Every school kid learns why a rattlesnake rattles. The venomous snake shakes the interlocking scales on the end of its tail as a warning to ward off predators. A new study finds that these crafty reptiles also trick their human listeners into thinking they’re closer than they really are.
Animals use all sorts of methods to defend themselves. Some rely on camouflage or playing dead. Others use physical or chemical features such as quills on a porcupine or the spray of a skunk.
Rattlesnakes quickly move their rattles, which are made of keratin—the same protein that makes up fingernails and hair. A snake gets a new segment on its rattle each time it sheds, but sometimes segments can break off.
“The accepted reason why rattlesnakes rattle is to advertise their presence: it’s basically a threat display: I am dangerous!” study senior author Boris Chagnaud of Karl-Franzens-University Graz in Austria, tells Treehugger.
“The snakes prefer to advertise their presence not to be preyed upon or stepped on. The advertisement likely saves them avoiding biting an approaching threat which results in an economy of venom, an important resource for the snake.”
But they don’t rattle all the time, he says. Whenever possible, they prefer to rely on their camouflage so they don’t reveal their presence to potential predators.
Studying How Rattling Changes
One day, Chagnaud was visiting the animal facility belonging to coauthor Tobias Kohl, chair of zoology at the Technical University of Munich. He noticed that the rattlesnakes changed their rattling as he approached them.
“You come closer to the snakes, they rattle with a higher frequency, you retreat, the frequency gets lower,” he says. “The idea for the study thus arose from a simple behavioral observation during a visit at an animal facility! We soon realized the snake rattling pattern was even more elaborate and led to a misinterpretation of distance, which we tested in a virtual reality environment on human subjects.”
The first part of the study was relatively low-tech, Chagnaud says. He and his team held experiments in which they projected a black circle in front of the snakes that increased in size and moved at different speeds. While the disc moved, they recorded the snakes’ rattling and videotaped them.
They found that as the potential threats got closer, the rattling rate increased to about 40 Hz and then switched to a higher frequency between 60 and 100 hertz.
“We quickly were able to show that the snake rattling was providing the information about distance before suddenly changing their modulation frequency to a higher one,” Chagnaud says. “We soon recognized that this change in frequency was a nice trick of the snake to change the perception of an approaching subject.”
The second element of the study was a little more difficult, he says. For that experiment, co-authors Michael Schutte and Lutz Wiegrebe designed a virtual reality environment where human subjects moved about and were exposed to synthetic rattlesnake rattling noises.
“We used an array of loudspeakers to simulate a stationary sound source (our virtual snake) and included elevation and loudness cues into our VR environment,” Chagnaud says. “The results from our experiments clearly showed that the adaptive rattling leads human subjects to misinterpret the distance to the sound source, i.e. the distance to our virtual rattlesnake when our virtual snake was using the rattling pattern seen from their biological counterparts.”
The results were published in the journal Current Biology.
Random Rattling Development
One of the most fascinating parts of the study is the connection between the rattling sound and the perception of distance in humans, the researchers say.
“Snakes do not just rattle to advertise their presence, but they eventually evolved an innovative solution: a sonic distance warning device - similar to the one included in cars while driving backwards,” Chagnaud says. “But suddenly snakes change their game: They jump to even higher rattling frequencies which leads to a change in distance perception. Listeners believe they are closer to the sound source than they are.”
Interestingly, rattling like this is relatively random, the researchers believe.
“The rattling pattern has evolved in a random process, and what we might interpret from today’s perspective as elegant design is in fact the outcome of thousands of trials of snakes encountering large mammals,” Chagnaud says.
Snakes that were able to stop predators with their rattles were the most successful and thrived in the “evolutionary game,” he says.
“To see how well their rattling pattern activates our auditory system, first providing distance information and then fooling subjects to underestimate the distance was for me just truly amazing.”
