Why Do These Frogs Tap Dance?

Source: Holger Krisp/Wikimedia Commons

If you spend a lot of time with certain frogs, you’ll probably notice a behavior called toe-tapping: a rapid up-and-down movement of the middle toes of the hind legs.

People have known about toe-tapping for a long time; there are even videos of toe-tapping frogs on YouTube. Despite being so widely observed, there have been very few experimental investigations of the phenomenon, and its function is hardly clear. Often, the behavior seems to be associated with hunting and feeding, but there are also accounts of toe-tapping during courtship and in response to the advertisement calls of male frogs.

This common but understudied behavior piqued the interest of Lisa Schulte and Yannis König of Goethe University Frankfurt. In a new study, they systematically tested whether food or calls would trigger toe-tapping in green-and-black poison frogs kept in a colony at the Zoo Frankfurt.

The researchers presented the frogs with three kinds of stimuli: small prey (fruit flies), larger prey (crickets), and playbacks of male advertisement calls.

The frogs toe-tapped in response to both of the food items, leading Schulte and König to conclude that prey in general triggers toe-tapping.

Playback of advertisement calls, on the other hand, did not seem to elicit toe-tapping. However, Schulte says that toe-tapping could still play a role in these frogs’ courtship.

“Just using a playback call in the tank is not the same as their entire mating behavior,” she says. “These results do not exclude the possibility that toe-tapping is displayed during the physical part of courtship.”

Tap Dancing for Whom?

While this study supports the observational evidence that toe-tapping is involved in feeding, the researchers say there are still plenty of unanswered questions about the behavior, in this frog species and in others.

Source: Brian Gratwicke/Wikimedia Commons

Schulte says that with green-and-black poison frogs, it would be interesting to investigate the prey cues to which the frogs react, such as the prey's smell or movement. Would the frogs toe-tap in response to seeing videos of various potential prey items, for instance?

Then there is the question of why they do it. What function does toe-tapping serve?

Schulte says there are a few theories. One is that toe-tapping might act as a visual or vibrational lure for prey.

“We know this behavior from other species, such as deep-sea anglerfish that use a light to attract prey,” she says. “It may be the frogs do something similar by moving their little toes.”

Another hypothesis is that rather than attract prey, toe-tapping might agitate it and trigger it to move, making it easier for the frog to detect.

“If a fruit fly crawls, you can see the frog get excited and look at it,” says Schulte. “Then it just sits there and stares at the fruit fly until it moves again, and only when it moves again does the frog take it. My theory is that by toe-tapping, the frog might trigger the fruit fly to move again so that it can be sure the fly is a prey item that they want to eat.

“It’s such an interesting, understudied behavior, and it might be a form of communication between predator and prey.”