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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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  • $\begingroup$ Slit-pupil vertebrates are mostly nocturnal. Isn't? may be that allow to pass more light into retina at night? $\endgroup$
    – Always Confused
    Sep 19, 2016 at 18:52

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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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  • $\begingroup$ 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. $\endgroup$
    – Violet Giraffe
    Mar 13, 2013 at 13:19
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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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  • $\begingroup$ 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. $\endgroup$
    – Violet Giraffe
    Mar 11, 2013 at 19:55
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    $\begingroup$ Why can vertical pupils shrink more than circular ones? Isn't the lower limit on pupil size always 0? $\endgroup$
    – kmm
    Mar 11, 2013 at 20:49
  • $\begingroup$ @Kevin: how do you imagine a circular pupil shrinking to 0? Where will pupil tissue go? $\endgroup$
    – Violet Giraffe
    Mar 11, 2013 at 21:15
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    $\begingroup$ The pupil is an opening in the iris, not tissue: en.wikipedia.org/wiki/Pupil $\endgroup$
    – kmm
    Mar 11, 2013 at 21:47
  • $\begingroup$ @Kevin: I didn't realize... Still, muscular system for circular pupil seems less trivial and more restricting $\endgroup$
    – Violet Giraffe
    Mar 11, 2013 at 22:06
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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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  • $\begingroup$ How does a vertical pupil help to detect horizontal movement? $\endgroup$
    – Armatus
    May 17, 2014 at 23:02
  • $\begingroup$ 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. $\endgroup$
    – SmokeyVW
    May 18, 2014 at 0:07
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    $\begingroup$ 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. $\endgroup$
    – SmokeyVW
    May 18, 2014 at 0:17
  • $\begingroup$ @SmokeyVW thanks for the link to the bokeh examples pages. I had the question of how a cat (or a vertical line pupil animal in general) would see and that link showed exactly what I was looking for. $\endgroup$
    – Rodrigo Gómez
    Jan 12, 2015 at 22:41
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There was a study published this last year (2015) that suggests that the pupil style of an animal is related to its strategy for predation. This means that land animal pupils evolved as adaptations to the niche that they filled.

The study distinguishes between herbivorous, active, and ambush foraging behaviors. Ambush predators tend towards vertical pupils, where as active predators (those that chase down and kill their prey) tend more towards circular. The pupils of herbivorous foragers tended to be mostly horizontally-elongated as is the case with goats. Eyes also tend to be forward set in predatory animals and more at the side of the head in prey animals.

In Why do animal eyes have pupils of different shapes? Banks, et. al. Science Advances
07 Aug 2015: Vol. 1, no. 7, e1500391 DOI: 10.1126/sciadv.1500391 It is suggested that

Abstract
There is a striking correlation between terrestrial species’ pupil shape and ecological niche (that is, foraging mode and time of day they are active). Species with vertically elongated pupils are very likely to be ambush predators and active day and night. Species with horizontally elongated pupils are very likely to be prey and to have laterally placed eyes. Vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. This is advantageous for ambush predators to use stereopsis to estimate distances of vertical contours and defocus blur to estimate distances of horizontal contours. Horizontally elongated pupils create sharp images of horizontal contours ahead and behind, creating a horizontally panoramic view that facilitates detection of predators from various directions and forward locomotion across uneven terrain.

enter image description here
Figure 1. Why do animal eyes have pupils of different shapes? Banks, et. al. Science Advances
07 Aug 2015: Vol. 1, no. 7, e1500391 DOI: 10.1126/sciadv.1500391

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I see one possible advantage to round pupils. A slit pupil gives higher visual acuity vertically than horizontally because of diffraction and when a slit pupil is very nearly shut, there's high diffraction in the horizontal direction giving poor horizontal visual acuity. A round pupil on the other hand gives a creature the highest visual acuity in all directions for a given area of pupil. A smaller pupil is better for reducing chromatic aberration but chromatic aberration can be pretty much neglected because for any creature with evolutionary pressure for sharp vision in all directions, good accomodation will ensure at least one wavelength of light focuses onto the retina forming a sharp image because the sun emits a continuous range of wavelengths.

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