Rattlesnakes develop acoustic mechanism to ‘trick the human ear’

The sound of a rattlesnake’s tail has always been a feature of movies.

The sound results from the rapid vibration of hard keratin rings at the tip of the reptile’s tails.

Keratin is the same protein that makes up our nails and hair.

The main ability to make this frightening noise is that the snake can vibrate its tail muscles up to 90 times per second.

It relies on this powerful vibration to warn other animals and humans of its presence and not to approach it.

Despite this sound warning, rattlesnakes are still responsible for the majority of recorded bites against humans in the United States of America, which amount to 8000 bites each year.

Researchers have known for decades that a snake can change the frequency of the bell it emits, but there is little research on the significance of the shift in pitch.

In this study, scientists conducted experiments by moving a human-shaped object near a rattlesnake and recording its response.

The closer the object is to the snake, the higher the frequency of the vibrations up to about 40 Hz. This was followed by a sudden jump in sound whose frequency ranged between 60-100 Hz.

To find out what the sudden change in pitch meant, the researchers conducted further work with human participants and a hypothetical snake.

Participants perceived the bell’s increasing rate as getting louder and louder the closer they got.

The scientists found that when the sudden change in frequency occurred at a distance of 4 metres, the test subjects thought the snake was much closer, about 1 metre away.

The researchers believe that the switch in the sound is not just a simple warning, but a complex communication signal between these types of snakes.

“The sudden switch to high frequency mode acts as a smart signal that fools the listener about the actual distance between him and the sound source,” says lead author Boris Chagno, from Karl Franzens University in Graz, Austria.

He added that “the misinterpretation of distance on the part of the person and his avoidance of approach provides a margin of safety of the distance between the snake and the person.”

The study authors believe that snake behavior takes advantage of the human auditory system, which has evolved to interpret increased loudness as something moving faster and approaching.

“Evolution is a random process, and what we might interpret today as good design is actually the result of thousands of experiments with snakes facing large mammals,” said Dr. Shagno.

“Snake rumbles co-evolved with mammalian auditory perception by trial and error, leaving those snakes best able to avoid being trampled,” he added.

The study was published in the journal Current Biology.