A rattlesnake’s maracalike tail shake sends a clear message to any intruder foolish enough to approach: “Stay away!” When that message is ignored, the snakes pull an auditory sleight of tail, making it sound like they’re suddenly right there. According to a new study, they switch from a low- to a high-frequency rattle, convincing observers they are much closer than they actually are.
Rattlesnakes warn away enemies by shaking the interlocking hollow segments at the ends of their tails, which bump together and make a rattling sound. To better understand the variation in these rhythms, scientists mimicked an approaching threat by slowly moving a torsolike object toward a western diamondback rattlesnake (Crotalus atrox) sitting on a small platform—or increased the size of a black disk on a screen facing the snake (see video, above).
Of 25 snakes tested, all steadily increased their tail shakes as the torso approached, going from a slower, lower frequency rattle to a faster rattle until they reached a frequency of about 40 hertz (Hz). Then, as the torso got closer, they suddenly upped the ante of their rattles to between 60 and 100 Hz.
To find out how an intruder might react, the researchers simulated rattlesnake rattles in a virtual reality grassland for 11 people who were instructed to stop once their avatar was within 1 meter of the virtual snake. When participants heard a slow, steady rattle of 12 Hz, they often stopped as they approached the 1-meter mark. But when the rattles gradually increased to 20 Hz—and jumped to 70 Hz 4 meters away from the snake—participants stopped right after the jump about 20% of the time, the researchers report today in Current Biology. They conclude that the rapid shift functions as a sort of auditory trickery meant to startle humans—and other animals—into backing off.