04 July 2014

Horned Frogs' Tongues

Welcome back. There’s no getting around it. Catching your day’s meal on the run with your tongue is the epitome of fast-food dining in addition to being a pretty cool party trick. I know I couldn’t do it. I doubt The Ecuadorian Bat that I wrote about could do it, and you may recall that bat has the longest tongue relative to the size of its body of any mammal. It’s just not a mammal thing.

If you’ve been wondering how tongue-grabbing reptiles and amphibians accomplish this amazing feat with ease, you’re not alone. A recent study from Germany’s Kiel University (Christian-Albrechts-Universität Kiel) has filled in some key blanks.

Horned Frogs

The investigators conducted lab experiments with horned frogs (Ceratophrys sp.). For the record, horned frogs are not horned toads, which are really horned lizards (Phrynosoma sp.). Those are reptiles; horned frogs are amphibians, a whole different class of vertebrae. (The State of Texas and TCU might get that right some day.)

Horned frog. (multiple websites)

Although horned frogs are kept as pets and can be bred in captivity, they’re from South America. As frogs go, they’re big, generally 4 to 6 inches, weighing as much as a pound, with very large mouths. Their name comes from the horn-like upper lids above their eyes.

Horned frogs hunt their prey by hunkering down and waiting. When a meal is in range, they fire their projectile tongue. Unlike Saturday morning cartoons that used to show a tongue wrapping around prey like a bullwhip, the frog’s tongue is tipped with mucus that sticks to the bug, bird, worm, lizard, fish, rodent or whatever. In milliseconds, the tongue hits the target and reels it in to scarf it down. The action is made easier because frogs’ tongues are attached to the front of their mouths.

Tongue’s Adhesive Force and Contact Area

The researchers set out to gauge the tongue’s adhesive force and contact area. This was accomplished by inducing four horned frogs–2 adults, 2 juveniles--to fire their tongues against glass microscope slides that were connected to force sensors (transducers). How did they get the frogs to bite so to speak? By putting crickets on the other side of the glass slide at feeding time.

As each frog tried to capture a cricket, the impact and pulling force of the tongue on the glass slide were recorded until the frog released the slide. (After data collection, the cricket was a goner.) Multiple trials were conducted with each frog.

The resulting force-versus-time measurement graphs showed short, distinct peaks when the tongue hit the slide. The average peak force was about 3 times the frogs’ body weight. One juvenile frog had a hit with an impact of 6.5 times its weight!

The force-versus-time graphs also showed adhesive force peaks in the opposite direction--i.e., reeling in the glass slide--immediately after the impact peaks. Those forces averaged about 1.5 times the frogs’ body weight. Again, a juvenile frog took the prize with a peak adhesive force of about 3.4 times its weight.

Examining the glass slides, the researchers found the tongue contact area covered by mucus varied from almost nil to complete. Though one might think the mucus functions as glue, the adhesive force was stronger in trials where there was less mucus. There’s no question that the tongue capture involves a wet adhesive system; however, other tongue features (surface profile, material properties) must play a role. Another finding of note is that the total adhesive force increased with shorter impact durations.

Wrap Up

Overall, the study showed that frogs’ tongues can be best compared to pressure sensitive (self-stick) adhesives used on tape, postage stamps or Post-its. How closely the results with glass slides will compare to those with shapes and materials of actual frog prey remains for testing, but continued research could lead to improved adhesives, plus. Why? Put the measured forces into perspective.

Horned frog, Ceratophrys_ornata_anon
One study of 70 elite-level boxers (fighters not underwear) found their average punching force was 776 pounds. I’d guess that’s about 4 to 5 times their weight. That’s less than our star juvenile frog’s ballistic tongue power.

Comparing human pulling or lifting power to adhesive force is messier, but those tongues were reeling in 1.5 times their weight. Can you bench press 1.5 times your weight? How about 3.4 times your weight? One-handed? Now that would be a neat party trick. Thanks for stopping by.


Research paper on Nature’s Scientific Reports website:
Example articles on the study on the BBC and Los Angeles Times websites:
Related background on frogs and lizards:
Boxers’ punching power:

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