Currently, I'm creating an alien species with two eyes, but two pupils in each eye. Though I'm not sure how it would affect its vision, whether negatively, positively, or both, so I've been looking for an answer.

I previously asked this in the Biology community, though I was suggested to turn here instead.

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    $\begingroup$ I can't type a full answer now, but I'd like to leave this here as an inspiration for you and other people who would like to answer: en.m.wikipedia.org/wiki/Four-eyed_fish $\endgroup$ – The Square-Cube Law Mar 19 '18 at 21:02
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    $\begingroup$ The question is ill-defined. Please expand the description of the creature's eyes, because as it stands there is not enough information. Pedantic mode: The eye is an optical system, and as such it has a physical aperture, an entrance pupil (the physical aperture as seen from the object side), and an exit pupil (the physical aperture as seen from the image side). What we commonly call "the" pupil is the entrace pupil of the system, whereas an eye surgeon would be more concerned with the physical aperture. $\endgroup$ – AlexP Mar 19 '18 at 21:09
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    $\begingroup$ The pupils either adapted to different wavelength of light, and can share the same retina space, or pass the same (or overlapping) bands of spectrum, which would mean there have to be separate retinas for both pupils. In either case, having a separate eyes with one pupil seems like a simpler solution. $\endgroup$ – Alexander Mar 19 '18 at 21:13
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    $\begingroup$ ... If you simply mean "what would happen if the pysical aperture of the eye was made up of several smaller holes instead of one big hole" then the answer is nothing much; the image would be slightly blurred and the light sensitivity decreased in proportion to the opaque area. The physical aperture is way out of focus, so its image of the retina is very very blurred. Combined with the general low quality of the eye as an optical system, this would be of minor importance. $\endgroup$ – AlexP Mar 19 '18 at 21:18
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    $\begingroup$ Do they have two retinas per eye? That's the crucial bit. $\endgroup$ – user33150 Mar 20 '18 at 2:13

From the perspective of a human, the results are bad.

  • The muscles around the pupil are designed to dilate and contract the pupil, regulating the amount of light impacting on the retina. Two holes instead of one means the area between the two holes isn't being controlled as expected, and the result is poor light control on the retina.

But, if the eye is designed for something non-human (like a goat, frog, cat, or...) like two holes, then there could be some benefits.

  • If you have a two-chambered retina, it would allow you to receive something different in one compared to the other. This would only make evolutionary sense if what was coming through the pupil couldn't be dealt with by the construction of the retina. In other words, if you add cones to detect infrared light, that would NOT justify two chambers.

  • On the other hand, a second chamber would be useful for full-spectrum sight (or, perhaps, "normal" spectrum sight) if you're in a binary star system and one of those honkers is really, really, really bright. The second chamber would be more acute than simply shading a single chamber.

  • Or, if you really color outside the lines, the second chamber might heighten contrast, or heighten saturation, or some other Photoshoppy effect that would give them an advantage over just seeing plain old color.

So, if the eye is a two-chambered retina and the muscles of the eye are designed to manipulate two pupils, we have an ultra-cool solution with an amazingly high geek factor.

If, on the other hand, you're just thinking about human eyes with two holes. Nope, there's no advantage at all. In fact, it's all downhill.

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    $\begingroup$ two-chambered retina: Say the aliens are on a planet that is pretty dark, could they get the thermal/nightvision type of vision? youtube.com/watch?v=Mrb1TCgeDPo $\endgroup$ – Crettig Mar 20 '18 at 1:07
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    $\begingroup$ @Crettig, The easy answer is "sure." But remember, evolution wouldn't bother with two chambers if one chamber can do. If the good old rods & cones could adapt to be stimulated by a wider range of frequencies, then a single chamber would be adequate. On the otherhand, brightness (as an example) is something the rods and cones can't handle. So, if ultraviolet R&C suffered due to the ambient brightness of what we'd call "normal sight," that would justify a second chamber. There are a lot of opportunities here. $\endgroup$ – JBH Mar 20 '18 at 1:40
  • $\begingroup$ @JBH keep in mind evolution is a random process and many evolved traits could be optimised. $\endgroup$ – tox123 Mar 22 '18 at 22:13
  • $\begingroup$ @tox123, No doubt! But from the perspective of storytelling, having a cool reason is never a bad thing. $\endgroup$ – JBH Mar 22 '18 at 23:24

It's possible and the effects should be positive.

