Visual optics: the shapes of pupils

Land, Michael F (2006) Visual optics: the shapes of pupils. Current Biology, 16 (5). R167-R168. ISSN 0960-9822

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The eyes of many terrestrial vertebrates have slit-shaped pupils. A new hypothesis links this pupil shape to the way that vertebrate lenses compensate for chromatic aberration.

Human eyes have circular pupils, but a great many animals have pupils that are oval or slit-shaped. These slits/ovals may be oriented vertically (as in crocodiles, vipers, cats and foxes), or horizontally (as in some rays, flying frogs, mongooses and ungulates such as sheep and hippopotami). The explanation usually given for the use of a slit pupil is that it can exclude light more effectively than a circular pupil, and so slit pupils tend to be found in the eyes of animals with a crepuscular or nocturnal lifestyle that need to protect their eyes during daylight. The slit pupil of a cat, for example, can change the intensity on the retina 135-fold, compared to 10-fold in man. This has never seemed to be an entirely convincing explanation, however, as some animals (such as the tarsier) have circular pupils that can close down very effectively, to a diameter of about half a millimeter, and in many ungulates the rather rectangular pupils do not close to a narrow slit in bright light. A recent paper by Malmström and Kröger offers a much more intriguing explanation of the oval pupil; it relates to the way that vertebrate lenses have evolved to handle color.

Item Type: Article
Schools and Departments: School of Life Sciences > Biology and Environmental Science
Depositing User: Michael Land
Date Deposited: 06 Feb 2012 20:39
Last Modified: 26 Jul 2012 10:19
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