Frequency modulation of rattlesnake acoustic display affects acoustic distance perception in humans

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.