Ultraviolet vision in birds: the importance of transparent eye media

Olle Lind,Peter Olsson,Mindaugas Mitkus,Almut Kelber

doi:10.1098/rspb.2013.2209

Abstract

Ultraviolet (UV)-sensitive visual pigments are widespread in the animal kingdom but many animals, for example primates, block UV light from reaching their retina by pigmented lenses. Birds have UV-sensitive (UVS) visual pigments with sensitivity maxima around 360–373 nm (UVS) or 402–426 nm (violet-sensitive, VS). We describe how these pigments are matched by the ocular media transmittance in 38 bird species. Birds with UVS pigments have ocular media that transmit more UV light (wavelength of 50% transmittance, λT0.5, 323 nm) than birds with VS pigments (λT0.5, 358 nm). Yet, visual models predict that colour discrimination in bright light is mostly dependent on the visual pigment (UVS or VS) and little on the ocular media. We hypothesize that the precise spectral tuning of the ocular media is mostly relevant for detecting weak UV signals, e.g. in dim hollow-nests of passerines and parrots. The correlation between eye size and UV transparency of the ocular media suggests little or no lens pigmentation. Therefore, only small birds gain the full advantage from shifting pigment sensitivity from VS to UVS. On the other hand, some birds with VS pigments have unexpectedly low UV transmission of the ocular media, probably because of UV blocking lens pigmentation.

Highlights

To be able to see ultraviolet (UV) light, an eye has to meet two criteria: it has to possess UV-sensitive (UVS) photoreceptors and ocular media that transmit this part of the spectrum
The UV transmittance of the eye media determines our own insensitivity to short wavelength light, almost all studies on UV vision in animals focus on visual pigments
Few species have been investigated for ocular media transmittance (OMT) and comparative studies are only available for fishes [3,4,5] and jumping spiders [6]

Summary

Introduction

To be able to see ultraviolet (UV) light, an eye has to meet two criteria: it has to possess UV-sensitive (UVS) photoreceptors and ocular media that transmit this part of the spectrum. Eyes from Timor zebra finches (Taeniopygia guttata) and Bourke’s parrots (Neopsephotus bourkii) were acquired from animals euthanized in other research projects, eyes from burrowing owls were obtained from Copenhagen Zoo. Published OMT data on 10 bird species were kindly provided by Nathan Hart at The University of Western Australia The sampling base was 1 nm, three to five measurements were averaged for each eye, smoothed by an 11-point running average and normalized to the highest value within the range 300–700 nm From these data, we determined the wavelength at which 50% of the light incident on the cornea was transmitted to the retina, lT0.5, a commonly used indicator of UV transparency [7], and the slope of the OMT function. We chose the domestic chicken (Gallus gallus domesticus) for VS pigments and the budgerigar for UVS pigments (see the electronic supplementary material, table S1 for details)

Results

Findings

Discussion