Abstract
Biphonation, defined as the simultaneous production of two distinct, non-harmonically related fundamental frequencies, has traditionally been viewed as an anomaly or a by-product of vocal pathology. Recent studies have challenged this assumption and found that biphonic calls are present in the natural vocalizations of a wide range of taxa, including birds, amphibians and mammals. This phenomenon could play an essential role in communicating distinct pieces of information at short- versus long-distance, increase call complexity to allow more individually distinct calls, and provide cues to the sender's direction of movement. Proposed mechanisms underlying biphonation production include asymmetries in vocal fold oscillations, the addition of aerodynamic whistles, the involvement of secondary structures, and bilateral specializations. This scoping review underscores the adaptive significance of biphonic calls in non-human animals, highlighting their role in the evolution of vocal communication and suggesting avenues for future research.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.