The rattlesnake trick to fool our ears

Small details that go unnoticed by ordinary mortals can be a crucial clue to the attentive scientist. This happened when the researcher from the University of Graz Boris Chagnaud visited a zoo recently and observed that snakes rattlesnakes changed the tinkling frequency of their tails as he got closer or farther of them. The detail caught his attention and he decided to check what was happening.

Now, in a job published this Thursday in the journal Current Biology, Chagnaud and his team detail a series of experiments with a group of 13 rattlesnakes (Crotalus atrox) whose behavior they have studied for the first time since a double point of view: from that of the snake itself and its reactions to a threat, and from that of humans or animals that approach one of these snakes and hear its warning signal.

That rattlesnakes vigorously shake their tails to warn other animals of their presence is widely known and studied. In previous work it had been shown that noises vary in frequencyBut little was known about the relevance of this phenomenon or what message it sends to listeners. And that is what Chagnaud and his collaborators have tried to understand.

A change in frequency

Based on their first observation, in the first phase of the study Chagnaud and his team conducted a series of experiments that consisted of placing objects that appeared to move towards rattlesnakes and observing their reactions. One object they used was a human-like torso, and another was a black disc that was coming and that it seemed to be approaching as it increased in size.

As the potential threats loomed, what the researchers saw was that vibration rate increased at about 40 Hz and then abruptly switched to an even higher frequency range, between 60 and 100 Hz.

A rattlesnake about to sound its warning | Tobias Kohl

“Combining looming visual stimuli with acoustic measurements, we show that rattlesnakes increase their rattling rate (down to about 40 Hz) with decreasing distance from a potential threat,” they write. A system that reminds them of the acoustic signals emitted by the parking sensors of modern cars. In the presence of a threat, they say, rattlesnakes they change abruptly to a higher and less variable frequency of 60 to 100 Hz.

The snakes adapted the rattling frequency to the speed at which the object was approaching

But not only that. Additional results showed that rattlesnakes adapt their rattling rate in response to the object’s approach speed rather than its size. “In real life, rattlesnakes make use of additional vibratory and infrared signals to detect approaching mammals, so we would expect rattling responses to be even more robust,” says Chagnaud.

The receiver’s point of view

For the second part of the work, the one that concerns the way in which animals capture the sound of the snake, Chagnaud’s team designed a virtual reality experiment in which 11 participants were led to believe that they were walking through a meadow towards a hidden snake. The rattling speed of the virtual snake increased as the humans got closer and it suddenly leapt to 70 Hz at a virtual distance of 4 meters. In the interval, se asked listeners to indicate when the sound source appeared to be one meter away. And, consistently, the sudden increase in the frequency of the noises caused the participants to underestimate their distance to the virtual snake.

The frequency of the rattle caused participants to underestimate their distance to the virtual snake | Wikimedia Commons

According to the researchers, this reaction shows that the abrupt change in the rate of rattle generates a sudden perception and strong decrease in distance at receivers which, along with the low-frequency rattle, acts as an interspecies communication signal.

An “elegant” evolutionary solution

“Our data shows that the acoustic display of rattlesnakes, which has been interpreted for decades as a simple acoustic warning signal about the presence of the snake, is in fact a much more intricate interspecies communication signal”Says Chagnaud. “The sudden switch to high-frequency mode acts as an intelligent signal that tricks the listener into their actual distance to the sound source. The misinterpretation of the distance by the listener creates a margin of safety in the distance ”.

“The rattle of snakes coevolved with the auditory perception of mammals by trial and error”

“Not only did the snakes rattle to announce their presence, they developed an innovative solution: a sonic distance warning device similar to that found in cars while driving backwards, ”adds Chagnaud. “Evolution is a random process, and what we could interpret from today’s perspective as an elegant design is in fact the result of thousands of trials of snakes encountered with large mammals. The rattle of the snakes coevolved with auditory perception of mammals by trial and error, leaving the snakes that were better able to avoid being stepped on ”.

Reference: Frequency modulation of rattlesnake acoustic display affects acoustic distance perception in humans (Current Biology)


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