Eye shape and retinal topography in owls (Aves: Strigiformes)

Area of Science:

• Comparative physiology
• Evolutionary biology
• Neuroscience

Background:

• Vertebrate eye morphology, including eye shape and retinal topography, is influenced by environmental adaptations.
• Previous research has documented these adaptations in various vertebrates, but their extent within avian orders remains less understood.
• Owls (Strigiformes) offer a unique model for studying visual system variation due to their diverse activity patterns and habitats.

Purpose of the Study:

• To investigate interspecific variation in eye shape and retinal topography among nine owl species.
• To correlate these visual traits with species' activity patterns and habitat preferences.

Main Methods:

• Quantified eye shape using the ratio of corneal diameter to eye axial length.
• Analyzed retinal topography by examining retinal ganglion cell (RGC) density and visual streak organization.
• Utilized hierarchical cluster analysis to assess the influence of activity pattern and habitat versus phylogeny.

Main Results:

• Nocturnal owl species exhibit larger relative corneal diameters compared to diurnal species.
• All studied owl species possess a temporal RGC density area and a visual streak, but its definition and elongation vary.
• Species in open habitats or with diurnal activity show well-defined, elongated visual streaks (H:V ratios 2.33–3.88), while forest-dwelling or nocturnal owls have less defined streaks and lower H:V ratios (1.27–1.77).
• Hierarchical cluster analysis indicated that variation is primarily linked to activity pattern and habitat, not phylogeny.
• A fovea was confirmed in seven species, suggesting its potential presence in all strigid owls but absence in the barn owl (Tyto alba).
• Analysis revealed diverse RGC classes, including large RGCs similar to those in other vertebrates.

Conclusions:

• Interspecific variation in owl eye shape and retinal topography is strongly associated with their activity patterns and habitat preferences.
• These findings support similar adaptive trends observed in the visual systems of other vertebrate groups.
• The study highlights the role of ecological factors in shaping visual system evolution within avian orders.