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

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.”