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In various fantasy stories I have read and seen there is a trope known as Eye Colour Change Warning: TV Tropes!. In many cases the person or creature's eyes change color as a sign that they are activating their super powers. What I am curious about is:

Could a creature have eyes that rapidly change color based on their mood?

For example a character gets mad and their eyes turn red.

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    $\begingroup$ basically put a layer of chromatophores on top of the muscles that control the iris. Use it as a method of non-verbal communication. I think it's a wonderful idea... $\endgroup$ – Isaac Kotlicky Feb 12 '17 at 14:40
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    $\begingroup$ I know of several humans whose eyes get red because they have become very very mellow and want to eat pizza and giggle and listen to music. I say that should be considered a super power too. $\endgroup$ – Willk Feb 12 '17 at 19:05
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    $\begingroup$ My daughter had a blue parrot that had eyes that flashed a yellow ring around the pupil when offered something especially appreciated, e.g., a small branch snapped of a huckleberry bush loaded with berries. $\endgroup$ – user2338816 Feb 13 '17 at 5:34
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    $\begingroup$ You mean, like humans? $\endgroup$ – T. Sar Feb 13 '17 at 12:22
  • $\begingroup$ All social creatures need some kind of signal system. Changing eyes color can be an example. $\endgroup$ – kelin Feb 14 '17 at 7:59
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You are designing the creature, so yes

There is a skeptics SE question on this subject, relating to humans. This seems to support the claim that mental state can have some difference on an eye colour - mostly through anecdotal 'evidence' though. It doesn't necessarily mean human eyes can have rapid drastic changes though (blue to red instantly, for example, would require a very fast change in the amount of melanin and may not be healthy).

You could, however, have a different structure to your creatures entirely. Perhaps the chemicals that drive emotions, once in the blood stream, refract light differently and make the colours. Perhaps your creatures have various protective filters on their eyes - under stress a creature on a desert planet may have an automatic dust filter which comes down and this scatters or reflects red light more - making the eyes appear red.

There are many methods you could apply. Your question only asks for if they could, not how they would though so I won't go into that too much here.

Edit to add the how:

There are lots of ways to do this:

  • Moisture traps: This idea comes from the Charidotella egregia which traps water between different layers of thin reflective surfaces as described in this paper (even if only the abstract is available it describes this process fairly simply). The moisture changes the distance between these reflective films and, by varying the amount of moisture, the distance between reflective films can be controlled.

    • I like this method because it gives fine control over colour change and can be related to something more human - crying. Interestingly we have different types of tears I would imagine the salinity of your creatures tears could effect how easily the porous layers of film absorb and, therefore, produce different colours.
    • This is a very fast acting method.
    • This also requires little effort (no producing complicated dyes or such that this may be an evolutionary disadvantage).

  • Colour cells: Cuttlefish are, I've been finding out, rather amazing colour changers. Here is a quote from that article:

    Cuttlefish skin has been likened to a color television—it has a way of combining basic colors to form more complex hues and dynamic patterns. "It really is electric skin," Hanlon said, because it's all controlled by neurons in the brain that transmit impulses and information to the rest of the body. "A cuttlefish has maybe ten million little color cells in its skin, and each one of them is controlled by a neuron. If you turn some on, but leave others switched off, you can create patterns," Hanlon explained.

    • This method is brilliant colour control, fast acting and we can link it into the brain fairly easily since it is controlled by electrical impulses. It would require having these colour cells in the iris (perhaps as a method to better see in different lights). These could be related to emotion in many different ways but a few that occurred to me:
    • When sad humans often look down where we receive fewer distractions as we look inside ourselves. This could be substituted by a darker pigment blocking out more light.
    • Anger in humans can cause dilated pupils, perhaps to take in more light. In our creatures they could do this by changing to lighter irises, revealing the crisscross of blood-vessels behind the eye and creating a red eye look (as with albinos).

Those last couple of points could apply to either technique or any of those other answers have mentioned.

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    $\begingroup$ I forgot to include the if yes, then how in the question. If you want to elaborate further by all means, but you have already provided the information I needed and have given me ideas. I will keep the question open for a while longer just in case a better answer appears. $\endgroup$ – Anketam Feb 12 '17 at 15:27
  • $\begingroup$ @Anketam I added a few ideas I had already seen, hope they help (I'm particularly fond of the eloquent design of the moisture trap). $\endgroup$ – Lio Elbammalf Feb 12 '17 at 16:32
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    $\begingroup$ Fun fact: octopuses do color-camo too, and we totally don't understand it because as far as we know, they can't see color. (Cuttlefish might be able to see color by splitting the spectrum and picking up different wavelengths at different parts of their eyes, but we're not sure.) $\endgroup$ – MissMonicaE Feb 13 '17 at 18:58
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Some Chameleons change the colour of their skin to reflect their mood. For example: Darker colours mean that the chameleon is angry whilst lighter colours may be used to attract mates. They have a specialised layer of cells to allow them to do this so there is no reason why a creature couldn't have a layer of cells over their eyes to allow them to do the same thing.

