Science of saliva: How versatile spit helps frogs catch and deliver a meal

A frog's tongue can capture an insect five times faster than the blink of an eye. But what keeps that bug from falling off along the way? Saliva with unique properties, scientists say.

Candler Hobbs/Georgia Tech
Northern leopard frog grabs a cricket.

In 2011, an African bullfrog named Pacman Frog became a YouTube star by using his tongue to play a smartphone game.

The video has already received over 10.5 million views, and now has another claim to fame: inspiring researchers at the Georgia Institute of Technology to investigate the properties of a frog’s tongue and saliva.

“Further YouTube research yielded amazing videos of frogs eating mice, tarantulas and even other frogs,” wrote three of the study’s authors in an article for The Conversation. “The versatile frog tongue can grab wet, hairy and slippery surfaces with equal ease...What makes the frog tongue so uniquely sticky? Our group aimed to find out.”

The research team, led by Ph.D student Alexis Noel, began by making slow-motion videos of frogs snagging their prey.

The researchers found that the frog's tongue lashes out at an astonishing speed, able to capture an insect in less than 0.07 seconds, five times faster than the blink of an eye. 

“In addition, insect acceleration toward the frog’s mouth during capture can reach 12 times the acceleration of gravity," the researchers explain. "For comparison, astronauts normally experience around three times the acceleration of gravity during a rocket launch.”

How can the tongue keep its grip on the insect? To find out, the researchers focused on the frog’s saliva. After painstakingly scraping together a fifth of a teaspoon, they put it in an indentation machine used for measuring biological materials responses to force.

They found that frog saliva is a “non-Newtonian fluid.” In one of his lesser-known accomplishments, Sir Isaac Newton observed that some fluids – like water and honey – maintain a constant viscosity, or thickness, at a given temperature. Frog saliva isn’t one of those fluids. Instead, its viscosity decreases when shear stress – the force of two surfaces rubbing against one another – increases.

This means that, most of the time, frog saliva is relatively thick. But when rubbed between a fast-moving tongue and unfortunate insect, it gets runny and can coat the bug’s body. When the tongue slows down, it thickens again, gluing the insect to the tongue as it’s reeled back in.

The question that began with a YouTube video has been answered, but the study might have more practical uses. "Most adhesives that have been created are stiff, especially tape," explained David Hu, a faculty member in Georgia Tech’s School of Biological Sciences and co-author of the study, in a press release. "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."

By giving a better sense of how frogs fit into their environment, the study’s findings could also help preserve them.

“This work provides valuable insight into the biology and function of amphibians – 40 percent of which are in catastrophic decline or already extinct,” the researchers explained. “The knowledge gathered on unique functions of frog and toad species can inform conservation decisions for managing populations in dynamic and declining ecosystems.”

You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.