Frog Saliva Is ‘Specially Evolved’ to Capture Insects, Study Finds

Snatching flies out of the air is no easy task, but new research published this week in the Journal of the Royal Society Interface had revealed that frogs have a secret weapon in their arsenal which makes things easier – super-sticky saliva specially evolved to help them hunt insects.

Scientists have long suspected that frogs used their saliva to help catch their prey, but as part of their new study, a team led by Georgia Institute of Technology Ph. D. student Alexis Noel used a combination of high-speed photography and an instrument called a rheometer to confirm that the tailless amphibians are able to produce ultra-adhesive saliva when catching flies.

According to NPR, Noel and her colleagues gathered 15 frogs and scraped their tongue to collect saliva samples under prey-capturing conditions. They then analyzed those specimens and learned that when the frog’s tongue came into contact with a fly, the saliva’s properties change.

While frog saliva is typically thick like molasses, the force of the impact between the tongue and the fly causes it to liquefy and cover the prey’s body. Once it spreads out, the saliva thickens and allows the predator to pull the now-trapped insect into its mouth. What this means, the authors of the study said, is that frog spit is a non-Newtonian fluid, meaning it can change properties.

Protein content, structure of tongue responsible for this ability

In an interview with The Guardian, Noel called the process “incredible” and explained that the process is the result of shearing forces or forces that work in unaligned directions. In this case, frog saliva is known as a “shear-thinning fluid,” meaning that it typically has a thick consistency but is thinned dramatically by a shearing force, allowing it to enter into the fly’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,” she told the UK media outlet. Afterward, the frog rubs its tongue on the inside of its mouth, dislodging the fly and allowing the predator to swallow its prey – thanks to an assist from its eyeball, which helps it to liquefy the saliva.

What causes this unusual phenomenon to happen? Noel explained that it is due in part to a number of long-chain proteins contained in the frog’s saliva, which cause it to be more mucus-like than human spit. Of course, the bungee-cord-like properties of the amphibian’s tongue also play a key role in the process by allowing it to stay in contact with the insect, and the tongue’s softness helps keep the saliva as it returns to the frog’s mouth, she told NPR.

The study is said to be the first to use these techniques to study the viscosity of frog saliva, and the researchers believe that learning more about how frog tongues work could someday make it possible to develop reversible soft adhesives capable of working at high velocity – perhaps even a conveyor belt that could be used to pick up fragile components in a manufacturing facility.

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Image credit: A. Noel and D.L. Hu/Georgia Institute of Technology

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