Rattlesnakes ( Crotalus sp. ) Are a genus of poisonous vipers, endemic to the American continent, characterized by having horny cases at the end of their tails. These are filled with keratinous segments, capable of emitting sound when they collide with each other when shaken, as if it were a maraca. Something curious is that with each molt these snakes incorporate a new ring to the bell. However, since they can make up to two molts a year, or lose it completely, counting the rings is not an adequate method of knowing their age.
Generally, these vipers use the noise of their bell to make their presence known . By shaking it, they launch an aposematic warning, that is, they are saying to whoever approaches them ‘be careful, I’m close, I’m dangerous and you shouldn’t try to eat me’, something quite useful to avoid being stepped on or preyed upon by large mammals or birds.
The vibration of the sound they emit is usually well known by the rest of the animals with whom they share the habitat, when they hear it they can estimate the approximate distance they are from the snake and thus avoid crossing with it. However, these snakes have found a way to fool everyone who approaches them, making them believe that they are closer than it might appear. A recent study published in the journal Current Biology , carried out by scientists at the University of Munich (Germany) has discovered how they do it.
When rattlesnakes begin to feel threatened, their ‘maraca’ generates a continuous broadband sound of up to about 40 Hz. Suddenly, if the animal continues to feel provoked, it abruptly switches to a higher, less variable frequency of 60 at 100 Hz. A fact that German scientists discovered by testing 25 snakes by means of a projection in front of them, a black dot that was getting bigger on the screen, as can be seen in the video above. As a result, 100% of the snakes increased the frequency of the rattle as the spot grew larger, as it seemed like a possible danger approaching them.
This sudden variation, also simulated in a virtual reality meadow for 11 human volunteers, systematically affects the distance judgments on our part, making us believe that the reptile has come much closer than we initially believed, without even having moved from place. Participants stopped right after the frequency hopping 20% of the time. As a conclusion, the researchers say that this rapid change in frequency is very useful for rattlesnakes to scare and push back humans and other nearby animals.
References:
Forsthofer, M., Schutte, M., Luksch, H., Kohl, T., Wiegrebe, L., & Chagnaud, BP (2021). Frequency modulation of rattlesnake acoustic display affects acoustic distance perception in humans. Current Biology. Published. https://doi.org/10.1016/j.cub.2021.07.018
