Poison frogs’ strange behaviour confounds scientists

By Elizabeth Landau

NEW YORK: Faster than Gene Kelly tap-dancing in the rain, many species of poison dart frogs tap their middle toes on their hind feet so rapidly it can look like a blur. Three laboratories in different countries recently set out independently to understand why. Their studies all suggest that the presence of prey influences these frogs’ toe-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 kind of toe or foot movement while hunting.

The latest study, which was posted online last month but has not yet been published in a peer-reviewed journal, came from biologists at the University of Illinois Urbana-Champaign. The researchers observed colorful dyeing poison dart frogs tapping up to 500 times per minute, or more than three times as fast as Taylor Swift’s “Shake It Off.”

When the frogs saw fruit flies in a petri dish but could not reach them, they tapped less frequently. This suggests that the tapping could relate to their ability to capture their meal. But the team also found that toe-tapping had no relationship to the frogs’ success at catching prey. This “kind of 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 amphibians’ dance floor mattered. Dr. Fischer’s team found that frogs tapped their toes the most when perched on leaves in a tank, compared with being placed on agar gel, soil or glass.

Because leaves easily carry vibrations, that result supports the idea that the frogs could be tapping to encourage prey to move and to make the tasty bugs easier to detect. (These frogs snap their tongues only at live, moving insects.)

Another hypothesis many scientists have considered is that the toe-tapping vibrations could lure prey closer, similar to how turtles stick out their tongues to mimic worms and deep-sea angler fish attract meals with their glowing fishing-rod-like protrusion. But while Gulf Coast toads have been seen moving prey toward themselves with toe vibrations, this has not been shown in poison dart frogs.

A separate team of biologists set out to examine the vibrations made by the toe-tapping. They used an accelerometer to record the tapping of yellow striped poison frogs in a specially constructed tank. “Here we are very Caribbean, so we imagine the 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 recordings, the frogs accelerated their toe-tapping before flicking their tongues to attack prey. Frogs with longer middle toes were more likely to show this acceleration.

The Magdalena researchers would eventually like to study whether the frogs sense the movements of their prey and other organisms through vibrations, with the signal traveling from their hind feet to their inner ears.