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In my world, there is a species of human called Homo hematophagus (blood eating human).

Traditionally, these vampires are a cannibalistic race: In war they often drank the blood from their rivals' wounds. They have a blood drinking ritual after sacrificing animals. They tend to be much more nocturnal than common humans (Homo sapiens).

Blue-green colorblindness (also erroneously called blue-yellow colorblindness) is a genetic disease characterised by non-functioning (in the case of tritanomaly) or absent (in the case of tritanopia) blue-sensitive cones.

enter image description here (The percentage of real life humans who are colourblind.)

12.5 % of vampires are blue-green colourblind (10 % have tritanomaly, and 2.5 % have tritanopia). They are also much more prone to achromatopsia, and red-green colorblindness (which include protanomaly, protanopia, deuteranomaly, and deuteranopia, and blue cone monochromacy) than common humans.

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    $\begingroup$ Small nitpick: in tritanomaly cones are not "non-functioning" but merely less sensitive or sensitive to different wavelength than the most common form. The extent of tritanomaly can vary from very slight effect to almost complete tritanopia. $\endgroup$
    – jpa
    Aug 18, 2023 at 17:48
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    $\begingroup$ Is blood all they eat? Do they cook any food? There are African tribes that drink the blood of their cattle for sustenance, as opposed for any ritualistic/religious reasons. They also eat cooked meat, as do most humans for a healthy diet. Humans evolved to eat cooked food and therefore cooked by fire. Fire provides light, and with that light they can distinguish color that might be helpful in determining if food is safe to eat. Those with a certain kind of color blindness may have an advantage or disadvantage because of how food, or something else, looks by firelight. $\endgroup$
    – MacGuffin
    Aug 19, 2023 at 3:11

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They're nocturnal, and they're carnivores.

That's precisely why most mammals are dichromats to start with. We evolved from nocturnal ancestors, who lost the ancestral tetrapod tetrachromatic vision, and only a few of us have re-evolved trichromatic vision.

If they aren't eating plants, they don't need detailed color discrimination to tell ripe from unripe or rotten fruits and vegetables. If they hunt, they need pattern and motion sensitivity more than color sensitivity. And if they are active at night, it is better to sacrifice color sensitivity for luminosity sensitivity than to hold on to better color vision regardless.

So, yeah; you could easily justify much more than merely 12.5% colorblindness. There's no disadvantage to weed it out, and those who have it might well be better hunters and have better night vision.

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    $\begingroup$ "If they hunt, they need pattern and motion sensitivity more than color sensitivity." If they hunt then the ability to see if their prey is bleeding red blood, as opposed to being wet with water or something, would be important information for the success of the hunt. Blue-green color blindness would be bad for those trying to pick out fruits among leaves but perhaps beneficial to picking out animals among the leaves. $\endgroup$
    – MacGuffin
    Aug 18, 2023 at 18:03
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    $\begingroup$ @MacGuffin Typical prey doesn't end up bleeding until after being bitten... $\endgroup$ Aug 18, 2023 at 18:14
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    $\begingroup$ If hunting in packs the prey could be bleeding from wounds inflicted by others. If there's a dozen guys with spears uphill from me, and they toss spears into a herd of some prey animal, then when the herd come running downhill then it would be to my advantage to know which of the prey got speared (as they'd get winded sooner) versus the ones that were uninjured but ran with the herd to improve chances of escape. If the herd gets to the river at the bottom then they will all look wet coming out the other side but the wounded and injured will still bleed red and those merely wet would not. $\endgroup$
    – MacGuffin
    Aug 18, 2023 at 18:21
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    $\begingroup$ @MacGuffin that pretty rare and minor information compared to be better at seeing the prey in the first place. $\endgroup$
    – John
    Aug 19, 2023 at 12:52
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    $\begingroup$ @MacGuffin What has to happen for your information to be useful, The animals has to be injured by a spear, then enter water, then exit in your direction, still mixed in with enough other prey to need quick singling out. the injury is significant enough to leave to be bleeding enough to be visible from a distance soon after crossing water AND aid in taking down the prey but not so significant that it has any other impact on things like movement or behavior of the prey. How often is this situation going to arise once a generation? How often is being able to see better at night going to arise? $\endgroup$
    – John
    Aug 19, 2023 at 17:03
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The stereotype for colorblind people is that while on one hand they have a harder time matching socks, on the other they are less prone to be fooled by camouflage.

