The toe pad of the Amazon milk frog can hold up to 14 times the animal’s body weight. That’s like an average American man holding a Honda Civic… with his toes.
These tree frogs are pros at sticking their landings. And it’s a good thing, because the ability to catch and hold onto a twig or branch is the only thing that lies between the frog and a fatal fall. But little is known about how they do this.
Researchers at Kiel University in Germany used high-speed cameras to film the frogs landing on a stick. They found that the frogs grab on with uniquely designed toe pads and use cartwheels and other acrobatics to slow their momentum. And sometimes, they belly flop.
Concave, hexagonal cells and mucus on their toe pads allow them adhere to surfaces through capillary action. That’s that same force that enables liquid to be drawn through a straw and water striders to walk on ponds.
“The challenge of landing is immense, and failure can be crucial,” said Nienke Bijma, a Masters student at Kiel University in Germany. Even surviving a fall means a frog will have to expend energy climbing back up to its destination.
As an undergraduate, Bijma and two scientists with the university’s Functional Morphology and Biomechanics research group used high-speed cameras to study the mechanics of tree frog landings. Their results were published in April’s Journal of Comparative Physiology A.
“We were wondering how these frogs are able to land on narrow and often unpredictable substrates multiple body lengths away,” Bijma said in a recent interview.
Researchers filmed the frogs leaping toward and landing on a wooden stick.
By studying the slowed-down footage and the frogs’ reflection in an angled mirror, the team observed two separate landing techniques. One involved grabbing hold with a front or hind limb. In the other, they landed on their abdomen — an amphibian belly-flop.
The belly flop has upsides and downsides. “[O]vershooting the target is less likely, as the abdomen of the frog will immediately stop the flight…,” the study reads. “However, during abdominal landing, the abdomen has to dissipate all of the frog’s kinetic energy, which could potentially cause harm to visceral organs.”
But the frogs risk overshooting when grabbing hold with the front or hind limbs, making the stickiness of the toe pads critical.
The study found that once a frog’s toe-pad touched the wooden dowel, it stuck in place without sliding. The frogs then performed cartwheels and other acrobatic maneuvers, thanks to their limber anatomy. That and the friction created by additional body parts coming in contact with the stick allowed each animal to oscillate and slow down enough to fully perch.
Calculating each frog’s velocity and deceleration during the landing process, the researchers determined that the frogs’ toe pads can hold up to 14 times their body weight. An impressive feat considering one of the frogs was able to cling to the stick with only one digit.
And while more studies could assess how these animals land on different materials and terrains, the task might have to be left to future researchers.
Next up for Bijma: looking at how dung beetle locomotion could be used in robotics.