Science & Environment

Frog's tongue 'can lift three times own body weight'

Horned frog in the experimental set-up Image copyright Thomas Kleinteich
Image caption Each frog was separated from a tempting morsel by a glass slide, which was used to measure the forces exerted by its tongue.

New research shows that the pulling force of a frog's tongue can be up to three times the animal's own weight.

Zoologists placed the horned frog, a predator known to swallow whole mice, in front of a glass slide and tempted it with a tasty cricket.

Stronger pulling forces were measured when contact with the glass was briefer and less mucus was left behind.

The study, from the journal Scientific Reports, suggests the action of the tongue is similar to sticky tape.

"It's the first time we've ever measured how well frog tongues stick," said Dr Thomas Kleinteich, who performed the experiments at the University of Kiel in Germany.

Dr Kleinteich works in a group that studies biological adhesives, including gecko and beetle feet, with a view to finding new designs for sticky applications like boot soles, tapes and parcel closures.

"The thing that's interesting about frog tongues is that they're really fast," he told BBC News. "It only takes milliseconds."

The South American horned frog in particular, a popular pet, is known for its ability to snatch morsels up to half its own size - from locusts and fish to other amphibians and small rodents.

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Media captionSlow motion footage of the horned frog catching a cricket

In the wild, they lurk half-buried in wait for their prey, and then "they swallow pretty much everything that fits into their mouths," Dr Kleinteich said.

To study this combination of strength and speed, he bought four horned frogs from local pet shops. During their regular feeding routine, he presented each frog with an adult cricket behind a glass slide, attached to a transducer that recorded the forces exerted by the frog's tongue.

On average, these forces were larger than the weight of the frog itself, and in the case of one young amphibian more than three times larger.

After each trial, the equipment was removed and the frog got its treat. Dr Kleinteich ultimately needed twenty measurements from each frog, so the predators had to be kept happy.

Looking at the slides afterwards, the "tongue print" left behind on the glass slide offered more insights, including massive variation in the proportion of the contact area that was covered by mucus.

Image copyright Thomas Kleinteich
Image caption The tongue prints left behind showed varying degrees of mucus coverage

"The common belief is... that the mucus acts as some sort of superglue," Dr Kleinteich explained. "But what we found was actually that we got higher adhesive forces in trials where we found less mucus. That was quite interesting."

The mucus appeared to build up over time, so that cases where the tongue touched the glass for longer left more mess behind. "But during the initial contact, the mucus coverage was rather low," said Dr Kleinteich. "So to actually establish the contact, there might be very little mucus involved."

"It plays a role. It's definitely a wet adhesive system, it's not just structure and friction, because there is some fluid involved. But the key is the structure plus the mucus.

"It's not like having a liquid glue, it's rather like a sticky tape."

That comparison applies particularly to the way the tongue peels away from the glass. The researchers saw stringy "fibrils" of mucus stretching between the two surfaces, just like the ones you can see if you look closely at sticky tape peeling off a surface.

Image copyright Thinkstock
Image caption In the wild, horned frogs wait half-buried until "pretty much anything that fits into their mouth" strays nearby

The team is now using microscopes to examine the fine details of the tongue's surface and see more of its sticky secrets.

Dr Kleinteich said he enjoyed doing the feeding experiments. "It's fun," he said. "I used to do a lot of morphological, descriptive work with amphibians - I used to study dead, museum specimens. For me it was quite exciting to work with the living frogs and see how they behave."

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