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.
“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.”
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