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I've been to local zoo the other day and one lizard caught my attention: its pupils are circular, which, I thought, is not usual for reptiles. Turns out it is, but now I can't find any explanation on why some animals have one kind of pupil and others have the other. Lizards can have both, and so can snakes. The only difference I have found is that circular pupil can't shrink quite as much as a cat's-eye pupil, but that hardly explains why circular pupil even evolved in the first place as I don't see any advantage to it.

Ideas?

P. S. Fish only have circular pupils so that shape is older, right?..

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In bright light, a cat can also squint, thus approximating a circle with a square-like aperture. So a vertical pupil is not that much of a liability to focusing. The same cannot be said of a horizontal pupil (unless the eyelids are vertical). Additionally, a vertical pupil might optimize detection of horizontal movement, which is likely an advantage in hunting prey on the savannah.

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How does a vertical pupil help to detect horizontal movement? –  Armatus May 17 at 23:02
    
I think I'm in error there. It would be a horizontal slit according to wikipedia on pupil. If someone can confirm, I'll fix my response, or else delete that point. –  SmokeyVW May 18 at 0:07
    
With more digging, I found contradictory evidence. graphic-fusion.com/beyondbokeh.htm - so I don't know the correct answer. I am inclined to believe the photography based info more strongly. –  SmokeyVW May 18 at 0:17
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Your mention of cats hinted that vertical pupils have to do with night vision, and indeed they do.

The retinas of cats and other nocturnal animals are very sensitive to even the tiniest amount of light. This can make their eyes hurt when exposed to bright sunlight. So their pupils have to shrink as much as possible in the sunlight, and, as you pointed out, vertical pupils can shrink more than circular ones.

EDIT: People pointed out in the comments that cats have vertical eye pupils only in bright sunlight. In low-light conditions, the pupils expand to a circle.

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It was my first (and only) idea that nocturnal animals have vertical pupils. It didn't confirm, though, there are lots of nocturnal animals with circular pupils. So, not all nocturnal animals have vertical pupils. I didn't think of it the other way, though - all animals with vertical pupils might be nocturnal. Have to check it, thanks. P. S. I am somewhat familiar with photography and the second theory doesn't make sense to me. Depth of field (DOF) is something you only need to be concerned with at short distance. –  Violet Giraffe Mar 11 '13 at 19:55
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Why can vertical pupils shrink more than circular ones? Isn't the lower limit on pupil size always 0? –  kmm Mar 11 '13 at 20:49
    
@Kevin: how do you imagine a circular pupil shrinking to 0? Where will pupil tissue go? –  Violet Giraffe Mar 11 '13 at 21:15
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The pupil is an opening in the iris, not tissue: en.wikipedia.org/wiki/Pupil –  kmm Mar 11 '13 at 21:47
    
@Kevin: I didn't realize... Still, muscular system for circular pupil seems less trivial and more restricting –  Violet Giraffe Mar 11 '13 at 22:06
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Circular pupils are always functionally superior to vertical pupils; a slit does not correctly focus light from all directions whereas a circular pupil does. If you observe cats when they're hunting at dawn and dusk*, they have big, circular pupils; it's only when they're in bright light that the pupil shrinks to a slit. So why have vertical pupils at all? Because, as you mention they're capable of letting a more controlled range of light into the eye.

Thus vertical slits let you be active in a wider range of light conditions at the cost of poorer vision in bright light; whilst circular pupils continue to function well in brighter light but at the cost of not allowing such a large range of control over the amount of light entering the eye.

It is then easy to see why you get both kinds of pupil: circular pupils are favoured by animals that are typically active in bright light and need good vision under these circumstances; vertical pupils are favoured by animals that are primarily active in low light but need some ability to see in bright light.

*- it's often stated that cats are nocturnal, this is untrue; cats are crepuscular - that is they are most active at dawn and dusk.

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I have thought about it, but don't understand how exactly non-circular hole negatively affects vision. For example: a heavily used сamera lens can often have a broken iris, which takes irregular shape. The only clearly visible effect of this is the shape of unfocused background lights and other bright points. it doesn't worsen the sharp part of image in any immediately visible way. That's why I've discarded this reason when thinking of pupils. –  Violet Giraffe Mar 13 '13 at 13:19
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