Wednesday, September 15, 2010

Dancing in the Dark: Tree Frogs Illuminate the Art of Vibration as Communication

You and I may use “no trespassing” signs to deter would-be invaders from stomping on our turf, but what if we could use something much more primitive – and perhaps more jarring – to get our message across?  If we were male red-eyed tree frogs, we would shake invaders away.  

In a paper published in Current Biology in July, Michael Caldwell and his colleagues from Boston University were the first to establish shaking, or tremulation behaviors as a means of vibration-based communication among red-eyed tree frogs.  While decades of research have been dedicated to visual and acoustic signals among animals, scientists have generally overlooked vibration as a means of communication. Caldwell’s article demonstrates, for the first time, that the vibration in the male tree-frog’s aggression display is more important than the visual dance.   

Red-eyed tree frogs are among the most photographed animals in the world, thanks to their distinct appearance.  With bright red eyes, vibrant green skin, blue bellies, and orange hands and feet, they make for a compelling image.  And their rump-shaking territorial behavior makes for compelling video.

Under the cover of darkness, these colorful frogs stake their claim among the leaves and use a sound called a "chack" to call for females. If a second male frog should appear on the same branch, the contest is on and the shaking begins.  With hands and feet wrapped around the branch, the frog rises up on its limbs and begins shaking, engaging in what looks like a frog tantrum, as Caldwell puts it.  If the encroaching frog isn’t immediately deterred, a fearsome wrestling match ensues until one of the frogs gives in and hops away. 

Your trespassing will have consequences.  Adorable ones.       
According to Karen Warkentin, Boston University Professor and Senior Author on Caldwell’s article, frogs have long played an important role in research on animal communication, but research into vibrations as message-carriers is pretty new.     

Using infrared cameras to catch the choreography (the frogs are shy – they only dance in the dark), Caldwell found that the shaking is always at a constant rate of 12 hertz, or 12 times per second.  The regularity of the shaking secures it as a clear signal to the other frogs, so it’s not easily mistakable for the effects of wind or rain. 

Caldwell’s team made a robot with a faux-frog on it to test whether the real frogs were responding to the visual or vibrational aspect of the shaking.  They found that the frogs did not shake back when the robot frog moved but the branch didn’t, further establishing the vibrations felt in the branch are the cue to start the fight. 

“Michael showed that the adult frogs use vibrations in male-male aggression more than they use sound, and I’ll be really surprised if these are the only animals that do,” said Warkentin.  

Special thanks to Michael Caldwell and John Christy of the Smithsonian Tropical  Research Institute, Karen Warkentin of Boston University, Peggy Hill of the University of Tulsa, and Paul de Luca of the University of Toronto for helping inform this article.  

More images to come!  

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