By Dr. Becker
Recently two scientists in Germany decided to investigate the stickiness and other characteristics of frog tongues. (The researchers belong to a group that studies biological adhesives, including gecko and beetle feet, at the University of Kiel, so their interest in frog tongue stickiness isn’t as wacky it sounds!)
The scientists chose South American horned frogs for their experiment, because these particular frogs are known for their ability to grab prey up to half their body size, including locusts, fish, other amphibians, and even small rodents.
In the wild, the frogs sit perfectly still and well camouflaged, waiting for a meal to happen by. Using that powerful, sticky tongue, they snatch prey right out of the air or off the ground and, according to study author Dr. Thomas Kleinteich, “swallow pretty much everything that fits into their mouths.”1
Frog Tongues: Powerful Post-It Notes
The South American horned frog is a popular pet in Germany, so Kleinteich bought four from local pet shops for his study. At mealtime, he offered each frog a cricket or grasshopper, but there was a catch. The insects were behind glass slides, and the slides were attached to measurement devices designed to record the strength and speed of the frog’s tongue. After each trial, the slides were removed and the frogs got to eat.
The researchers learned that on average, the forces exerted by the frogs’ tongues against the glass slides were greater than the weight of the frogs themselves. But also of surprising interest was the appearance of the frogs’ tongue-prints on the slides. The scientists observed a great deal of variation in the contact area that was covered by mucus. Their assumption had been that the mucus acted like very strong glue. But what they found was the opposite – the stronger the adhesive forces of the frogs’ tongues, the less mucus there was on the slides.
The longer a frog’s tongue was attached to a slide, the greater the mucus accumulation, but during the initial contact of tongue to slide, mucus coverage was minimal. This led Kleinteich to theorize there might actually be very little mucus involved in establishing contact between tongue and target. The mucus does play a role as part of the tongue’s “wet adhesive system.” However, Kleinteich concluded, “It’s not like having a liquid glue, it’s rather like a sticky tape” (Post-It notes, actually).
Like sticky tape, when a frog’s tongue pulls away from the slide, fine strings of mucus stretch between the two surfaces, similar to the very fine strings of adhesive we see when we remove a piece of tape from a surface it has been stuck to.
Ballistic Tongues Help Sit-and-Wait Predators Catch a Meal
These study results, which were published in June 2014 in the journal Scientific Reports,2 may help explain the tremendous speed of frogs’ tongues, which flick in and out of their mouths in just milliseconds.
Scientists have assumed the frogs’ tongue speed is primarily intended to catch hard-to-catch prey. But Kleinteich’s study results suggest there may be another reason. The greater the speed of the flick, the more impact with and adhesion to the target… which could result in a bigger meal for Mr. Frog.
The ability to capture large prey could be an evolutionary adaptation for the horned frog, which simply sits and waits for meals instead of actively pursuing them.
“One meal could be their energy budget for an entire year,” says Kiisa Nishikawa, an amphibian biologist at Northern Arizona University in Flagstaff, Arizona. Nishikawa wasn’t involved in the German study, but she does have a horned frog as a housemate. The frog went into hibernation for the winter in November 2013, and as of early June 2014, he still hadn’t eaten!3