Occurrence and Independent Evolution of Bilateral Ear Asymmetry in Owls and Implications on Owl Taxonomy

R. A. Norberg

Abstract

On the basis of the literature and my own examination of living and/or dead but fresh owls of 16 species, bilateral asymmetry of external ears in owls is surveyed and ear structure briefly described. Consideration of the probability of origin of various structural similarities and dissimilarities in the ear leads to the conclusion that ear asymmetry has evolved independently in at least five lines, represented by the respective genera (1) Tyto, (2) Phodilus, (3) Bubo, Ciccaba, Strix, (4) Rhinoptynx, Asio, Pseudoscops, and (5) Aegolius. Bubo, Ciccaba, and Strix probably represent more than one line of origin of ear asymmetry. Available evidence suggests that bilateral ear asymmetry in owls serves to make the vertical directional sensitivity patterns different between the two ears for high frequencies, thus making possible vertical localization based on binaural comparison of intensity and spectral composition of sound. When an owl localizes prey by hearing, the direction of the source usually forms a shallow angle with the ground. Therefore, a certain angle of error usually converts into a longer distance along the ground for a vertical error than for a horizontal error. This is a crucial factor that calls for good vertical localization ability of owls which rely on hearing for localization of food. Selection pressure for improvement of the ability of vertical localization of sound is believed to lie behind the evolution of all types of bilateral ear asymmetry in owls. On the basis of comparative ear structure the current subdivision of family Strigidae into subfamily Buboninae and Striginae is rejected. The external ears of Rhinoptynx and Pseudoscops are described for the first time and shown to be very similar to those of Asio otus, demonstrating affinity between these three genera.