I could find no scientific evidence for it, but there are plenty of pages in the internet saying that about camouflage. I do happen to have a relative who is colorblind. My father does seem to see more details and patterns in images that are too colourful, despite mistaking the colours.

Mantis shrimp have three colour regions in each eye... One side sees the bluer hues better, another sees reds better, and the central band seems to be best for green and yellowish hues. They have trinocular vision in each eye separately and focus with different regions to filter out different prey. So... perhaps your vampires only care about differentiating red tones (blood and human skin) and don't care about the rest, so full spectrum vision is not as advantageous and therefore natural selection does not filter the colorblind out. It just happens that blue-green colorblindness is not advantageous either so it is less common due to being a recessive (non-dominant) condition.

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    $\begingroup$ My understanding is the camouflage thing is merely you have to redesign camouflage for dichromats. Witness the tiger has effective camouflage for its prey. $\endgroup$
    – Joshua
    Aug 18, 2023 at 22:19
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    $\begingroup$ "trinocular" means "three eye". I think you mean "trichromatic". $\endgroup$ Aug 19, 2023 at 1:37
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    $\begingroup$ @Acccumulation trinocular may also mean "like having three eyes" and it does apply to each individual eye of a mantis shrimp: en.wikipedia.org/wiki/Trinocular_vision $\endgroup$ Aug 19, 2023 at 2:43
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If some of the cone cells in the retina of the OP's nocturnal vampiric humans have been displaced by more rod cells, thus leading to colourblindness relative to Homo sapiens, then it could be considered advantageous to have more rod cells, and therefore significantly better night vision under the right circumstances.

Since Homo hematophagus is more nocturnal, they have less use for vision in brightly-lit areas, which are where cone cells excel anyway. As cone cells are of little use at night, having a lot more rods is advantageous for a nocturnal species.

I think that not only would this mutation be viable, it would probably be selected for amongst the vampires, and would gradually increase in prevalence.

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Is there such a thing as blue-green colourblindness?

Our eye's blue detector is very old. It came before the rods and the red-green detectors. It is almost as old as multicellular life. The blue opsin has been theorised to have served as a precursor to photosynthesis.

Our blue cones are somewhat separate to our green and red cones. There are less of them in our central vision, and you can identify them in a stained retina under a microscope by their distinctive nerve connections. We have no blue detectors in the centre of our vision. There are some people with defective blue vision. There are genetic variations (see Oliver Sacks' Island of the Blind) that knock out all three detectors, leaving only the rods. But I know of no cases of blue and green cones being knocked out, leaving the red. it is possible to get some unfortunate with deuteranopia and tritanopia, but they don't go together.

I would be suspicious of evolutionary explanations, particularly short-term ones. Old-world monkeys developed three-colour vision millions of years ago. This is sometimes said to give an advantage when picking fruit. But red fruit are available in season when there is lots for everyone. Something that gave a survival advantage might be expected to work when food was scarce, or for some other reason such as recognising camouflaged predators. What do we actually use our blue vision for, anyway?

Try something else. We have a macular yellow dye over our central vision. There is still debate as to what it is for. It probably protects our vision against UV damage, and filters out blue (which we have no sensors for in our macula) leaving our red and green sensors without interference from the blue. Not having it (macular degeneration) is a bad thing, but the amount of macular dye varies from person to person. If vampires had much more macular dye (and the amount we have varies a lot) then they would not see blue.

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    $\begingroup$ Re: "Is there such a thing as blue-green colourblindness?": The OP is using the term "blue-green colorblindness" to refer to what's usually called "blue–yellow colorblindness", which is rare, but certainly exists. (The OP apparently prefers "blue-green" because "blue-yellow colorblindness" sounds like it would make blue and yellow appear alike, which it does not.) $\endgroup$
    – ruakh
    Aug 18, 2023 at 20:35
  • $\begingroup$ @ruakh Ah. Thanks for the clarification. 'Blue colourblindness' would have done. There are anomalous 'red-green' colour blindness where people cannot distinguish the two well, but blue is very much on its own. $\endgroup$ Aug 19, 2023 at 8:04

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