South American horned frogs have a pretty frumpy body shape. Their extremely round bodies and large mouths have earned them the nickname “Pac-Man frogs,” due to their resemblance to the video game character of the same name. This stocky body type actually works perfectly for their hunting style. Rather than needing a long, lean body to climb and track down food like some frogs, these guys just partially bury themselves in the dirt and lazily wait for something to walk in front of their face. When suitable prey is near enough, the frog’s super-sticky tongue darts out to quickly ensnare it.
While the use of a sticky tongue has been working for amphibians and reptiles for millions of years, researchers didn’t really know much about the adhesive performance or the amount of force exerted onto the prey until Thomas Kleinteich and Stanislav Gorb of Christian Albrecht University in Kiel, Germany decided to investigate. Their results were published in an open-access format in Scientific Reports.
Kleinteich chose to use Pac-Man frogs for this study, due to their voracious appetites and sedentary behavior. A microscope slide attached to a force transducer was placed in front of the frog, and either crickets or worms were placed behind the glass. When the frog’s tongue dashed out to grab the meal, it hit the plate which allowed Kleinteich to analyze the force, adhesive power, and mucus level associated with each strike.
Four frogs needed to be tested 20 times each, so the plate was removed and the frog was allowed to get the meal after each attempted strike, in order to prevent the frog from getting frustrated and giving up on trying to get the food. Additionally, the researchers did not take more than a few measurements at each feeding in an attempt to keep each frog naïve about the experiment.
The researchers found that most strikes were enough to pull more than the frog’s body weight, and one of the juveniles tested exerted enough force to pull three times more. This allows the frog to go after pretty much anything that can fit in its mouth. The researchers also key behind this hunting prowess was not from the most expected source.
Though many have assumed that the mucus acts as a glue, it appears that the mucus and the texture of the frog’s tongue work in concert to trap prey like a tape. Additionally, they found that lower amounts of mucus were associated with better adhesion. However, the strikes that made longer contact with the glass yielded more mucus, so the ability to attack larger prey likely requires the use of quicker, and thus stickier, strikes.
The next phase of research involves examining the frog tongues on a microscopic level to better understand how the structure aids in adhesion.