Imagine walking through a field of tall grass gently rustling in the hot, dry wind of the American West. Suddenly, you hear an unmistakable whispery rattling. You look around, certain a rattlesnake is right under your feet and ready to strike. But it’s nowhere to be found.
Melissa Amarello, cofounder and executive director of a nonprofit organization called Advocates for Snake Preservation, says this has happened to her when studying rattlesnakes in the wild. “They sound like they are right here,” she says. “And they are not.”
New research published in Current Biology today (August 19) may provide an explanation. Scientists based in Germany and Austria found that western diamondback rattlesnakes (Crotalus atrox) quickly ramp up their rattling frequency when a potential threat appears to be getting closer to them. The sound produced by the switch to a higher-frequency rattle is perceived by human listeners as being louder, tricking them into thinking the snake is closer than it really is.
“I think this is a really cool study,” says David Pfennig, an evolutionary biologist at the University of North Carolina, Chapel Hill, who was not involved with the research. Pfennig says the observations are an example of phenotypic plasticity, or the ability of organisms to change their features in response to their environment. The snakes don’t just rattle at a set frequency, he says. “They can modulate that. And they can change that frequency depending upon their current environmental circumstances—in this case, the perceived distance from the threatening organism to them.”
According to Ludwig Maximillian University of Munich graduate student and coauthor Michael Forsthofer, the study was sparked by the authors’ observation that the sound of the diamondback’s rattling seemed to change when human researchers in the lab moved closer to them. Forsthofer says the question of how the snakes react when approached has been a question in the field for decades that has been addressed with some creative methods, including gently bopping snakes on the head with chainmail-gloved hands or using people with perfect pitch to identify the tone the rattlesnake is rattling at. According to the new study, although previous work noted variability in rattling patterns, researchers didn’t look at what might be causing that variability, or whether there could be additional information encoded in different rattling patterns beyond “there’s a snake close by.”
Initially, Forsthofer tried a similarly creative approach: manually pulling a human-sized model torso on wheels towards a snake in the lab and recording the sound of the animal’s rattle. Although this was a naturalistic stimulus, he says, he couldn’t ensure the torso was moving at the same speed every time. Additionally, the rolling torso made noise, and “if you want to analyze the sound the snake makes, it’s not very helpful to have another sound on top of that,” he explains.
Instead, Forsthofer and his colleagues settled on a visual cue that was less realistic but easier to control: an optical illusion. Outside the snake’s testing enclosure was a screen on which the researchers projected an image of a black circle that got bigger or smaller, appearing to move closer or farther away from the snake. While the snake was watching the “moving” circle, the researchers recorded the sound of its rattling and then measured the frequency.
They found that when the circle began “approaching,” the snakes would start rattling at a relatively low frequency of less than 40 Hertz, but would slowly increase the frequency as the circle got closer. If the circle still appeared to be approaching after the initial warning rattles, the snakes quickly switched to a high-frequency rattle of 60 to 100 Hertz.
Amarello, who was not involved with the study, says people tend to think animals such as snakes that are only distantly related to humans don’t have anything going on inside their heads other than basic instincts such as feed and breed. She says it’s really interesting “that a snake is actually paying attention to how close a threat is,” and that there’s some point in that trajectory where it changes how it’s rattling.
Snakes on a headset
Both Pfennig and Amarello tell The Scientist that rattlesnakes can’t hear their own rattles, and Pfennig says it’s thought that rattlesnake rattling is to alert potential predators to stay away—as opposed to communicating with other snakes or luring in prey.
Amarello says some people interpret the chilling sound of a rattle as a sign the snake is angry. “That’s not what it means at all,” she says. “This is a thing that rattlesnakes do when they are scared to death, when they are sure that lying still and being camouflaged isn’t going to work anymore.”
According to Pfennig, rattling is an example of what’s known as an honest signal, because the rattlesnake is signaling to a potential predator of another species that it’s genuinely dangerous. This benefits both parties, as the snake gets left alone and the predator doesn’t have to deal with a painful and venomous bite.
Because the researchers saw that the slow increases in low-frequency rattling corresponded with how close the stimulus was to the snake, they hypothesized that the abrupt shift in rattling frequency may serve to make it sound like contact with the snake was imminent.
As testing this hypothesis on humans in real life could be unsafe, the scientists turned to another technological solution: virtual reality. “Something that virtual reality is very handy for is when you can just create manipulations that would be impossible in the real world,” says coauthor Michael Schutte, who recently got his PhD from Ludwig Maximillian University of Munich.
Schutte and his colleagues recruited human volunteers to don virtual reality headsets that made them feel as though they were moving through a tall grassy landscape toward a snake—while actually sitting in a room facing a vertical array of loudspeakers. The speakers either played constant low-frequency rattling or low-frequency rattling that slowly intensified until it quickly shifted to high-frequency when the participant was within 4 meters of the virtual snake, which they could hear, but couldn’t see. In both cases, the sound got louder as subjects approached the snake. Participants were instructed to press a button to stop the automatic approach when they thought the rattlesnake was a meter away from where they stood in the virtual world.
The researchers found that when they played the high-frequency rattling, participants underestimated the distance between themselves and the snake. Participants hearing the high-frequency rattling also had significantly fewer misses (failing to stop within a meter of the snake) compared with people who heard the constant, low-frequency rattling—suggesting that, at least in the virtual world, the high-frequency rattling was an effective warning.
As it turns out, says Pfennig, the rattler’s honest signal becomes a little bit dishonest when the snake speeds up the rattling to make it seem closer than it really is.
Schutte and Forsthofer acknowledge that because both arms of the experiment involved a virtual stimulus, there are some limitations to what conclusions they can extrapolate to the real world, although Forsthofer says the snakes’ reaction to the circle threat seemed to jibe with how they reacted to humans approaching them in lab.
Additionally, Pfennig says that rattlesnakes and humans likely haven’t lived in the same habitats for long enough for this rattling behavior to have evolved to be specific to humans. He wonders how the high-frequency rattling affects other animals that are natural predators of rattlesnakes, such as coyotes, or animals such as bison that aren’t predators but could accidentally step on a rattlesnake.
“It would be really interesting to see if that trickery works on other species,” says Amarello. “Something that is an animal that they’re more likely—and more often—deterring with their rattle.”