Tetrapod vocal evolution reveals faster rates and higher-pitched sounds for mammals.

Using the voice to produce sound is a widespread form of communication and plays an important role across diverse species and contexts. Variation in the rate and mode of sound production has been extensively studied within orders or classes, but understanding vocal signal evolution ultimately requires comparison across all major lineages involved. Here we used phylogenetic comparative methods to investigate the evolution of dominant frequency and its association with body mass across a balanced set of 873 species of mammals, birds and frogs. Our results show that all vocal systems share the same general feature of the negative allometric relationship between body mass and dominant frequency, but that mammals clearly deviate compared to frogs and birds. We found mammals to vocalize at much higher frequencies and their signals evolved 4- to 6-fold faster compared to other tetrapod clades. 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, and discuss the functional drivers that allowed their shared ancestors to evolve a richer array of frequencies than other tetrapods.