It's possible to have two pupils in the same eye with each adding their own benefit. As linked by Renan, the four eyed fish has four pupils in two eyes (so it's a bit of a misnomer). The upper pupil of the eye is adapted for vision in air while the lower pupil for vision in water. That is, the refractive index of the lens in front of each respective pupil is adapted for the different refractive index of the medium they're seeing in. It also has two separate retinas inside a single eyeball, one for each pupil.

For your own creatures it would likely work in a similar fashion, for vision in different mediums or even different colors or spectra (assuming it als has two retinas).

If you introduced a creature with multiple pupils that had no evolutionary advantage, a reader may question if you really thought it through at all.

  • $\begingroup$ Was going to answer just to suggest multiple spectra (infrared / night vision being the most obvious ones), but this pretty much covers my thoughts. Also wanted to note that the ability to see different parts of the EM spectrum adds a lot more story possibilities than just a minor feature change with no real effect on the race's capabilities or culture. $\endgroup$ – brichins Mar 20 '18 at 19:05
  • $\begingroup$ different parts of the spectra is not a bad idea, if they pupils are at right angles to each other it could use the same eyeball to get two different complete visual systems. $\endgroup$ – John Mar 21 '18 at 3:40
  • $\begingroup$ Lots of natural creatures actually see in ultraviolet for various practical purposes. I'd suggest this as an interesting alternative use for additional mechanisms in the eye. See also: askabiologist.asu.edu/colors-animals-see and my previous comments about a related question: worldbuilding.stackexchange.com/a/64325/13536 $\endgroup$ – flith Mar 21 '18 at 8:06
  • $\begingroup$ Talking about evolutionary advantages... Would four pupils require more energy from the brain to process the images? If so, would it be significant? $\endgroup$ – xDaizu Mar 21 '18 at 15:47
  • $\begingroup$ @xDaizu It depends on the arrangement. As in, are the different views always used at the same time or just in different mediums like air and water. But in either case it seems unlikely that the increase would be linear because there would certainly be some overlap in structures. $\endgroup$ – Samuel Mar 21 '18 at 15:53


Not sure about how this would affect an alien, but in humans this is a medical condition with the following side effects:

  1. blurred vision in the affected eye
  2. poor, dim, or double vision in the affected eye
  3. oblong shape of one or all additional pupils
  4. issues with glare
  5. a bridge of iris tissue between the pupils

Although, Pliny the elder wrote that it may give you magic powers, but that has yet to be determined

There are people of the same kind among the Triballi and the Illyrians, who also bewitch with a glance and who kill those they stare at for a longer time, especially with a look of anger, and that their evil eye is most felt by adults; and what is more remarkable is that they have two pupils in each eye.

enter image description here

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    $\begingroup$ This is a disfigurement of a normal human eye, not the normal form of an evolved feature. Why do you think an alien would just have a disfigured human eye rather than one evolved to have multiple pupils? It doesn't seem reasonable to draw conclusions from this. Who is Pliny the elder? $\endgroup$ – Samuel Mar 19 '18 at 22:05
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    $\begingroup$ @Samuel IIRC, he was a Jew with Roman citizenship who wrote histories for the Romans. He was (barely) a contemporary of Jesus, and wrote a LOT, including a lot about the early Christians, though he himself was not a Christian. He is important because as a non-Christian he has more credibility among non-believers, and he therefore helps establish the authenticity of Jesus as a real historical figure. $\endgroup$ – Joel Coehoorn Mar 19 '18 at 23:14
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    $\begingroup$ Upvote for citation of Pliny the Elder! $\endgroup$ – Willk Mar 20 '18 at 0:10
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    $\begingroup$ @Samuel I posted the mythology part as I found it to be genuinely an interesting tidbit while googling. It could be added into the alien's history/religion/folk takes/historical archives. Maybe the aliens teach that anger at a one pupil being will cause it to die and that's the reason that all beings (while not true and totally being a product of evolution, religions have taught weirder things) on the aliens world currently have two pupils. When the aliens encounter an earthling and the death stare doesn't work as intended,OH NO WHAT HAS THE ALIEN COUNCIL ELSE BEEN LIEING ABOUT? inappropriate? $\endgroup$ – Crettig Mar 20 '18 at 1:04
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    $\begingroup$ I want to upvote for "but that has yet to be determined" (citing Pliny the Elder should be de rigeur, so no upvote for that) but going from an unusual feature in humans with downsides to infer about a hypothetical evolved feature is akin to going from the fact that cornu cutaneum can be malignant to inferring that all horned animals have a higher risk of cancer. $\endgroup$ – Jon Hanna Mar 20 '18 at 14:14

Your creature occupies a middle trophic level. It is a predator but is preyed on by larger predators.

