Evolution of 'earlessness' in the true Toad family (Bufonidae), The

Abstract

Anurans (frogs and toads) have a tympanic middle ear to transmit airborne sound from the environment to their inner ear sensory cells. Yet, many bufonid (true toad) species have independently evolved earlessness, the lack of a tympanic middle ear, despite the importance of acoustic communication in most toad mating systems. My thesis aims to determine why middle ear structures are so evolutionarily labile in the Bufonidae family by comparing development, sensory, and morphological data of eared and earless toads within a phylogenetic context. I show that the middle ear forms very late in the development of toads and takes many months past metamorphosis to become fully functional. Adult earless species are typically less sensitive to high frequency sound and more sensitive to low frequency vibrations compared to eared toads. I also find the skulls of eared and earless are very similar, indicating the middle ear is lost without change to other developmentally or genetically linked skull features. I conclude that alternative hearing pathways allow earless species to retain some hearing sensitivity, and discuss roles for development and behavior in shaping the evolutionary lability of ear structures.