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I’m in the process of making my own original series where I have all types of biologically feasible cryptids and monsters in it. In this series, I’m planning on having vampires being a species that broke off of humans at some point in our evolution. I was thinking homo erectus but more on that in a future question.

But basically I’m having them be a more carnivorous species than us, that evolved to be nocturnal. And some of the adaptations I came up with are cooler body temperatures, pale skin, and most interestingly, black sclera. Does anybody have any idea if it’s even possible for sclera to be black? I was thinking it’d be caused by a mutation that helped hide the whites of their eyes (seeing as how we evolved to show so much of our own sclera so others could tell where we were looking so with this, it’d be harder for their prey to tell where they’re looking). Or do you think it’d be more possible to make them voluntarily turn their sclera black when they’re hunting, similar to how chameleons and cephalopods can?

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    $\begingroup$ Welcome to WorldBuilding.SE! You seem to be asking two separate things - where your vampires should branch off from humanity in the evolutionary tree, and whether it's possible to evolve black sclera. Questions should only contain one question, so I'd suggest picking one to focus on. $\endgroup$ – F1Krazy Aug 18 '20 at 9:57
  • $\begingroup$ Agreed: pick one question and ask it! You're allowed to ask more than one question. Also check out Anatomically Correct Vampires and Anatomically Correct Vampire Teeth to see if those help you at all! $\endgroup$ – elemtilas Aug 18 '20 at 10:43
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    $\begingroup$ Ok, I reworded it to focus on just the one question. Sorry, this is my first post so I just got a little carried away. $\endgroup$ – TheSuperSaiyan44 Aug 18 '20 at 11:45
  • $\begingroup$ It's certainly possible for human sclera to change color. For instance, one of the more obvious possible symptoms of hepatitis is the sclera becoming yellow. $\endgroup$ – jamesqf Aug 18 '20 at 16:46
  • $\begingroup$ my ex-pet monkey (get released to jungle) also has black sclera like this image, since other answer image look dark brownish to me than true black. $\endgroup$ – Li Jun Aug 18 '20 at 17:07
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Possible? Most likely, since other primates, like gorillas, have this trait, although they also have cases of white scleraenter image description here

Simply having a pigmented sclera sounds a lot easier of a trait to have than having eyes changing color, since that would require special chromatophores covering the entire sclereal region and acting like those found in an octopus' skin.

In addition, there's yet another solution for your nocturnal vampires: large pupils. enter image description here

This is a leopard seal, a powerful predator. To make most of the light available in their dark aquatic environment, they have large eyes and pupils capable of dilating to a very large size, thus creating a bigger entrance for the already scarce light. Despite these black eyes, their sclera seems to usually be either white or reddish white. This trait is also shared by many nocturnal predators and could be a great alternative considering your vampires also hunt in low light conditions and would allow them to look more human (pupil contracts revealing a more or less normal looking eyes, maybe with a slightly larger pupil, allowing them to pass more easily as humans should the need arise).

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    $\begingroup$ Nocturnal doesn't even need to be part of it. I thought about it a little bit and I'm actually having trouble coming up with an animal species other than humans that has a visible white area around their eye at rest. (Dogs, cats, horses, sheep, cows, chickens, bears...no, nope, also no. Actually most mammals seem to have very dark colored eyes altogether, except for cats and goats. Maybe a few individuals with light eyes or a visible white, but mostly none.) $\endgroup$ – user3067860 Aug 19 '20 at 14:24
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    $\begingroup$ Eyes with a slightly larger pupil, making them look cute and helping them lure human prey. Muahahaha. +1 $\endgroup$ – Ivana Aug 19 '20 at 21:00
  • $\begingroup$ @Ivana considering how often we see people wanting to pet Apex predators, I wouldn't be surprised if vampires took advantage on how stupidly we can act when near something we consider cute. $\endgroup$ – ProjectApex Aug 19 '20 at 22:24
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OF COURSE, BUT...:

As all the other answers to this question have pointed out, it is certainly possible to have black sclera, and it can certainly evolve. It is not uncommon for animals to have a sclera matching their eye color. The question is, do you want it to? For your nocturnal variant on Homo sapiens, there are some hunting advantages to dull eyes, but if your primary prey is humans (like vampires are traditionally portrayed) then it may be a bad idea.

Giving a reason something evolved is always a bit of conjecture, but the theory is that the sclera enhances communication among people. If you want to communicate with people, or if vampires are communal hunters like humans, then this could interfere with hunting. If humans aren't their primary prey, then being able to coordinate with humans is a valuable skill, and anything interfering with that is bad.

Have you ever googled black sclera and seen the pictures of the people using contacts? Even if they aren't deliberately trying to look evil, they do. I've used this in stories as a trait to make a character more alien than their fully human counterparts. If you are seducing your prey, you don't want to scare them away. Looking scary isn't usually helpful unless terror is the hunting style (like maybe for werewolves). If the goal is to work alongside humans, the same rule applies.

Although I can't say exactly how the musculature would work, I can imagine a vampire whose eyes have adjustably SIZED pupils, and that contract to have a small (human-sized) appearance in the presence of light. There would be something slightly off about how their eyes adjusted to changing light conditions, but it wouldn't be grossly obvious. In the dark, the pupils would expand to fill the area occupied by sclera, covering the whites, and this would take the place of dilating pupils. Your vampires would still have a "tell" to observant humans, but it wouldn't be quite so — evil — in appearance.

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    $\begingroup$ Fear can cause people to freeze. So if a vampire can change their appearance at will thy can use that to their advantage. Look like a human to move about sociably and then when they go for the attack the cause their sclera to darken to instill fear/look bad ass. They could use Chromatophores (like octopus) to be able to quickly change the color at will. $\endgroup$ – DarcyThomas Aug 19 '20 at 4:12
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Chimpanzee sclera are very dark.

chimp eye

Scleral pigmentation leads to conspicuous, not cryptic, eye morphology in chimpanzees

Chimps are our closest relatives. They have sclera considerably darker that their irises, as depicted. It is not clear why they have light irises set off against dark sclera and we are vice versa. But if they can do it, then it is reasoble for another closely related hominid like your vampires to do it.

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