Abstract
ABSTRACTMost vertebrates have UV-sensitive vision, but the UV sensitivity of their eyes is limited by the transmittance of the ocular media, and the specific contribution of the different media (cornea, lens) has remained unclear. Here, we describe the transmittance of all ocular media (OMT), as well as that of lenses and corneas of birds. For 66 species belonging to 18 orders, the wavelength at which 50% of light is transmitted through the ocular media to the retina (λT0.5) ranges from 310 to 398 nm. Low λT0.5 corresponds to more UV light transmitted. Corneal λT0.5 varies only between 300 and 345 nm, whereas lens λT0.5 values are more variable (between 315 and 400 nm) and tend to be the limiting factor, determining OMT in the majority of species. OMT λT0.5 is positively correlated with eye size, but λT0.5 of corneas and lenses are not correlated with their thickness when controlled for phylogeny. Corneal and lens transmittances do not differ between birds with UV- and violet-sensitive SWS1 opsin when controlling for eye size and phylogeny. Phylogenetic relatedness is a strong predictor of OMT, and ancestral state reconstructions suggest that from ancestral intermediate OMT, highly UV-transparent ocular media (low λT0.5) evolved at least five times in our sample of birds. Some birds have evolved in the opposite direction towards a more UV-opaque lens, possibly owing to pigmentation, likely to mitigate UV damage or reduce chromatic aberration.
Highlights
The majority of animals, including most vertebrates, can see ultraviolet (UV) light, with wavelengths shorter than 400 nm
We present new measurements from ocular media transmittance (OMT) of 30 species of birds (Fig. 1A-E) and used published data on 40 additional species
High variation in OMT We found high variation in the total ocular media transmittance and lens transmittance amongst our sample of birds
Summary
The majority of animals, including most vertebrates, can see ultraviolet (UV) light, with wavelengths shorter than 400 nm (for a review see Cronin and Bok 2016). Colour vision is based on four types of cone photoreceptors, expressing visual pigments sensitive to long (LWS, peak sensitivity 560 - 570 nm), medium (MWS, 497 - 509 nm), short (SWS2, 427 - 458 nm) and very short (SWS1, 355-426) wavelengths (Hart 2001, Hart and Hunt 2007). While owls and some other birds have lost SWS1 (Höglund et al 2019; Kelber 2019), most birds can be categorized as either ultraviolet-sensitive (UVS), with an UV-sensitive SWS1-based visual pigment maximally sensitive to wavelengths between 355 and 373 nm, or violet-sensitive (VS), with SWS1 sensitivity peaking in the violet range between 399 and 425 nm (Hart 2001, Ödeen and Håstad 2003, Håstad et al 2005a, Ödeen and Håstad 2013). UVS birds are generally smaller and have more transparent ocular media than VS birds (Lind et al 2014)
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