The rattlesnake’s warning sound, a grating ch-ch-ch, is one of the most spine-tingling noises in nature—because if you can hear it, you’ve already wandered too close.

But according to a new study published August 19 in the journal Current Biology, that communication is more complex than anyone realized.

By analyzing the warning vibrations of western diamondback rattlesnakes (Crotalus atrox), scientists learned that the serpents rattle at lower frequencies of up to 40 hertz, or more slowly, when a threat is far away. But when an intruder gets too close for comfort—a distance that differs depending on the individual snake—the rattlers abruptly shifted to a faster, higher-frequency warning signal of between 60 and 100 hertz.

When test subjects were asked to listen