Some birds appear to have sunglasses
Although I myself did not have to wear glasses (except perhaps sunglasses) until I turned 55, millions of people on Earth may view this simple ‘tool’ to improve the resolving power of their eyes as one of the greatest inventions ever made. Photographers, microscopists, and opticians can tell you that the aperture of an optical system not only controls the amount of light admitted (and thus the brightness of the image you see) but also the resolving power of the system.
In terms of their optical performance, the eyes of humans, although not the absolutely very best in the animal kingdom, are in the top group and a resolution of 2 minutes of arc (1/20 of a degree) is possible: feeding a thin thread through the eye of a needle is, after all, not an easy but definitely possible task. But under bright light the pupil contracts and this reduction in the optical system’s aperture works against maximal resolution and not only reduces the amount of light admitted.
Hawks, vultures and eagles, which fly in the brightest sunlight and peer downward hundreds of metres not only have more narrowly spaced photoreceptive cells in their retinas than humans, but they also keep their pupils wider open than us. This they do in spite of the high ambient light levels to minimise edge artefacts caused by narrow apertures. It can be calculated that to achieve maximum resolution, the pupil in human beings ought to be 3 mm wide. In the eagle with its similarly-sized eyes it is 8 mm even in sunlight, which corresponds to a human pupil at its maximum dilation at night!
Why does the eagle eye show no reduction in pupil size comparable to that seen in the human eye? A decrease in the diameter of the pupil, which is effectively a reduction of the eye’s aperture, would worsen the eye’s resolving power. In order to keep the pupil at its optimum width for resolution and, at the same time, prevent photoreceptor damage through overstimulation with bright and intensive light, a kind of neutral density filter between pupil and retina ought to be present.
According to a number of comparative vision physiologists birds possess this filter structure in the form of the “pecten”, a peculiar, frequently pigmented and folded tissue flap that projects into the eyeball principally of daytime-active species of birds. Nocturnal birds either possess a very small pecten or, as in owls, none at all. The pecten may therefore seem to act like internally-worn “sunglasses”, cutting down the intensity of the light without affecting the setting of the aperture of the eye’s optical system, and because of its pigmentation protecting the retina against the deleterious effects of ultraviolet radiation.
Not everyone is convinced of this interpretation of the pecten and other possible functions for the structure have been suggested. The pecten is a highly vascularized structure (meaning it is well supplied with blood vessels). Could it be involved in the supply of nutrients to the eye or is it perhaps involved in monitoring the vitreous humour’s pH or pressure? Does it have multiple functions? It’s still not clear and the fact that no mammal has this peculiar structure in its eyes doesn’t make it any easier to assign a definitive function to it.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
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