Beyond SWS1 Cones: Canonical and Noncanonical Routes of Ultraviolet Sensitivity in the Avian Retina

Ultraviolet sensitivity in birds is generally attributed to short-wavelength-sensitive type 1 single cones, but ultraviolet radiation may influence the retina through several additional pathways. We reviewed molecular, optical, physiological, and behavioral evidence for canonical and putative noncanonical ultraviolet detection in the avian retina. Under photopic conditions, short-wavelength-sensitive type 1 cones provide an established chromatic pathway, whereas secondary ultraviolet absorption bands of other cone pigments may contribute at high irradiance. Under scotopic conditions, particularly in rod-dominated owls lacking functional ultraviolet- or violet-sensitive type 1 cones, the secondary ultraviolet absorption band of rhodopsin provides a plausible achromatic pathway. Ultraviolet-sensitive opsin 5 in inner-retinal neurons and Müller glia may mediate local regulatory or non-image-forming responses. We also examine fluorescence of oil droplets in double cones as a possible route into the luminance system. Previous reports described ultraviolet-induced visible fluorescence in chicken retinal oil droplets and fluorescence of chicken double-cone droplets under blue excitation, although physiological activation of the adjacent visual pigment by droplet fluorescence has not been established. These mechanisms are not mutually exclusive, and their relative contributions probably depend on irradiance, adaptation state, ocular-media transmission, receptor abundance, and behavioral context. Receptor-specific physiological and behavioral experiments are required to distinguish between them.