The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals

Abstract

Although ultraviolet (UV) sensitivity is widespread among animals it is considered rare in mammals, being restricted to the few species that have a visual pigment maximally sensitive (λmax) below 400 nm. However, even animals without such a pigment will be UV-sensitive if they have ocular media that transmit these wavelengths, as all visual pigments absorb significant amounts of UV if the energy level is sufficient. Although it is known that lenses of diurnal sciurid rodents, tree shrews and primates prevent UV from reaching the retina, the degree of UV transmission by ocular media of most other mammals without a visual pigment with λmax in the UV is unknown. We examined lenses of 38 mammalian species from 25 families in nine orders and observed large diversity in the degree of short-wavelength transmission. All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity. Other mammals, however, such as hedgehogs, dogs, cats, ferrets and okapis had lenses transmitting significant amounts of UVA (315-400 nm), suggesting that they will be UV-sensitive even without a specific UV visual pigment.

Keywords: lens; mammal; retina; transmission; ultraviolet sensitivity; vision.

Figure 1.

The absorption spectra of the visual pigments of the ferret and the spectral transmission of its lens. The absorption maxima of the visual pigments (rods—505 nm; cones—430 and 558 nm) are taken from Calderone & Jacobs [28] and the visual pigments templates of Govardovskii et al. [29] (solid lines) have been fitted to them using the methods described in Hart et al. [30]. The lens transmission (dotted line) is taken from this study. As all the visual pigments absorb significant amounts of UV radiation and the lens transmits in this part of the spectrum, the ferret is likely to perceive such short wavelengths.

Figure 2.

Average spectral transmission of three bovine lenses before (solid line) and after (dashed line) four days of freezing.

Figure 3.

Representative average spectral transmission curves at short wavelengths of the lenses from 10 mammalian species. Most curves are the averages of all available lenses. However, for the black rat and meerkat individuals of a variety of lens sizes were scanned; the data shown for the two species are for young and old animals, respectively. From left to right at 50% transmission they are (n, lens axial diameter in millimetres); young black rats (2, 3.8), cat (6, 7.0), okapi (2, 7.0), cattle (8, 11.1), rabbit (2, 6.7), Arabian oryx (1, 10.3), squirrel monkey (2, 4.6), Alaotran gentle lemur (1, 5.9), adult meerkat (1, 3.4) and prairie dog (7, 3.6). All scans were zeroed at 700 nm.

Figure 4.

Lens transmission as a function of lens size/age in rodents. (a) Spectral transmission of 11 black rat (R. rattus) lenses ranging in axial length between 3.7 and 5.2 mm. (b) Wavelength of 50% transmission as a function of lens size for all the lenses shown in (a). The data are fit by y = 43.992x + 149.68 (R² = 0.9062). The dashed line is an approximation of the relationship expected if pathlength were the only factor affecting transmission. (c) Average wavelength of 50% lens transmission (±1 s.d.) of mice (M. musculus) of known age; 40 (n = 3), 70 (n = 8), 265 (n = 4) and 564 (n = 6) days. The data are fit by y = 0.0443x + 311.1 (R² = 0.9634).