The Question: I saw the post about frogs going silent. But my question is how frogs sense when someone is approaching, and go quiet immediately, all at once?
Submitted by: Peggy, USA

singing frogThe Short Answer: Peggy is referring to this post about frogs that had previously been singing night after night stopping. But Peggy’s question is different. Anyone who has ever tried to approach a bunch of croaking frogs has experienced that frustration that the frogs seem to know we’re coming. And often, as Peggy points out, they all seem to know we’re coming at once. Dr. Peter Narins, who studies acoustical behavior in frogs, at the University of California, Los Angeles, calls it “The zone of silence that surrounds herpetologists.”

It might be comforting to Peggy to know that it happens even to people who make it part of their mission in life to sneak up on frogs.

So the question then is how the frogs know when a herpetologist (or an amateur nature lover) is approaching. One answer is that herpetologists and nature lovers are giants. Imagine any movie you’ve ever seen that featured giants. With every step, they make the earth shake. Now compare the size of your average herpetologist to even the largest frogs. People are giants. And we make the earth shake with every step, even when we’re trying very hard to be sneaky. Frogs can not only hear every snapping twig or rustling leaf, they can feel the vibrations of the earth.

The simplest answer to Peggy’s question then is probably just that all the frogs in the vicinity feel the vibrations through the ground and they all stop what they’re doing to say, “What the heck is that??” In a movie with giants, this would be the point where the screaming starts and all the people run in different directions. In the world of frogs it’s when everyone shuts up.

Another possibility: Another possibility is that frogs are very much attuned to each other when they are singing. Many animal species in which males sing to females engage in synchronous singing, where the males all croak or sing nearly simultaneously. In some frogs and other animals, it’s been shown that females are impressed by the first male they hear sing. Because each male is vying to be the one to sing first, they all end up singing in the same rhythm. If one male tries to come in a little early, the others nearby, who are all listening very closely, change their timing, too, and everyone is singing together again. If a male tries to get in the middle of a gap in the notes, so that he might stand out, he runs the risk that to the listening females, he might not sound like the first … he might sound like the last. The end result of this competition to be first is a bunch of male frogs very closely synchronizing their songs.

Another possibility is that in some animals if males sing a relatively continuous song as opposed to short bursts, females seem to need a break in the music in order to be able to separate the song from background noise and respond to it. And if all the males are trying to sing for as long as possible, pause for as short a time as the females need, and then sing for as long as possible again, you can end up with synchrony because if you are the one male who is still singing while the females’ hearing is in the pause phase, you will be out of luck.

All of this is to suggest that males are listening to each other very closely. So another possibility for the “zone of silence” is that if you walk near a chorus of frogs and one frog hears or sees you and stops singing to hide, all the other frogs in the area might be saying to themselves, “Whoa, what made Fred shut up so quickly?” and they all shut up, too.

Dr. Narins believes the most likely answer is the simplest one: that all the frogs feel the earth shake and hear the twig snap at the same time. “We have shown that frogs are extraordinarily sensitive to substrate vibrations, and that when they sense these, they may rapidly cease vocalizing. Whether they all sense the vibrations and react to the common stimulus, or whether the first one who ceases calling acts as the stimulus for his neighbor to cease calling, resulting in a chain reaction that propagates through the population, is not known. But in my experience, since the seismic signal is often accompanied by an airborne component (a person approaching produces footsteps and the simultaneous audible crunching of leaves, for example), the former seems the more plausible explanation. But we have also shown that the seismic signal alone is sufficient to change the calling pattern of frogs.”

So the trick it seems, if you want to sneak up on frogs, is to walk softly and act as little like a giant as possible.

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