Faster than Gene Kelly tap dancing in the rain, many poison frog species tap the middle toes of their hind legs so quickly they appear to be a blur.
Three labs in different countries recently independently set out to understand why. All their studies suggest that the presence of prey influences these frogs’ foot tapping, but the purpose of all that fancy footwork is still mysterious. The research could help explain similar behavior in other frogs and toads, as dozens of species make some type of toe or toe movement while hunting.
The latest study, published online last month but not yet published in a peer-reviewed journal, was conducted by biologists at the University of Illinois Urbana-Champaign. The researchers observed colorful frogs dyeing poison darts by tapping up to 500 times per minute, which is more than three times faster than Taylor Swift’s “Shake It Off.”
When the frogs saw fruit flies in a petri dish but couldn’t reach them, they tapped less frequently. This suggests that the touch may relate to their ability to catch the meal.
But the team also found that toe touching had no relationship to the frogs’ success in catching prey. That “confused us, and that’s what we’re still thinking about,” said Thomas Parrish, who worked on the study as an undergraduate with Eva Fischer, a biology professor.
While some mysteries remained, it became clear that the amphibian dance floor was important. Dr Fischer’s team found that the frogs tapped their toes more when they were perched on leaves in an aquarium, compared to when they were placed on agar gel, soil or glass.
Because leaves easily carry vibrations, this result supports the idea that frogs might tap to encourage prey to move and to make it easier to spot the tasty insects. (These frogs click their tongues only at live, moving insects.)
Another hypothesis being considered by many scientists is that the vibrations of touching the toes might lure prey closer, similar to how turtles stick out their tongues to imitate worms and deep-sea anglerfish lure meals with their luminous fishing rod-like protrusion. But while Gulf Coast toads have been seen moving prey toward themselves with toe vibrations, this has not been demonstrated in poison dart frogs.
A separate group of biologists decided to examine the vibrations produced by tapping their toes. They used an accelerometer to record the heartbeat of yellow-striped poison frogs in a specially constructed tank.
“Here we are very Caribbean, so we imagine frogs playing drums,” said Luis Alberto Rueda-Solano, an author of the study at the University of Magdalena in Colombia. The study, published last November in the journal Evolutionary Ecology and led by Natalia Vergara-Herrera, found that in about 37 percent of the recordings, the frogs accelerated their foot tapping before moving their tongues to attack prey. Frogs with longer middle fingers were more likely to show this acceleration.
The Magdalena researchers would eventually like to study whether frogs sense the movements of their prey and other organisms through vibrations, with the signal traveling from their hind legs to their inner ears.
“It’s a potentially really interesting example of a predator using sensory cues to manipulate prey behavior — at least that possibility exists,” said Reginald Cocroft, a biologist at the University of Missouri who collaborated on the study.
Does frog meal size matter? In a separate study published in early 2023, Lisa Schulte and Yannis Köning of Goethe University Frankfurt in Germany experimented with green and black poisonous frogs at the Frankfurt Zoo, showing that both crickets and smaller fruit flies could to make amphibians beat.
But the calls of other frogs don’t inspire foot tapping, suggesting that the behavior isn’t just a general expression of excitement, Dr. Schulte said.
Dr. Schulte noted complementary findings from each group’s studies, which indicate some relationship between toe tapping and poison dart feeding of frogs.
All three groups plan to follow up on their findings, advancing the science to understand whether toe tapping helps these frogs catch their dinner, or whether they’re just doing it for fun.