SHAKE, rattle and strike. It is one of the most terrifying sounds in the animal kingdom, but how the rattlesnake evolved its chilling warning signal is a mystery. 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. Now his study suggests the rattle evolved long after the tail-shaking behaviour. His team 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 (The American Naturalist, doi.org/bqpv). “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.” This article appeared in print under the headline “What came first: rattle or shake?