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    $\begingroup$ I was about to write the same response, most of the time chameleons do not use color for disguise but a way to show their mood. $\endgroup$ – Cem Kalyoncu Feb 12 '17 at 18:07
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Yes.

There are many animals on Earth that can and do change color (though not eye color specifically, I think). It wouldn't be a stretch at all to extend this to eyes (and/or patches around the eyes, maybe, to enhance the idea). The interesting thing would be to explain how and why this change happens. (I'm assuming, since this is Worldbuilding, that you're thinking of building a world with creatures like these yourself.)

  • Is it universal? Do some members of the species not change eye color, or change to different colors than most others?
  • Is it voluntary? Can they cheat? Can an individual force itself to change eye color even though it doesn't feel the associated emotion, or to suppress the change even though the emotion is present?
  • Is it used for other purposes besides showing emotion? Artistically, erotically?
  • Can it be suppressed or damaged permanently by accidents, surgery, etc.?
  • How did it evolve? Was it a useful trait to signal emotion in a social species? Was it a form of mating display? Was it a (maybe undesirable) side consequence of another biological process?

The detailed mechanism need not be explained, but as I seem to remember, in general there are ways for color to appear in a living organism:

  • Structures can be colored by pigments inside cells (and these cells can expand or contract to spread or mute a given color).
  • Structures can contain reflective surfaces that appear to change color as they face light at different angles (iridescence).

Either of these could very well work inside eyes, though vision can be impaired if the coloring structures stand between the retina and the incoming light, of course.

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    $\begingroup$ see my comment - you wouldn't want or need these obstructing the lense itself - just put chromatophores into the iris and the surrounding sclera, leaving the pupil free... $\endgroup$ – Isaac Kotlicky Feb 12 '17 at 14:43
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    $\begingroup$ @IsaacKotlicky Hadn't seen your comment, but anyway, that's why I wrote "if". You could also have the chromatophores behind the retina if the equivalents of rods and cones don't suck up all the light. $\endgroup$ – pablodf76 Feb 12 '17 at 14:49
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    $\begingroup$ that's... a bit similar to how cat eyes (and other nocturnal animals) work - reflect the light within the eyeball in order to increase detection rate of photons... and then allow them some way of "controlling" the color of the reflected light... hmmmn... perhaps with a naturally occurring metamaterial that can shift colors based upon angle of incidence, so "rotating" the structure in cell (by flexing a "muscle", for instance) would change the resulting color... I like this... $\endgroup$ – Isaac Kotlicky Feb 12 '17 at 20:25
  • $\begingroup$ @IsaacKotlicky Ah yes, of course: a movable, color-shifting tapetum lucidum. :) $\endgroup$ – pablodf76 Feb 12 '17 at 21:10
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Yes. You don't even have to look that hard to find examples. Personally my eyes shift from brown to green based on my blood pressure/stress level. My optometrist finds it to be fascinating, apparently it's not that common, but I share the trait with a few family members. My mother's eyes, for example, manage to throw a blue-grey color into the mix. Changing eye color isn't any more unbelievable in a story than changing skin color and there are lots of creatures that can do the latter.

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    $\begingroup$ I have the same thing going on: From a green/golden color to golden/brown to even very dark-brown if I get really angry. It is tied to adrenaline and blood-pressure levels as far as I can tell. Tiredness is also a part of it. From what I have been told it is apparently not that unusual especially if one of your parents had blue eyes (my father) and the other brown (my mother). $\endgroup$ – Tonny Feb 13 '17 at 14:34
  • $\begingroup$ My sister has eyes like this ("mood eyes"). Mine are straight blue, but my wife claims she can tell what's going on with me by what shade they are. Point is, people actually do this now. The question just wants it more extreme than is normal. $\endgroup$ – T.E.D. Feb 13 '17 at 16:25
  • $\begingroup$ mine go greener (from brownish-green) by adding alcohol $\endgroup$ – Burki Mar 17 '20 at 11:58
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Heterochromia iridum which is where one eye colour is different from the other can be acquired after birth due to injury or sickness.

The colour change can be caused by deposits in the eye such as iron from an injury or sickness

So you could use a similar mechanism to change eye colour due to mood without it sounding far-fetched.

