Fast-flicking frog tongues are a biological high-speed adhesive system. They stick immediately to different sorts of surfaces and capture quick, distant, and often tiny prey at rapid velocities. Now, using high-speed recordings, researchers reveal that the frog tongue is basically a muscle-powered adhesive tape. The work was published in Royal Society Open Science this week.
For frogs, the forces acting on their tongues during impact and retraction can be well beyond the body weight of the amphibians themselves. However, previous work on frog tongues have focused on the extremely fast projection behavior, and not so much on the processes occurring at the interface between the tongue and the prey. To study the mechanism of adhesion, Kiel University’s Thomas Kleinteich and Stanislav Gorb filmed three horned frogs, Ceratophrys cranwelli and Ceratophrys hybrids, at 2,000 frames per second through illuminated glass. These South American burrowing frogs are sit-and-wait predators. In their terrariums in the lab, the frogs typically don’t move much and stayed half buried in their substrate.
For each experiment, a glass slide was placed inside the terrarium, and a cricket was positioned immediately behind the glass and held in place with tweezers. When the frogs tried to catch the cricket, their tongues attached to the glass slides. The team also micro-CT scanned entire frogs as well as tongues dissected from the frogs. The latter were also examined under a scanning electron microscope.
The tongue rolls over the target during attachment, they found, but during the pulling back phase, the tongue retractor muscle acts perpendicular to the target surface – which prevents peeling during tongue retraction. When the tongue does detach, fibrils of mucus form between it and the glass. These fibrils commonly occur in pressure-sensitive adhesives. That means frog tongues could be a biological analog to engineered materials – like everyday sticky tape.
The distribution of muscle fibers, in addition to the layer of connective tissue underneath the surface, optimize the adaptability of the tongue to uneven substrates, the team writes. This prevents the tongue from peeling off, allowing for a higher stickiness. While the strike is almost instantaneous – achieving maximum tongue-to-target surface area in just 20 milliseconds, Science reports – the process of peeling the tongue off might take more than a second.