partially supported by National Science Foundation (NSF) Graduate Research Fellowship (DGE-1148903) and NSF career award PHY-1255127. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsors.

Georgia Institute of Technology

Related Frogs Articles:

Study examines pesticides' impact on wood frogs
A new study looks at how neonicotinoid pesticides affect wood frogs, which use surface waters in agricultural environments to breed and reproduce.
Frogs have unique ability to see color in the dark
The night vision of frogs and toads appears to be superior to that of all other animals.
Male poison frogs become cannibals after taking over territories
Systematic 'infanticide' of unrelated young occurs in several animal species.
Seven new species of night frogs from India including 4 miniature forms
Scientists from India have discovered seven new frog species belonging to the genus Nyctibatrachus, commonly known as Night Frogs.
Reversible saliva allows frogs to hang on to next meal
A Georgia Tech study says a frog tongue's stickiness is caused by a reversible saliva in combination with a super soft tongue.
Disentangling the myth of the singing bushmaster viper with the help of tree frogs
Legend spread among both colonists and native Americans from the Amazon region and Central America has it that the largest viper in the western hemisphere, the bushmaster, sings.
UNIST unveils the genomic mechanism of African clawed frogs
A team of international researchers, affiliated with Ulsan National Institute of Science and Technology has discovered how African clawed frogs got extra pair of genes.
Temporary extinction reprieve for some frogs
Australian scientists have good news for frog conservation -- there may be longer than expected time to intervene before climate change causes extinction of some species.
Stowaway frogs being stopped by border security
An analysis of stowaway frogs coming into Australia has shown that strict biosecurity measures at borders and within the country are reducing the risk of introduction of new diseases by up to 50 percent.
Male frogs have sex on land to keep competitors away
Researchers have assumed that natural selection drove frogs to take the evolutionary step to reproduce on land as a way for parents to avoid aquatic predators who feed on the eggs and tadpoles.

Related Frogs Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Jumpstarting Creativity
Our greatest breakthroughs and triumphs have one thing in common: creativity. But how do you ignite it? And how do you rekindle it? This hour, TED speakers explore ideas on jumpstarting creativity. Guests include economist Tim Harford, producer Helen Marriage, artificial intelligence researcher Steve Engels, and behavioral scientist Marily Oppezzo.
Now Playing: Science for the People

#524 The Human Network
What does a network of humans look like and how does it work? How does information spread? How do decisions and opinions spread? What gets distorted as it moves through the network and why? This week we dig into the ins and outs of human networks with Matthew Jackson, Professor of Economics at Stanford University and author of the book "The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviours".