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T. Malmstrom, R.H. H. Kröger; Relationship between lens optics and pupil shape in terrestrial vertebrates . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1742.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Multifocal crystalline lenses compensate for chromatic aberration in animals having pupils that are large relative to the focal lengths of the optical systems. In a multifocal eye, all refractive zones of the lens have to be usable when the pupil constricts. A slit pupil can include all refractive zones and protect the sensitive retinae of nocturnal and crepuscular animals in bright light. We studied terrestrial vertebrates from different phylogenetic groups to map out how widespread multifocal lenses are. Furthermore, we studied whether there is a correlation between multifocal lenses and slit pupils. Methods: The optics of the lens were investigated by eccentric slope–based infra–red videorefractometry. Pupil shapes were recorded using a digital still camera. We studied a total of 15 species of primates, carnivores, ungulates, rodents, reptiles and amphibians. Lens optics and pupil shapes were determined in two or more species in each phylogenetic group. In the selected pairs of species, one species had a round pupil and the other one a slit pupil. Results: Both monofocal and multifocal optics were found in each group. All species having slit pupils showed rings in videorefractive images characteristic for multifocal optics. Two species (Elaphe taeniura, a snake, and Acomys cahirinus, a rodent) were found to have multifocal lenses and round pupils. The lynx had an intermediate pupil shape between the round pupil of the sibirian tiger and the slit pupil of the domestic cat. The lynx's lens is also intermediate between the tiger’s clearly monofocal lens and the domestic cat’s clearly multifocal lens. Conclusions: Multifocal lenses are abundant in terrestrial vertebrates and the slit pupil appears to be an adaptation to such lenses in most cases. The fact that even closely related species may differ in lens optics and pupil shape indicates that evolutionary changes of the eye can be relatively fast. The properties of the lynx eye suggest that even intermediate forms are of adaptive value, which is an unexpected result. It is an open question why large cats do not need multifocal lenses.
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