Red-Eyed Rump Shaker

A photo of a red-eyed treefrog taken
by Carey James Balboa at Wikimedia.

At night, male red-eyed treefrogs gather on saplings over Central American forest ponds to show off their stuff for the ladies, producing self-advertising “chack” calls. Despite the fact that they gather in groups, they defend their calling territories from flirtatious male competition. Females assess the available males and usually mate with a single male, who mounts her and clings on for dear life in a behavior called amplexus until she lays eggs that he then fertilizes. Occasionally, multiple males will try to mate with the same female at the same time, which usually results in two ticked-off male frogs.

What does an angry red-eyed treefrog do, you may ask? A mildly irritated treefrog will likely produce territorial “chuckle” calls, to let rivals know this is his favorite calling-plant and they’d better step-off. But a really ticked-off red-eyed treefrog rapidly lifts and lowers his hind end in a behavior called tremulation.

 
"Step off, I'm doing the hump!" Video by Michael Caldwell.

 Not much is known about this tremulation behavior. Is it something they do just to release anxiety or is it a communication signal? If it is a communication signal, is it a visual signal or a vibrational signal or both? And what exactly might it communicate?

Michael Caldwell, Karen Warkentin and Gregory McDaniel from Boston University, and Gregory Johnston from Flinders University in Australia, set out to ask the red-eyed treefrog if the tremulations were a communication signal and what they may mean. But without Dr. Doolittle’s powers of talking to the animals, how can scientists determine what and how animals are communicating?

First, the researchers observed natural interactions between males at choruses in the wild and recorded everything they did. They found that male red-eyed treefrogs will often approach another male while making “chuckles” and “chacks”. These males also tremulated in every aggressive interaction observed. Some of these males kicked with their back legs and some encounters even escalated to wrestling. Eventually (usually anywhere from a minute to an hour later, but occasionally several hours later), one of the males would submit by fleeing the plant or remaining silent and motionless. The dominant male would then resume his self-advertising “chack” calls. So males use tremulation in aggressive contexts with other males, but does that mean that it is a signal?

Males that won encounters tremulated more and used more
"chack" and "chuckle" calls than did males that lost encounters.
Figure from Caldwell et al. 2010 Current Biology paper.

The researchers then conducted staged contests by placing pairs of calling males on the same sapling. In these staged contests, males showed all the same aggressive behaviors the researchers had observed in natural conditions, and most ended in a wrestling match. The males that won their encounter produced more calls and more tremulations than did males that lost their encounter (Check out the graph above). So tremulations are used in the context of aggression with other males and winners tremulate more than losers. It looks like these tremulations are an aggressive communication signal, but to know for sure, we need to know if other males respond to them. And are tremulations a visual signal, a vibrational signal, or both?

So the researchers had to get creative and take it one step further: They put a robotic frog on a vibrating shaker that could mimic the visual display of a tremulation. They attached a separate vibrating shaker to the plant to mimic the vibrations of a tremulation. Now, they could look at the effects of the visual and vibrational components of the tremulation behavior separately!



Robofrog! Notice the jointed limbs and the metal rod sticking out of the robot's
belly. That rod is connected to a shaker that moves the robot so it looks like
he is performing a tremulation display. A separate shaker is connected to the
sapling to send the vibrational component of the display. This way, the
researchers can expose frogs to the visual component and the vibrational
component of the tremulation display separately. Photo by Michael Caldwell.

The researchers compared male red-eyed treefrogs that were exposed to (1) nothing, (2) a frog robot that does nothing, (3) a frog robot that “tremulates” with both plant vibrations and visible movement, (4) “tremulation” vibrations in the plant, without the frog robot, (5) a frog robot that moves it’s butt up and down but doesn’t produce vibrations, and (6) white noise vibrations in the plant (this is just a generic vibration).

Males responded aggressively to the imitated tremulation vibrations, visual or combined but not to any of the other treatments. This suggests that tremulations are a communication signal that rival males respond to. Interestingly, males only tremulated in response to tremulation vibrations. This suggests that the vibrational component is important to sending the full aggressive signal.

Males have aggressive responses to the visual display alone,
the vibration alone, and the visual display combined with the
vibration. But males only tremulated in response to vibrational
signals. Figure from Caldwell et al. 2010 Current Biology paper.

The sensitivity to soundless surface vibrations is widespread among animals, but we know very little about vibrational communication, especially in vertebrates. Michael, Gregory, Gregory and Karen have cleverly shown us that male red-eyed treefrogs use vibrational signals in contests with each other. How many other species will we discover using this silent channel of communication if we just listen?

Want to know more? Check this out:

Caldwell MS, Johnston GR, McDaniel JG, & Warkentin KM (2010). Vibrational signaling in the agonistic interactions of red-eyed treefrogs. Current biology : CB, 20 (11), 1012-7 PMID: 20493702