When in the role of predator it uses the vertical pupil to maximize depth perception and perform accurate ambush attacks. When under threat by predators it constricts the vertical pupil and uses the horizontal pupil to maximize field of view while looking for danger.


A 2015 study[16] confirmed the hypothesis that elongated pupils have increased dynamic range, and furthered the correlations with diel activity. 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.

Depicted: vertical predator pupil (cat), horizontal prey animal pupil (goat).

cat pupil

goat pupil

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    $\begingroup$ Important to note that these are mutually exclusive functions. If the two pupils were in the same eye (Like this perhaps: vignette.wikia.nocookie.net/naruto-fandom/images/5/5c/…) the effects would cancel each other out at best, and interfere with vision at worst. As Willk says, the alien would have to shift between them by contracting one pupil completely to obtain each effect separately - the two enhancements are not simultaneous $\endgroup$ – automaton Mar 21 '18 at 15:30
  • $\begingroup$ basically fight and flight modes, nice $\endgroup$ – XenoDwarf May 30 '20 at 6:08

Your alien is related to the mantis shrimp.

The mantis shrimp in all its glorious glory

They have the best eyes in the animal kingdom, with not two, but three pupils in each eye. Humans have binocular vision - we perceived depth by focussing two pupils on the same point. Mantis Shrimp have trinocular vision giving them better depth perceptions than humans using only a single eye. Not to mention that they see in the infrared and ultraviolet range and discern polarized light in ways that we can't. These phenomena aren't a result of multiple pupils, but rather because of a larger number of photoreceptors - twelve instead of the three that humans have. In short, their vision is something we can't quite even imagine.

They have other amazing qualities, like being able to create shockwaves underwater that stun prey even if their bullet-fast striking appendages miss. Oh, and the Department of Defense is studying the structure of its shell for the sake of designing better body armor. But I digress.


Correction by Samuel confirmed - their eyes are indeed compound, like those of insects, so their "pupils" that we see in the photo are an optical illusion - which I think is still pretty cool. Details in this article, which also tells how they can distinguish between right- and left-circular polarization of light, and may even use this as a communication method. Their vision is still trinocular, however, as each eye has three distinct sections.

I wasn't going to, but LSerni in the comments already did. Obligatory link to The Oatmeal, though the notion of them being able to see so many colors was disproved - many of the extra photoreceptors are used for some of the above purposes.

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    $\begingroup$ Every time I read about these things I'm just happy they hide in holes all their life $\endgroup$ – Roman Mar 20 '18 at 22:22
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    $\begingroup$ @Roman I'm happy they haven't allied with the tardigrades yet, because when they do they will be unstoppable. $\endgroup$ – tox123 Mar 20 '18 at 22:26
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    $\begingroup$ @tox123 Oh god, space capable mantis shrimp. $\endgroup$ – Roman Mar 20 '18 at 23:10
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    $\begingroup$ Can someone who can draw please create some concept art for this? A marine space navy. $\endgroup$ – Roman Mar 20 '18 at 23:11
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    $\begingroup$ @Roman you might find this interesting, if you don't already know - theoatmeal.com/comics/mantis_shrimp $\endgroup$ – LSerni Mar 20 '18 at 23:12

I'm no biologist, but a pupil typically isn't just the aperture, it's also the area the Lense fits to.

If you have two pupils, would you not have two lenses? In which case having radically different lenses would enable very different behaviours of the eye without sacrificing the benefits of one over the other.

Eg: Extremely good distance vision AND extremely good close vision in the same eyeball without having to refocus dramatically. Sort of a built in bifocal lense.

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    $\begingroup$ Separately polarized lens would be kinda cool too, a half closed eye blocks the horizontal or vertical polarization, but a fully open eye sees normally. Good for glare like seeing fish underwater, but more importantly able to watch 3d movies without the darn glasses. $\endgroup$ – user25818 Mar 20 '18 at 23:55

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