Tetrapod vocal evolution: higher frequencies and faster rates of evolution in mammalian vocalizations

Using the voice to produce sound is a widespread form of communication and plays an important role in contexts as diverse as parent-offspring interactions and courtship. Variation in the tempo and mode of vocal signal evolution has been studied in a phylogenetic context within orders or classes, but understanding vocal signal evolution ultimately requires comparison across all major lineages involved. Here we used comparative analyses to investigate the evolution of dominant frequency (i.e., the frequency with the highest energy content) and its association with body weight across 873 species of mammals, birds and frogs. In agreement with previous studies, we found that the negative allometric relationship between body weight and vocal frequency is a general feature of vocal systems. In addition, we found mammals to consistently vocalize at higher frequencies, and evolved their vocalizations at around 6-fold faster rates than those of birds and frogs. Although all three groups strongly rely on vocal communication, our findings show that only mammals have extensively explored the spectral acoustic space. We argue that such high vocal diversity of mammals is made possible by their unique hearing system, which evolved in the context of a small, parental-caring, nocturnal and insectivore ancestor, and has allowed them to detect, and therefore to evolve, a richer array of frequencies than other tetrapods.