Emu and Kiwi: The Ear and Hearing in Paleognathous Birds

Palaeognathae are undoubtedly the most ancestral of all living birds and are central to understanding the early evolution of birds, especially of auditory processing. The large paleognathous birds are characterized by low-frequency vocalizations whereas the smaller paleognathous birds vocalize at higher frequencies with hearing in those that have been tested matched to their vocalizations. Data from the emu suggest that the evolution of avian auditory hair cell types started with the loss of afferent innervation to the short hair cells, followed by a differentiation of the efferent neurons supplying tall and short hair cells. It remains unclear when the full differentiation of the physiological properties of tall and short hair cells occurred. Both paleognathous species whose hearing has been studied show evidence for auditory specializations. In the emu, this specialization corresponds to an overrepresentation of the lower end of their hearing range, as seen in a logarithmic frequency representation on its basilar papilla. In kiwi, the opposite is true, with a specialization at the higher end of their hearing range, supported, for example, by an overrepresentation of hair cells with the same or very similar morphology. The auditory brainstems of emu and kiwi show many features similar to that of the gallinaceous birds, with differences corresponding to the specializations of the peripheral auditory system.