Rattlesnakes are native to the Americas and are famous for the rattling noise they make by vibrating the tail tip segments when danger approaches. Previous research has shown that the frequency of this acoustic signal changes abruptly as a threat gets closer, but the significance of this change was not explained.
In a new study published in the journal Current Biology, researchers propose that the sudden change to a higher frequency has the effect of making approaching predators (and humans) think that they are closer to the snake than they really are.
Study senior author Boris Chagnaud of Karl-Franzens-University Graz visited an animal facility where rattlesnakes were housed. He noticed that the characteristic rattling increased in frequency as he walked towards a snake but decreased again when he walked away. Chagnaud and his colleagues designed some ingenious experiments to test the function of this frequency change.
Initially, the researchers used objects that appeared to move closer to a snake by increasing in size. One of the objects looked like a human torso, while another was a large, black disk. As these potential threats appeared to come closer to the snake, the rattling rate increased gradually to approximately 40 Hz and then suddenly jumped to an even higher frequency of between 60 and 100 Hz.
Subsequent experiments showed that rattlesnakes modulate their rattling rate in response to the speed at which an object is approaching, rather than its apparent size. “In real life, rattlesnakes make use of additional vibrational and infrared signals to detect approaching mammals, so we would expect the rattling responses to be even more robust,” says Chagnaud.
To test the effect of this abrupt change in rattling frequency on human listeners, the researchers enlisted the help of 11 participants who moved through a virtual grassland environment where a virtual snake lay concealed. They were asked to estimate their distance from the hidden snake as its rattling gradually increased in frequency.
When the individuals were at a virtual distance of four meters from the sound, its frequency abruptly jumped to 70 Hz. This sudden change in frequency caused the participants to perceive that they were much closer to the virtual snake than they actually were.
“Our data show that the acoustic display of rattlesnakes, which has been interpreted for decades as a simple acoustic warning signal about the presence of the snake, is in fact a far more intricate interspecies communication signal,” explains Chagnaud.
“The sudden switch to the high-frequency mode acts as a smart signal fooling the listener about its actual distance to the sound source. The misinterpretation of distance by the listener thereby creates a distance safety margin.”
“Snakes do not just rattle to advertise their presence, but they evolved an innovative solution: a sonic distance warning device similar to the one included in cars while driving backwards.”
“Evolution is a random process, and what we might interpret from today’s perspective as elegant design is in fact the outcome of thousands of trials of snakes encountering large mammals. The snake rattling co-evolved with mammalian auditory perception by trial and error, leaving those snakes that were best able to avoid being stepped on.”
By Alison Bosman, Earth.com Staff Writer