Editing Pupil (section)

==Animals==
[[File:Sepia eyelid shape.theora.ogv|thumb|The W-shaped pupil of the cuttlefish expanding when the lights are turned off.]]
Not all [[animal]]s have circular pupils. Some have slits or ovals which may be oriented vertically, as in [[crocodile]]s, [[Viperidae|vipers]], [[Felidae|cats]] and [[fox]]es, or horizontally as in some [[Batoidea|rays]], [[flying frog]]s, [[mongoose]]s and [[artiodactyl]]s such as [[elk]], [[red deer]], [[reindeer]] and [[hippopotamus]], as well as the domestic [[horse]]. [[Goat]]s, [[sheep]], [[toad]]s and [[octopus]] pupils tend to be horizontal and rectangular with rounded corners.  Some skates and rays have crescent shaped pupils,<ref name="Murphy and Howland, (2005)">{{cite journal|author1=Murphy, C.J.  |author2=Howland, H.C. |name-list-style=amp |title=The functional significance of crescent-shaped pupils and multiple pupillary apertures|journal=Journal of Experimental Zoology|volume=256|page=22|doi= 10.1002/jez.1402560505|year=1990|issue=S5 |bibcode=1990JEZ...256S..22M }}</ref> [[gecko]] pupils range from circular, to a slit, to a series of pinholes,<ref>{{Cite journal|last1=Roth|first1=Lina S. V.|last2=Lundström|first2=Linda|last3=Kelber|first3=Almut|last4=Kröger|first4=Ronald H. H.|last5=Unsbo|first5=Peter|date=2009-03-01|title=The pupils and optical systems of gecko eyes|journal=Journal of Vision|volume=9|issue=3|pages=27.1–11|doi=10.1167/9.3.27|pmid=19757966 |doi-access=free}}</ref> and the [[cuttlefish]] pupil is a smoothly curving W shape. Although human pupils are normally circular, abnormalities like [[coloboma]]s can result in unusual pupil shapes, such as teardrop, keyhole or oval pupil shapes.

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 [[cat]] ''(Felis sylvestris domesticus)'' has vertical slit pupils, its large relative the [[Siberian tiger]] ''(Panthera tigris altaica)'' has circular pupils and the [[Eurasian lynx]] ''(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 [[red fox]] ''(Vulpes vulpes)'' has vertical slit pupils whereas their large relatives, the [[gray wolf]] ''(Canis lupus lupus)'' and domestic [[dog]]s ''(Canis lupus familiaris)'' have round pupils.{{citation needed|date=June 2020}}

=== Evolution and adaptation ===
One explanation for the [[evolution]] of slit pupils is that they can exclude light more effectively than a circular pupil.{{citation needed|date=August 2017}} This would explain why 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. 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.<ref name="Walls et al., (1942)">{{cite book |first=G.L. |last=Walls |title=The vertebrate eye and its adaptive radiation |publisher=Hafner |orig-year=1942 |year=1967 |oclc=10363617 |series=Cranbrook Institute of Science Bulletin |volume=19}}</ref>  For example, the cat's slit pupil can change the light intensity on the retina 135-fold compared to 10-fold in humans.<ref name="Hughes, (1977)">{{cite book |last=Hughes |first=A. |orig-year=1977 |chapter=The topography of vision in mammals of contrasting life style: comparative optics and retinal organisation |editor-first=F. |editor-last=Crescitelli |title=The Visual System in Vertebrates |chapter-url=https://books.google.com/books?id=qCDqCAAAQBAJ&pg=PA613 |date=2013 |publisher=Springer |isbn=978-3-642-66468-7 |pages=613–756 |series=Handbook of Sensory Physiology |volume=7/5}}</ref>  However, this explanation does not account for circular pupils that can be closed to a very small size (e.g., 0.5&nbsp;mm in the [[tarsier]]) and the rectangular pupils of many [[ungulate]]s which do not close to a narrow slit in bright light.<ref name="Land, (2006)">{{cite journal|doi=10.1016/j.cub.2006.02.046|pmid=16527734|author=Land, M.F.|title= Visual optics: the shapes of pupils|journal=Current Biology|volume=16|issue=5|pages=R167–8|year=2006|doi-access=free|bibcode=2006CBio...16.R167L }}</ref>  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.<ref name="Malmstrom" />  It has also been suggested that in ambush predators such as some snakes, vertical slit pupils may aid in camouflage, [[Disruptive coloration|breaking up]] the circular outline of the eye.<ref name="Brischoux et al., (2010)">{{cite journal|doi=10.1111/j.1420-9101.2010.02046.x|pmid=20629855|author=Brischoux, F., Pizzatto, L. and Shine, R.|title=Insights into the adaptive significance of vertical pupil shape in snakes|journal=Journal of Evolutionary Biology|volume=23|issue=9|pages=1878–85|year=2010|doi-access=free}}</ref>

=== Activity pattern and behavior ===
In a study of [[Australia]]n [[snake]]s, pupil shapes correlated both with [[diel]] activity times and with foraging behavior. Most snake species with vertical pupils were [[nocturnal]] and also ambush foragers, and most snakes with circular pupils were [[diurnality|diurnal]] 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.<ref name="Brischoux et al., (2010)" />

A 2015 study<ref>{{cite journal |last1=Banks |first1=Martin S.  |last2=Sprague |first2=William W.  |last3=Schmoll |first3=Jürgen |last4=Parnell |first4=Jared A. Q.  |last5=Love |first5=Gordon D. |title=Why do animal eyes have pupils of different shapes? |journal=Science Advances |volume=1 |issue=7 |pages=e1500391 |year=2015 |doi=10.1126/sciadv.1500391 |pmid=26601232 |pmc=4643806 |bibcode=2015SciA....1E0391B |doi-access=free }}</ref> 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.

<gallery widths="240" heights="180" caption="Animals with non-circular pupils">
File:Horizontal Rectangular Pupil of a Goat.jpg|A [[goat]] with horizontal rectangular pupils
File:Taeniura_grabata_eye.jpg|A [[stingray]] with crescent pupils
File:Crocodylus siamensis closeup.jpg|A [[crocodile]] with thin vertical slit pupils
File:Cuttlefish_eye.jpg|A [[cuttlefish]] with W-shaped pupils
File:Gecko-oeil.jpg|A [[gecko]] with 'thin string of pearls' pupils
File:Catpupil03042006.jpg|A [[cat]] with thick vertical slit pupils
</gallery>