The yellow colour of the lens of the grey squirrel (Sciurus carolinensis leucotis)

Abstract

SUMMARY1. The absorption spectrum of the lens of the grey squirrel (Sciurus carolinensis leucotis) has been measured, revealing an absorption maximum at 370 nm. Peak densities in the intact lens ranged from 12 to 20.2. This peak is not present in the lenses of horse, cow, pig, dog, cat, ferret (Mustelo furo), or guinea-pig (Cavia porcellus).3. The pigment responsible for this absorption is water-soluble and aqueous extracts have been examined. Protein-free aqueous extracts show an additional maximum at 265 nm, which can only partially be accounted for by the presence of ascorbic acid.4. The absorption spectrum of extracts of lens material from the ground squirrel (Citellu8 mexicanus) also had maxima at 265 and 370 nm.5. Chromatography of the protein-free solution separated two yellow components, both of which had a yellow fluorescence. The faster component had a very similar absorption spectrum to the original protein-free solution.6. Possible functions of the yellow pigment are discussed.

The yellow colour of the lens of man and other primates

“…Whilst we do not have any direct evidence indicating the function of these pigments in the primate lenses, it is possible that they serve optical or metabolic functions, as suggested by Cooper & Robson (1969) in the case of the squirrel lens pigment.…”

“…The pigment that is responsible for the colour of the human lens does not appear to be identical with that of the baboon lens (it has a quite different fluorescence) although its absorption spectrum is clearly very similar. Again it is not clear whether the pigment of the baboon lens is the same substance as the pigment of the squirrel lens (Cooper & Robson, 1969) although they are certainly very similar.…”

“…The methods adopted for the measurement of the absorption spectra of whole lenses, thin layers of lens material, solutions of lens material, dialysates and chromatographic eluates, were the same as those used for our investigation of the squirrel lenses (Cooper & Robson, 1969).…”

Contributions of the mouse UV photopigment to the ERG and to vision

“…At least partly this presumption reflected the widespread, but mistaken, view that the lenses of all mammals absorb strongly in the UV, thereby providing pre-retinal filtering that would seem incompatible with the presence of UV photopigment. Although strong absorption of the short wavelengths is indeed characteristic of the lenses of a majority of the primates, as well as those of some other diurnal species, the lenses of many mammals transmit light with high efficiency well down into the UV [3][4][5]. In such animals lens absorption would seem to pose no obvious barrier to UV-based vision.…”

Spectral properties and retinal distribution of ferret cones

“…There are no direct measurements available to provide information as to the potential influence of any of these features on the measured spectral sensitivity, but several observations may be relevant. In an early study involving measurement of the absorption properties of squirrel lens (Cooper & Robson, 1969), a passing comment is made to the effect that no signs of specific absorption of light in the visible or near UV regions of the spectrum were detected in the lenses of a number of mammals, including that of the ferret. To the extent there is some small preferential absorption of short-wavelength lights by the ferret lens, it would tend to shift the estimated peaks of the three mechanisms toward shorter wavelengths, having its greatest influence on the S-cone mechanism and with proportionally smaller shifts for other two mechanisms.…”