Nonlinear phenomena make animal calls alarming for human listeners.

Animal vocalizations are extremely diverse, and evolutionary approaches to understanding this diversity assume some mapping between their acoustic form and communicative function, with specific features serving universal roles. Here, we investigate whether irregular vocal production with nonlinear phenomena contributes to the alarming quality of vertebrate calls. We resynthesized 98 calls of birds and mammals from 18 species, adding frequency jumps, subharmonics, amplitude modulation, or chaos. Human listeners then rated how alarming they found these calls in an immersive setting mimicking a forest at night. Chaos consistently made the calls more alarming, but other tested NLP did not, confirming that chaos is particularly suitable both for signaling alarm and, potentially, for intimidation in agonistic interactions. While our results suggest that nonlinear phenomena may have a broader function in the mammalian vocal repertoire, follow-up studies should now investigate whether these perceptual effects induced by nonlinear phenomena extend to receivers in non-human species.