Death rattle Tristan Savatier/Moment Open/Getty

Shake, rattle and strike. It is possibly one of the most terrifying sounds in the animal kingdom, but how the rattlesnake evolved its chilling warning signal is a mystery. Now a study suggests the rattle evolved long after the tail-shaking behaviour.

The evolution of the rattle has baffled scientists because, unlike other complex physical traits like eyes or feathers, it has no obvious precursor or intermediate stage.

“There is no half-rattle,” says David Pfennig at the University of North Carolina at Chapel Hill.


One theory is that ancestral snakes shook their tails to warn off predators, and the noise-making rattle – which is made of a series of hollow, modified keratin scales – evolved later as a more effective signal that took advantage of the pre-existing behaviour. This may be why many rattle-less snakes also shake their tails.

To test the idea, Pfennig and his colleague prodded 56 species of venomous and non-venomous snakes with a fake rat on a stick and recorded their defensive tail shakes.

They found that the more closely related a snake was to the rattlesnake, the more similar its tail shake was in speed and duration.

Effective signal

“This suggests the defensive tail vibration came first, perhaps as a physiological response to stress, and that became a reliable cue to predators that the snake was about to strike,” says Pfennig. “When the rattle evolved, it became an even more effective signal.”

How exactly the rattlesnakes then got their noisemaker is a more difficult question, says Pfennig. There are two possibilities, he says. One is that some snakes were genetically pre-disposed to retain some extra skin on their tails when they shed, which made noise when they shook it and so this trait was selected for until it developed into the rattle.

The other, more controversial idea, is that the snakes developed a callus on their tail from shaking it against the rough ground, and if the propensity to develop a callus was affected by genetic variability, it would be selected for until the structure underwent “genetic assimilation”, and the rattle would form without the need for irritating the skin.

Pfennig can’t say which explanation is more likely.

This research does not mean that behavioural changes always come before physical evolution, says Mary Jane West-Eberhard at the Smithsonian Tropical Research Institute, but it suggests that they can – and it might be quite common.

“It should give us a renewed appreciation of the importance of environmentally sensitive behaviour for evolution,” she says.

Journal reference: The American Naturalist, DOI: 10.1086/688017

Read more: Watch a rattlesnake plan attack by clearing path for its strike; Adapt first, mutate later: Is evolution out of order?