How do frogs demonstrate their bravery? By quivering like a coward. New research reveals that male red-eyed treefrogs (Agalychnis callidryas) shake a branch with their hind legs to signal a willingness to brawl with a rival. It's the first time researchers have seen this form of communication in tree-dwelling animals, and they say birds, lizards, and other creatures may similarly send signals through the branches.
Late at night in the wet jungles of Central America, red-eyed treefrog males sit on the branches of thin saplings and produce a sound called a “chack” to attract females. But when a rival homes in on a calling site, the two males pose aggressively and sometimes engage in fearsome wrestling matches. The fights can last hours, and, in some cases, both males end up locked in a grueling embrace, dangling by their hind legs from a branch.
During one of these fights, behavioral ecologist Michael Caldwell of Boston University (BU) observed an odd performance: the frogs briskly wiggling their butts up and down. Using a small accelerometer set on a tree branch, he and colleagues found that the victors in wrestling contests had vibrated more often and for longer than losers during the battle, and they were more likely to produce the final vibration. “So the vibrations allow a male to signal to anyone else on his calling site that he owns it and he’s ready to fight for it,” says Caldwell.
Please download  the latest version of the free Flash plug-in.
Because the frogs have excellent night vision, Caldwell wanted to know if they were responding to the visual sight of a shaking rival or the vibrations themselves. He set a robotic red-eyed frog on a wooden frame near to but not touching the tree branch of a chacking male and placed a shaker on the real frog’s branch to produce vibrations at 12 Hz, the typical frequency made by the frogs. In some tests, the robo-frog bounced up and down while the shaker remained off, providing a visual display without actually vibrating the branch that the real frog was on. In others, the researchers combined visual bouncing and vibrations, or they just turned on the shaker alone. Although the males responded aggressively in all of these cases, they only shook their branch in trials where vibrations were played, the researchers report  today in Current Biology. Caldwell concludes that the vibrations allow males to detect one another even when they can’t see each other, such as when a leaf or branch is in the way.
Previously, scientists have discovered animals ranging in size from scorpions to elephants  using vibrational communication on the ground, but no one had documented tree-dwelling animal communicating in such a way. Biologist Karen Warkentin of BU, a co-author on the paper, suggests that other creatures living in trees, such as birds, frogs, and lizards, might also use this mode of communication. “Vibrational sensitivity is very ancient—it predates hearing—so it would not be surprising to find that this is something that is relatively common.”
Entomologist John Sloggett, an independent researcher who works in the Netherlands, says the finding is surprising because red-eyed treefrogs are so common and well studied. “It’s really quite nice when you get something new coming out of a creature that we think we know a lot about already.”