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 a wide range species as well as contexts. Variation in the rate and mode of sound production has been extensively studied across different 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 weight 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 weight and dominant frequency, but that mammals clearly deviate compared to frogs and birds. We found mammals to vocalize at much higher frequencie,s 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.