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Rump-Shaking Red-Eyed Treefrogs

May 27, 2010

This post was chosen as an Editor's Selection for ResearchBlogging.org
Ensuring paternity is not easy for male red-eyed treefrogs. At night, males perch themselves on the branches of saplings and make a sound called a “chack.” Each male hopes that a female will find his chack to be the sexiest chack of all—if she can even distinguish his chack from those of the other males.

If a male is lucky enough to get chosen by a female, he’ll hop on her back and “ride” her until they reach the perfect leaf (which must be over a body of water). As they travel, other males throw themselves at the male in an attempt to knock him off his newfound lady-friend and take his place. When they finally reach the perfect leaf, the female, carrying her male and a whole bunch of wanna-be mates that have latched onto the chosen one, attaches herself to the underside of the leaf. The female holds on to the leaf and waits while the males duke it out.

Photo: Wikimedia Commons, Brian Gratwicke

Eventually, she’s left with one partner—who may or may not be the one who seduced her. The duo begins a form of pseudocopulation known as amplexus where the female releases her eggs one-at-a-time and the attached male fertilizes them. Every so often, the female will descend to the water body below (with the male still attached) to re-hydrate herself so that her eggs have enough water to survive. While she hydrates, her male tries to maintain his position as other males try once again to dislodge him. When she’s ready, the female returns to her leaf and continues releasing eggs and the male on her back—whoever that may be—fertilizes them.

With such intense competition, it makes perfect sense that a male would want to kick the sh*t out of another male who encroaches on his branch when he’s busy chacking for the ladies. But it would be rude to attack a competing male without so much as a warning, so the males have evolved a warning of sorts…

Before a male attacks, he performs a “tremulation display.” In other words, he shakes his booty.

It goes something like this: One male shakes his booty. Then the other male shakes his booty. The booty shaking increases in intensity, which results in the whole branch vibrating in some cases. At some point they move past the simple rump-shake to the wrestle-and-rump-shake. This can last for hours, and ends when one frog gets knocked off the branch (or one frog decides the chacking location is way overrated and takes off).

The winner, according to data recorded by an accelerometer placed on a branch, tends to be a better rump-shaker than the loser. During the fight, the winning male vibrates more often than the loser, vibrates for longer periods than the loser and usually has the last butt jiggle.

To try to understand this silly, but apparently effective behavior, scientists used a robotic red-eyed tree frog and a vibrator (i.e. something that produces vibrations at 12 Hz). They placed the robo-frog on a nearby branch within sight of the real frog and the vibrator on the chacking male’s branch. In some trials, the robo-frog bounced up and down while the vibrator was off. In others, the vibrator shook the chacking male’s branch while the robo-frog sat still. In the last set of trials the vibrator vibrated and the robo-frog boogied.

The chacking male wasn’t particularly fond of either the robo-frog or whatever was making his branch shake, but he didn’t get his groove on unless the vibrator shook his branch. Apparently the boogieing robo-frog wasn’t all that inspiring. These results suggest that the frogs vibrate to communicate their existence to one another. If a frog can see a rival male, he may not need to vibrate. But if there’s a leaf blocking his view, he’ll vibrate to let the other guy know he’s there.

The authors of this paper wonder if the red-eyed treefrog’s use of vibrational communication suggests that other tree-dwelling animals use vibrations to communicate. We wonder if the red-eyed treefrog’s butt-shaking and the masked birch caterpillar’s butt-dragging suggests a pattern of animals’ using their butts as a first line of defense.

Caldwell MS, Johnston GR, McDaniel JG, & Warkentin KM (2010). Vibrational Signaling in the Agonistic Interactions of Red-Eyed Treefrogs. Current biology : CB PMID: 20493702

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