Rattlesnake rattles trick human ears

Date:
August 19, 2021
Source:
Cell Press
Summary:
Rattlesnakes increase their rattling rate as potential threats
approach, and this abrupt switch to a high-frequency mode makes
listeners, including humans, think they're closer than they actually
are, researchers report.



FULL STORY ========================================================================== Rattlesnakes increase their rattling rate as potential threats approach,
and this abrupt switch to a high-frequency mode makes listeners, including humans, think they're closer than they actually are, researchers report
August 19th in the journal Current Biology.


==========================================================================
"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," says senior author Boris Chagnaud
at Karl-Franzens-University Graz.

"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." Rattlesnakes vigorously shake their tails to warn other animals of their presence. Past studies have shown that rattling varies
in frequency, but little is known about the behavioral relevance of this phenomenon or what message it sends to listeners. A clue to this mystery
came during a visit to an animal facility, where Chagnaud noticed that
rattling increased in frequency when he approached rattlesnakes but
decreased when he walked away.

Based on this simple observation, Chagnaud and his team conducted
experiments in which objects appeared to move toward rattlesnakes. One
object they used was a human-like torso, and another was a looming black
disk that seemed to move closer by increasing in size. As the potential
threats approached, the rattling rate increased to approximately 40 Hz
and then abruptly switched to an even higher frequency range, between
60 and 100 Hz.

Additional results showed that rattlesnakes adapt their rattling rate in response to the approach velocity of an object rather than its 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," Chagnaud says.

To test how this change in rattling rate is perceived by others,
the researchers designed a virtual reality environment in which 11
participants were moved through a grassland toward a hidden snake. Its
rattling rate increased as the humans approached and suddenly jumped
to 70 Hz at a virtual distance of 4 meters. The listeners were asked to indicate when the sound source appeared to be 1 meter away. The sudden
increase in rattling frequency caused the participants to underestimate
their distance to the virtual snake.

"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," Chagnaud
says. "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." ========================================================================== Story Source: Materials provided by Cell_Press. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Forsthofer and Schutte et al. Frequency modulation of rattlesnake
acoustic display affects acoustic distance perception in
humans. Current Biology, 2021 DOI: 10.1016/j.cub.2021.07.018 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210819113023.htm

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