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Forsthofer, Michael; Schutte, Michael; Luksch, Harald; Kohl, Tobias; Wiegrebe, Lutz and Chagnaud, Boris P. (2021): Frequency modulation of rattlesnake acoustic display affects acoustic distance perception in humans. In: Current Biology, Vol. 31, No. 19: 4367-

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The estimation of one's distance to a potential threat is essential for any animal's survival. Rattlesnakes inform about their presence by generating acoustic broadband rattling sounds.(1) Rattlesnakes generate their acoustic signals by clashing a series of keratinous segments onto each other, which are located at the tip of their tails.(1-3) Each tail shake results in a broadband sound pulse that merges into a continuous acoustic signal with fast-repeating tail shakes. This acoustic display is readily recognized by other animals(4,5) and serves as an aposematic threat and warning display, likely to avoid being preyed upon.(1,6) The spectral properties of the rattling sound(1,3) and its dependence on the morphology and size of the rattle have been investigated for decades(7-9) and carry relevant information for different receivers, including ground squirrels that encounter rattlesnakes regularly.(10,11) Combining visual looming stimuli with acoustic measurements, we show that rattlesnakes increase their rattling rate (up to about 40 Hz) with decreasing distance of a potential threat, reminiscent of the acoustic signals of sensors while parking a car. Rattlesnakes then abruptly switch to a higher and less variable rate of 60-100 Hz. In a virtual reality experiment, we show that this behavior systematically affects distance judgments by humans: the abrupt switch in rattling rate generates a sudden, strong percept of decreased distance which, together with the low-frequency rattling, acts as a remarkable interspecies communication signal.

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