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  • $\begingroup$ How is changing ones mood similar to an injury? Can you elaborate on that? $\endgroup$ – JDługosz Feb 13 '17 at 3:49
  • $\begingroup$ @JDługosz The included link describing heterochromia iridum mentions something as subtle as iron deposits appearing in the eye as cause for a colour change. The questions itself says asks about an undefined creature which I assumed is fictional could possibly produce a high amount of iron in moody situations. $\endgroup$ – Puddler Feb 13 '17 at 4:03
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    $\begingroup$ You should include that in your answer. In general, don’t rely on links but paraphrase the important content. $\endgroup$ – JDługosz Feb 13 '17 at 4:04
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Is it something that happens in the real world?

No, but many people think it does because they are translating "my eye appear to be a different color" to "my eyes are a different color". Those things may seem alike, but they're different. The basics of how it happens that when we're in this or that mood we hold our head in different ways and it can effect things like how blood shot our eyes are or the direction of light/shadow hitting our eyes. The result is that that Eyes can appear to change color, because of the colors around them.

Is it something that could be possible?

Yes indeedy it can be done. How? Basically the same way pupils can dilate. Just have a similar thing that comes over the pupil (in the standard 3 colors) that developed to be able to modify what light goes in the eye so that some things will stand out more or less because certain wave patterns will be blocked out. It would be a rather useful trait for a predator on Earth... but I don't know how the prey would evolve to counter it.

You could say that it is controllable in wild animals, but in the humanoid animals you could say that they lost the ability to actively control it resulting in the various lenses closing/opening when they are focused, in a rage, etc.

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  • $\begingroup$ But it does happen in the real world, as several comments here say. Not universal or even common, but not that rare if you have "hazel" or green-brown eyes. $\endgroup$ – nigel222 Feb 13 '17 at 20:36
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    $\begingroup$ Since color is a mater of appearance if something appears to be a different color it by definition is a different color. The sky is sometimes blue, sometimes black, sometimes red, simply because it "appears" to be a different color, regardless of why it appears that color is a different color. It's like saying the pixels of your TV are black. Sure they are when they are off or aren't a different color, but otherwise they are a different color because they appear to be a different color. $\endgroup$ – Justin Ohms Feb 14 '17 at 2:50
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    $\begingroup$ @JustinOhms That is true from a certain stance, but doesn't actually help and it is total nonsense from an objective standpoint. We see colors as lighter or darker sometime even when the color is the exact same due to how the mind processes. And the object of the question is to get eyes to be a different color, not appear to be a different color. If we wanted to make the color appear to change we could release pheremones that cause the brain to process eyes differently somehow thus changing the color but that changes the question so it's important to discriminate "appear" and "actual change". $\endgroup$ – Durakken Feb 14 '17 at 3:20
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    $\begingroup$ @Durakken While its true that perception does play a part especially in those with hazel or grey eye colors... eyes do change color for many reasons, including pregnancy, puberty, chemical and hormonal changes en.wikipedia.org/wiki/Eye_color#Changes_in_eye_color $\endgroup$ – Justin Ohms Feb 14 '17 at 20:15
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    $\begingroup$ @JustinOhms It says "theoretically" based on the fact that the color of eyes is based on melanocytes which makes eye color change as a person ages or go through extreme chemical changes melanin in the eyes can darken/lighten. While you can get a good start from this, it does not fit the criteria of a full color change imo, because you're changing the saturation level rather than the (I forget the word for the base part of color, but that word goes here) which may be accepable to the OP, but I'd still want something that can change the "base color". $\endgroup$ – Durakken Feb 14 '17 at 23:08
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To add another possibility:

Many animals have a "third eyelid". Perhaps this creature has one which carries some color, and in some moods holds it closed. You could even speculate that the filtering or protection might be functional to justify this behavior.

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In terms of creating a creature, I'd say you likely can. However, going through examples in our world (aka the more boring side of the spectrum), I'd suggest instead to have either special rings or patterns around the eyes, since eyes are very delicate and specialized organs, while the skin would likely be a better point to change, or at least easier to do so. The trait could've appeared as a way to scare predators (tigers have "fake eyes" on the back of their heads, so I think it's plausible) and ended up as something linked solely to the individual's subconscious, directly connected to the parts of the brain responsible for emotions and mood. Such patterns/rings/regions would be rich in chromatophores and, being linked to the subconscious, would change color similarly to how chameleons change their skin according to mood. Also, one could be born with a mutation that allowed them to control these regions to an extent (just like how some people can flap their ears).

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