Cranial asymmetry and inner ear evolution in extant owls and comparative bird species

Abstract

Hearing is an important sensory function in birds, which is associated with behavioural and ecological adaptations. Among birds, owls have exceptional hearing ability allowing species such as the Barn Owl (Tyto alba) to capture prey in complete darkness using only auditory cues. This exceptional hearing ability is attributed to two distinct skeletal adaptations: (1) an elongated cochlear duct of the ear; and (2) cranial asymmetry. These adaptations are conspicuous features of the bird skull, potentially permitting us to infer hearing ability in extinct species along the bird line. However, quantitative datasets suitable for testing their form-function relationships have until now been lacking. This dissertation aims to address this gap by investigating patterns of variation in these skeletal adaptations linked to hearing ability among owls and comparative bird species. I collated digital 3-D reconstructions of bird skulls and their labyrinths and used a geometric morphometric pipeline to assess variation in their shapes and sizes. I identified three independent origins of elongate cochlear ducts, twice among owls and once in the oilbird. Surprisingly, although the superficial bones of the hearing region showed asymmetry, the endosseous labyrinth did not, a finding perhaps attributable to strong developmental constraints on neural architecture. I also found that birds with asymmetry did not have proportionally long cochlear ducts. These results suggest cochlear duct elongation and cranial asymmetry in owls evolved as a distinctive auditory specialisation missing in other birds’ perceptions of ambient sound. Finally, I incorporated natural history observations about the bird species studied here, finding that auditory foraging is associated with increased cochlear duct length while other foraging styles are not. This work increases our understanding of the functional and morphological diversity of bird auditory adaptations, provides a framework for understanding the evolution of auditory specialization within the birds, and contributes to the broader discussions on sensory evolution in vertebrates.