The evolution of an “owl-like” auditory system in harriers: Anatomical evidence

Hearing is crucial for predators to detect hidden prey. Owls are perhaps the best example of predators with extreme auditory abilities and anatomical specializations that enhance prey localization by sound alone. Such specializations include enlarged acoustic meatus, asymmetrical ears, and enlarged auditory brain nuclei. Harriers (Circus spp.), medium-sized diurnal hawks, have similar sound-based hunting strategies and abilities to localize sound sources like owls, but whether they have the same anatomical adaptations remains unknown. We provide a quantitative analysis of the harrier's auditory system, combining histology and microCT scanning to analyze both brain and skull anatomy. Harriers differ greatly from other hawks in having enlarged acoustic meatus and expanded auditory brainstem nuclei (nucleus magnocellularis and nucleus laminaris). These are up to 3× and 12× larger, respectively, than in other hawks and are comparable in size to those of owls. However, harriers do not show other traits found in owls such as obvious ear asymmetry, an enlarged endosseous cochlear duct, or an expansion of other auditory nuclei that are enlarged in owls. These data suggest the ability of harriers to localize sound sources arises, in part, from the expansion of two auditory brainstem nuclei that are crucial to calculating interaural timing differences and determining the azimuthal location of sounds. The extent to which harriers can also accurately calculate the elevational component of a sound remains uncertain, but based on our analyses, it seems unlikely that they are as accurate as asymmetrically eared owls. We conclude that the anatomy of the harrier auditory system is “owl-like,” and their ability to accurately locate prey via auditory cues is likely enhanced by their unique hunting behavior.