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why do pupils dilate in the dark

Not all animals have circular pupils. Some have slits or ovals which may be oriented vertically, as in,
and, or horizontally as in some, and such as, and, as well as the domestic. , and pupils tend to be horizontal and rectangular with rounded corners. Some skates and rays have crescent shaped pupils, pupils range from circular, to a slit, to a series of pinholes, and the pupil is a smoothly curving W shape. There are rare cases of humans with very unusually shaped pupils, such as peanut-shell-shaped pupils, as if the sides of the pupils are pinched together in the middle, like a vertical peanut shell, being wide at the top and bottom of the pupil, but narrow in the middle. [ There may be differences in pupil shape even between closely related animals. In felids, there are differences between small- and large eyed species. The domestic (Felis sylvestris domesticus) has vertical slit pupils, its large relative the (Panthera tigris altaica) has circular pupils and the (Lynx lynx) is intermediate between those of the domestic cat and the Siberian tiger.

A similar difference between small and large species may be present in canines. The small European (Vulpes vulpes) has vertical slit pupils whereas their large relatives, the (Canis lupus lupus) and domestic (Canis lupus familiaris) have round pupils. One explanation for the of slit pupils is that they can exclude light more effectively than a circular pupil. [ This would explain why slit pupils tend to be found in the eyes of animals with a or lifestyle that need to protect their eyes during daylight. Constriction of a circular pupil (by a ring-shaped muscle) is less complete than closure of a slit pupil, which uses two additional muscles that laterally compress the pupil. For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans. However, this explanation does not account for circular pupils that can be closed to a very small size (e. g. , 0. 5Pmm in the ) and the rectangular pupils of many which do not close to a narrow slit in bright light.

An alternative explanation is that a partially constricted circular pupil shades the peripheral zones of the lens which would lead to poorly focused images at relevant wavelengths. The vertical slit pupil allows for use of all wavelengths across the full diameter of the lens, even in bright light. It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, breaking up the circular outline of the eye. In a study of, pupil shapes correlated both with activity times and with foraging behaviour. Most snake species with vertical pupils were and also ambush foragers, and most snakes with circular pupils were and active foragers. Overall, foraging behaviour predicted pupil shape accurately in more cases than did diel time of activity, because many active-foraging snakes with circular pupils were not diurnal. It has been suggested that there may be a similar link between foraging behaviour and pupil shape amongst the felidae and canidae discussed above. confirmed the hypothesis that elongated pupils have increased dynamic range, and furthered the correlations with diel activity.

However it noted that other hypotheses could not explain the orientation of the pupils. They showed that vertical pupils enable ambush predators to optimise their depth perception, and horizontal pupils to optimise the field of view and image quality of horizontal contours. They further explained why elongated pupils are correlated with the animal's height. Cats' impressive night vision is due partially to a part of their eye called the tapetum. This reflective layer is under the retina, which contains cells called rods and cones; these absorb light and transform it into information the brain can process. When light reaches the tapetum, it reflects back to the rods and cones. Therefore, cats' eyes have double light exposure. This also gives cats their characteristic "shine" at night. Cats can also dilate their pupils significantly, maximizing the amount of light coming through at night.

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