Adaptation in the Avian Middle Ear-The Columellar Annular Ligament in Aquatic and Diving Birds

The effective detection of environmental vibrations such as sound waves depends on the transmission of tympanic membrane motion through the middle ear to the inner ear hair cells. In birds, the bony element of the middle ear is the columella; its distal end joins the cartilaginous extracolumella and tympanic membrane, while its basal expansion (the footplate) interfaces with inner ear fluid at the oval window, where it is held in place by the stapedial (columellar) annular ligament. Variation in footplate and oval window geometry can alter the annular ligament's size and shape, thereby influencing middle ear mechanics. Previous studies have noted relatively small footplates in aquatic birds as compared to their terrestrial relatives, and suggested that the adaptive significance of these may relate to their influence on the relative size of the annular ligament. Here, I examine a taxonomically and ecologically broad sample of bird species to test the hypothesis that aquatic lineages have convergently evolved proportionally larger annular ligaments. Results show larger ligaments are characteristic of aquatic, and particularly diving species, while narrower ligaments occur in high-frequency specialists. These patterns are polyphyletic and, alongside their strong ecological associations, indicate repeated functional adaptation of the middle ear. Larger ligaments are consistent with reduced system stiffness and enhanced low-frequency transmission, a hypothesis which is plausible for pelagic seabirds. In contrast, the extreme ligament enlargement in diving taxa is unlikely to be related to hearing, and instead may play a role in protection from barotrauma.