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Rattlesnakes use sudden high-speed rattling to fool humans, and probably also animals, into believing that they are closer to the venomous vipers than they really are.

Like warning beeps in cars that get faster as drivers back up towards objects, rattlesnakes start their warning rattle at a slow, gradually increasing pace, then suddenly switch to a constant, high-frequency rattle. This gives the impression that contact is imminent – but in reality, the snakes could still be a full metre away, says at the University of Graz in Austria.

This auditory illusion works as a “smart signal”, providing a safety margin that probably protects the snake from getting stepped on or wasting valuable venom, he says.

“As you continue to approach the snake, he’s telling you, ‘You’re getting closer, you’re getting closer,’” says Chagnaud. “And then suddenly he says, ‘OK, now you’re way too close!’ even though you’re still a metre or so away. And that’s the trick of the snake.”

While visiting rattlesnake terrariums at the Technical University of Munich a few years ago, Chagnaud was intrigued to find that the snakes changed their rattling speed as he approached and backed away. He collaborated with researchers in Munich to test 30 of their Western diamondback rattlesnakes (Crotalus atrox) as they reacted to an approaching dummy human torso set on sliding rails, and to a growing black circle made by light projections onto a screen.

In both tests, the team found that the snakes’ rattling frequency would gradually speed up to about 40 rattles per second, then suddenly jump to an unchanging, high-frequency rattle ranging from 60 to 100 rattles per second depending on the snake.

The team then ran virtual reality tests on 11 people as they moved through a virtual grass field with different sounds. The researchers asked the participants to push a button when they believed they were within reach of the sound’s source and noted that the listeners were easily fooled by the sudden jump in rattle speed. “They thought they were 1 metre away from the [virtual snake], when actually they were more like 4 metres away,” says Chagnaud.

The results suggest the snakes have evolved a remarkable ability to deceive humans and animals in their processing of auditory clues about distance. “They’re not trying to [give fair warning to] save us from being bitten,” he says. “They’re advertising their presence to save themselves.”

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