Nonlinear phenomena in mammalian vocal communication: an introduction and scoping review.

Nonlinear phenomena (NLP) are common elements of mammalian vocalizations. Resulting from irregular sound production, they contribute to perceived harshness and are often present in calls conveying urgency or arousal. Initially dismissed as by-products of vocal production, NLP are increasingly recognized for their adaptive potential. However, NLP have never been the subject of a comprehensive review across vertebrate taxa. Here, we introduce NLP and examine developments in NLP studies in mammals. We found 220 papers published between 1962 and 2023, with publication rates increasing with time. The studies covered a wide range of taxonomic groups but were dominated by artiodactyls, carnivores, bats, rodents and primates. Tinbergen's questions offer a framework for future investigations, highlighting that while much research has been conducted on adaptive function, our understanding is still lacking in terms of ontogeny, mechanisms and evolution. The existing literature is a testimony to the importance of NLP in animal vocalizations. With the use of novel tools for analysis and playback studies, NLP research can become more cohesive and impactful, fostering better understanding among researchers. We look forward to a new age of NLP research, which we anticipate will lead to a paradigm shift in our understanding of vocal communication in mammals.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.