The loud rattle of a rattlesnake is a clear warning signal: don’t come any closer or I’ll bite you. However, if we approach, the snake slowly increases the speed of its tail rattling, then abruptly switches to a high frequency – and this is no coincidence, as experiments have now confirmed. They reveal that the reptile is making use of an acoustic deception: The sudden increase in the rattle frequency deceives people or animals as they approach the actual distance to the snake. The danger seems closer than it is. Ideally, the enemies flee early and the snake creates a kind of safety zone around it.
Even if the rattlesnake only has to fear a few animals thanks to its highly potent venom, it also has predators: coyotes, dogs, cats, birds of prey and, last but not least, humans can pose a deadly danger to the pit viper. Usually she escapes this by her camouflage coloring and by hiding. But if a potential threat comes too close to her, she uses her tail rattle as a threat and warning signal. This rattle consists of loose horn scales that are left over from past moults and that hit each other when the tail end vibrates. Depending on the circumstances, this rattling can vary in strength and speed. Observations suggest that the closer a potential enemy gets, the faster the rattlesnake rattles.
Warning signal in two stages
Michael Forsthofer from the Ludwig Maximilians University of Munich (LMU) and his colleagues have now investigated in more detail what is behind the rattlesnakes’ rattling and what function its tempo changes could have. Specifically, they wanted to know how exactly the rattle changes in response to the distance to the potential predator and, conversely, what it does to an approaching living being. To do this, they first stimulated some Texas rattlesnakes (Crotalus atrox) by moving a large black circle or a human dummy towards them at different speeds. It was found that the snake initially produces a rattle at a low frequency that gradually increases to around 40 Hertz. The intensity of this change depends on the distance and the speed of approach, as the researchers found. “It is a bit reminiscent of the acoustic signals of a car when parking,” they write.
But if a certain distance is not reached, the rattlesnake shifts up a gear, so to speak: Its rattling now changes abruptly to a higher, almost constant frequency of 60 to 100 Hertz. But why? To find out, the scientists confronted some human test subjects with a virtual rattlesnake. With the help of VR glasses, they transferred their test subjects to a grassy landscape in which the rattle of a snake hidden in the grass suddenly sounded. Depending on the passage, this rattle either changed continuously or there was a switch to high-frequency rattle when the snake’s virtual location was approached up to four meters. During the tests, test subjects were asked to indicate when they believed they were within three feet of the rattlesnake.
Acoustic illusion
It was found that the test subjects assessed their distance from snakes very differently. In the passages in which the rattling changed abruptly, they usually believed at this point that they were only a meter away – although the real distance was four meters. If, on the other hand, the rattle remained the same or only swelled evenly, the distance estimates were more precise. According to Forsthofer and his colleagues, this indicates that the rattlesnake is exploiting a deep-seated peculiarity of our perception: Because we interpret increasing volume or frequency as distance signals, the sudden frequency jump leads us to believe that we are closer than previously thought. “The sudden change to high-frequency mode deceives the listener about the actual distance to the sound source,” explains senior author Boris Chagnaud from Karl-Franzens-Universität Graz.
In principle, the rattlesnake uses a sophisticated acoustic illusion to create a safety margin. “For decades, the acoustic signals of the rattlesnake were interpreted as a simple warning signal of the presence of the snake. Our data now show that this is a far more complex inter-species communication signal, ”says Chagnaud. “The rattlesnakes not only rattle to indicate their presence, but have developed an innovative solution: an acoustic distance warning similar to the one in our cars.” to be discovered or trodden on.
Source: Michael Forsthofer (Ludwig Maximilians University Munich) et al., Current Biology, doi: 10.1016 / j.cub.2021.07.018