Tree frogs shake off rivals

Tree frogs use a kind of seismic vibration to shake off unwelcome rivals, new research finds.

Central American red-eyed tree frogs use vibration to help them secure mates. (Photo: Greg Johnston)

IN WHAT APPEARS TO be a tantrum, red-eyed tree frogs of Central America use vibration to ward off unwelcome male competitors, new research has found.

The male frogs shake the branches they're sitting on to improve their chances of finding a mate by keeping rivals at bay. This is the first evidence that tree-dwelling vertebrates use vibration to communicate - but the strategy might be more widespread than anyone thought, says Greg Johnston of Flinders University, Adelaide, who was involved in the work.

Greg originally set out to investigate why the frogs (Agalychinis callidryas) are so brightly coloured. He suspected they might be using the colour to communicate, so he established a study site at a breeding pond near Gamboa in the Panamanian rainforest to watch their behaviour.

Other researchers had noticed that males struck a particular pose, which  exposed their brightly-coloured sides, and shook when they got close to each other.  "It looks like they're having a tantrum," says Greg. "I saw it many times - and I noticed that each time the frogs shook, so did the branch they were on."

But this happened only if two males were on the same branch. If they were close together, but on different branches, the shaking didn't happen. "I wondered if the shaking was a vibrational - or seismic - signal, and the pose only coincidentally showed off the colours ," says Greg, whose work is published in the journal Current Biology.

Seismic research

During the initial study season, Greg and colleague Karen Warkentin, who were both then working at the Smithsonian Tropical Research Institute in Panama, used a miniature seismograph to measure the vibrations, and it seemed they were indeed being used for communication. The pair's colleagues at Boston University then did the definitive experiments to test the idea, using a robot frog. 

They found that the shaking was indeed an aggressive display to warn other males to stay away.

The male frogs generally defend a small tree, or a branch of a larger tree, near a pond which females head to when they come out of rainforest canopy to breed. In most frogs, females choose males based on their call, but there's no evidence these frogs follow the norm - the females seem to breed indiscriminately with whoever's near the water. Many other male frogs make sounds to deter other males, but the vibration strategy has the benefit of being undetectable by a predator, unless they happen to be on the same branch.

It has been suggested that some other tree-dwelling frogs and lizards may use vibration to communicate, buy there's no sound evidence for this yet, says Greg. "We don't know yet if any Australian frogs communicate through vibrations, but seismic signals may well be a widespread message of communication that we are just starting to really understand."

 Ross Alford, an expert on frogs at James Cook University in Brisbane, welcomes the findings. "The field experiments are a pretty convincing demonstration that the vibration itself serves as a signal ... This is a nice example of where some simple, basic, curiosity-driven research has opened up a whole area of investigation," he says.