Scientists have discovered that rattlesnakes make an abrupt change in the frequency of their rattles to trick predators like humans into thinking they’re closer than they actually are.
The new study, published in Current Biology, suggests that western diamondback rattlesnakes may use their buzzing booties as a tool for deception.
“Our data show 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 far more intricate interspecies communication signal,” says study co-author Boris Chagnaud, a biologist at Austria’s Karl-Franzens-University Graz, in a press release. Chagnaud likens the increase in rattle frequency to a reversing car that beeps faster and faster as you get closer to an object.
It’s no secret that rattlesnakes use their rattles—hollow keratin structures made out of the same stuff as human finger nails—to make their presense known, but how they employ different rattle frequencies for defense is less understood.
Chagnaud got the idea for the study after noticing that a snake in an animal care facility he was visiting rattled faster as he approached its enclosure. Chagnaud wondered how the snakes might be using their rattle frequency to send a message to those nearby. The message? “Hey. I’m sitting here. Don’t step on me because I’m going to bite you,” says Chagnaud to the New York Times’ Sabrina Imbler.
To investigate their question, the research team designed a series of experiments in which an object—including a humanlike torso and large black disk—would approach a western diamondback rattlesnake. As the object got closer, the snake jumped from a low-frequency rattle of around 40 hertz to one closer to 70 hertz, according to Science’s Rachel Fritts. The exact distance the snake made the switch to a faster rattle varied by individual.
The team was curious if the higher-frequency rattle really sounded closer to humans, and sent 11 people into a virtual reality grassland peppered with hidden snakes. At lower-frequency rattles, participants could estimate the virtual snake’s distance with reasonable accuracy. Then, when they came within four meters of the serpents, the rattle frequency jumped. When participants were asked to press a button when they thought they were one meter away from a virtual snake, they consistently underestimated the distance.
"Evolution is 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,” says Chagnaud in a press release. “The snake rattling co-evolved with mammalian auditory perception by trial and error, leaving those snakes that were best able to avoid being stepped on."
The higher frequency rattles may seem closer to humans because of a sound-perception quirk that makes the individual rattle sounds blend into a single note. That note seems louder despite being the same amplitude, reports Jason Bittel for National Geographic.
"Like other snakes, rattlesnakes, of which there are numerous species in North America, are more interested in being undetected than confronting any other animal other than their prey," says Whit Gibbons, a herpetologist who was not involved in the study, to CNN’s Megan Marples.
Rattlesnakes’ deceptive tactics may help them avoid close encounters, which should be welcome news to humans.