Asymmetrical positions of owl ears are consistent across species with this trait
the verdict
INSUFFICIENT LEANING
refutedsupported
the weight of evidence
2 sources for · 0 against
Retrieved evidence acknowledges ear asymmetry in certain owl species (such as the barn owl) while noting that other owl species (like the great horned owl) have symmetrical ears, meaning the trait is not uniformly consistent across all owls.
ABSTRACT The Night Parrot Pezoporus occidentalis is a critically endangered, elusive, nocturnal bird that is difficult to study in the field. Much is unknown about its biology, including how it navigates at night. A previous investigation of the eye and brain revealed that it likely has poor nocturnal vision, but its other sensory abilities, and overall skull anatomy, have not yet been investigated. This osteological study, based on micro-CT-scans of the holotype, describes and compares the anatomy of the Night Parrot skull, and identifies its possible ecological adaptations. Specialised ear anatomy suggests that the Night Parrot has enhanced acoustic abilities, in contrast with its diurnal congener P. wallicus and the only other nocturnal parrot, the Kakapo Strigops habroptila. The Night Parrot’s uniquely enlarged exoccipital bones may amplify sound, and bilateral ear asymmetry, not identified in any other parrot species, likely enhances its directional hearing, as in many species of owl. Enlarged exoccipitals displace the zygomatic processes and constrain orbit size, suggesting an evolutionary trade-off between hearing and vision. The auditory abilities of the species require deeper investigation, but our anatomical observations indicate that the potential impacts of noise disturbance may warrant consideration in Night Parrot conservation strategies as a precaution.
In this study we have investigated the processing of auditory cues for sound localization in the great horned owl (Bubo virginianus). Previous studies have shown that the barn owl, whose ears are asymmetrically oriented in the vertical plane, has a 2-dimensional, topographic representation of auditory space in the external division of the inferior colliculus (ICx). As in the barn owl, the great horned owl's ICx is anatomically distinct and projects to the optic tectum. Neurons in ICx respond over only a small range of azimuths (mean = 32 degrees), and azimuth is topographically mapped. In contrast to the barn owl, the great horned owl has bilaterally symmetrical ears and its receptive fields are not restricted in elevation. The binaural cues available for sound localization were measured both with cochlear microphonic recordings and with a microphone attached to a probe tube in the auditory canal. Interaural time disparity (ITD) varied monotonically with azimuth. Interaural intensity differences (IID) also changed with azimuth, but the largest IIDs were less than 15 dB, and the variation was not monotonic. Neither ITD nor IID varied systematically with changes in the vertical position of a sound source. We used dichotic stimulation to determine the sensitivity of ICx neurons to these binaural cues. Best ITD of ICx units was topographically mapped and strongly correlated with receptive-field azimuth. The width of ITD tuning curves, measured at 50% of the maximum response, averaged 72 microseconds. All ICx neurons responded only to binaural stimulation and had nonmonotonic IID tuning curves. Best IID was weakly, but significantly, correlated with best ITD (r = 0.39, p less than 0.05). The IID tuning curves, however, were broad (mean 50% width = 24 dB), and 67% of the units had best IIDs within 5 dB of 0 dB IID. ITD tuning was sensitive to variations in IID in the direction opposite to that expected for time-intensity trading, but the magnitude of this effect was only 1.5 microseconds/dB IID. We conclude that, in the great horned owl, the spatial selectivity of ICx neurons arises primarily from their ITD tuning. Except for the absence of elevation selectivity and the narrow range of best IIDs, ICx in the great horned owl appears to be organized much the same as in the barn owl.
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