Birds - same thing, but different? Convergent evolution in the avian and mammalian auditory systems provides informative comparative models

Birds have been and continue to be enlightening, comparative models in auditory research. This review highlights their particular appeal as a vertebrate group that evolved independently a similar division of labour to that seen in the mammalian cochlea, between classic sensory hair cells and hair cells specialising in amplification. Through studying both the similarities and differences between the avian and mammalian inner ear, profound insights into the principles of operation of such a divided system may be gained. For example, the prevailing model of the relationship between basilar-membrane displacement and afferent rate-level functions in mammals is reinforced by characteristic differences observed in birds, which correlate with known differences in basilar-papilla mechanics. Furthermore, birds arguably represent the most extreme case of hair cells using bundle motility for mechanical amplification at high frequencies, up to about 10 kHz. They should thus be informative for elucidating the operation and possibly the limitations of this ancestral amplifying mechanism at high frequencies.