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Frogs Shake the Tree

Red-eyed tree frogs mating.

Red-eyed tree frogs mating. Males defend their territory, giving them access to females, by vibrating tree branches. Credit: Greg Johnston

By Stephen Luntz

A new form of animal communication has been revealed with the discovery that male red-eyed tree frogs send signals by shaking the branches of the trees in which they sit.

“Unlike most species of frogs, the red-eyed tree frog doesn’t show any evidence of females choosing a mate with the loudest or prettiest voice,” says Dr Gregory Johnston of Flinders University’s School of Biological Sciences. Instead, females seem happy to mate with every male into whose territory they wander, making, in Johnston’s words, “having territory really important”.

Johnston went to Central America to learn why the iconic frog is so much more brightly coloured than most frog species. He thought the colours might represent some form of signalling, particularly since the male frogs expose a particularly brightly coloured patch when other males encroach on their territory.

However, Johnston noted that this occurs whenever males shake the branch on which they sit, apparently as a deterrent to other males. He used miniature seismographs to demonstrate that the shaking was real, but lacked time to take the research further. However, a former colleague, now based at Boston University, sent PhD student Michael Caldwell to uncover what was going on.

Using a small robotic frog that could expose its coloured regions, shake the branch or do both at once, Caldwell was able to demonstrate that the branch-shaking is an important way for a frog to signal its size and strength, hopefully scaring off competitors. If the intruder is on another branch, or the shaking is not strong enough, other signals may be deployed. Where the frogs are evenly matched they may end up wrestling for the territory.

Although other species are known to communicate through vibrations other than sound, this is the first time that the discovery has applied to a tree-dwelling vertebrate. “It may be really common.” Johnston says. “It’s just that we’ve never noticed it before.”

Moreover, it may explain why frogs are disappearing from ponds near roads in the Central American rainforest. “The frogs are in national parks, but these huge logging trucks use the roads between unprotected areas,” Johnston says. “With massive trucks lumbering past, the frogs can’t hear themselves think.” Or maybe they fear unimaginably powerful rivals.

Signalling through shaking may be less obvious to predators than bright colour schemes, although this leaves unanswered the question of why the frogs are so colourful.