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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.

Von Willebrand disease is a blood disorder where the blood does not clot properly. It is usually of genetic origin. The chromosome involved is chromosome 12. The symptoms include recurrent nosebleeds, bleeding gums, easy bruising, and menorrhagia.

Fortunately, vampires have an EXTREMELY high resistance to prions.

10 % of vampires have this genetic disease, compared to 1% of real world humans. I wonder if there is any selective evolutionary advantage for a mammalian species from the Homo genus to have such a high rate of von Willebrand disease?

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    $\begingroup$ It looks like you're asking for us to brainstorm a list of causes. In general questions asking "Why?" aren't a good fit for this site since most things have many equally valid possible causes. Questions with many equally valid answers aren't permitted on this site, $\endgroup$
    – sphennings
    Oct 11, 2022 at 23:36
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    $\begingroup$ He does offer a question about whether there is any (implied: selection) advantage from the disease at the end of the body. $\endgroup$
    – user86462
    Oct 12, 2022 at 0:38
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    $\begingroup$ @sphennings That's most unfair. Everyone can and will connect the dots between sexual attractiveness and sex. I aimed to answer the question in the body, and did. Yes, I did decide to use humour and riff on the idea of bleeding gums and bruising and nosebleeds being sexy. There's no rule against levity. Who wants a forum with a 100% chance of utter seriousness about sexy gingivitis? $\endgroup$
    – user86462
    Oct 12, 2022 at 7:15
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    $\begingroup$ You can remove the stuff about dominant and recessive genes. I has nothing to do with the question. $\endgroup$
    – Daron
    Oct 12, 2022 at 10:12
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    $\begingroup$ And I assume "scarifying animals" should be "sacrificing animals". $\endgroup$
    – Daron
    Oct 12, 2022 at 10:27

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Natural anticoagulant

Reducing blood clotting is such a useful feature that we give anticoagulant medication to substantial proportions of the elderly population in long-lived Western societies. This is because the risk of blood clots (strokes, heart attacks etc.) outstrips the dangers of blood thinning (increased bleeding) in the elderly. You simply need to find a reason why this is even more of a concern for your species, such that thinner blood is on balance beneficial even to reproductive and pre-reproductive age individuals. For example:

  • Their diet is really, really likely to encourage cardiovascular events - lardy cakes, high salt, HFCS, the delicious fatty bone marrow of their vanquished enemies, you name it
  • The negative effects of bleeding are not as severe: menstruation is infrequent (so heavy periods are less of a problem), their dental hygiene is good (so their gums don't bleed in the first place), they have natural (non-platelet based) regenerative powers that offset the bruising and nosebleeds
  • There are other genetic or environmental factors that increase the chance of early cardiovascular events, such as deep-vein thrombosis (which is a condition that is treated with blood thinners even in young people): they might have a tendency to congenital narrow veins in the limbs, or they may lie completely still for very long periods (say, for example, in a coffin) which is known to increase the chance of blood clots (this is why they encourage you to get up and walk in long-haul flights)

All in all, the degree to which blood clots is a balance between too little (excessive bleeding and bruising) and too much (clotting and strokes). Evolution has found a balance point for humans, and we call deviations from this point "diseases", like vW. Your vampires would have a different balance point, and may call "our" degree of blood clotting a hypercoagulation disease.

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    $\begingroup$ Lardy cakes are amazing. I'm not a brit, but have a great british cookbook. yum $\endgroup$
    – crthompson
    Oct 13, 2022 at 17:22
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    $\begingroup$ Lardy cakes should feature. $\endgroup$
    – user86462
    Oct 20, 2022 at 9:31
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They are a recovering species

The affliction isn't an advantage. The Homo Haematophagus is simply recovering from a large wipe out event. Afterwards the genetic diversity is so low that there is too little choice. The remaining population has Von Willebrand as a large genetic chance within themselves. Even when recovering to larger numbers the genetic diversity will take a long time to get large again. It is a genetic bottleneck that forces most offspring to have the affliction.

One example can be that Homo Haematophagus was hunted down in large quantities, while at the same time suffering from prions in their population. The remainder flees or hides. Of the remainder practically only the ones strong to prions survive, but many do have Von Willebrand. They hold their head down long enough to regain their numbers, most now afflicted with Von Willebrand.

Though not a full parallel we can see low genetic diversity in many species because of near extinction events. Iirc one of the big cats in Africa has suffered a near extinction event, causing most of the lineage to be traced back to a small number of the species. This includes genetic defects.

Do note that low genetic diversity can also wipe out the species. Genetic problems like Von Willebrand can more easily spread within the population and further mutations can make this worse. But if they survive, any genetic makeup will stay with them for a long time. Regardless whether it is good or bad.

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    $\begingroup$ Genetic Bottleneck makes sense to me. If one of the few survivors had Von Willebrand, then before too long, practically everyone will be related to them and have it. $\endgroup$
    – Ruadhan
    Oct 12, 2022 at 7:38
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    $\begingroup$ I believe they were assaulted by a very persistent clan of religious zealots with whips. $\endgroup$ Oct 13, 2022 at 14:28
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They smell GREAT

To other vampires, of course. You think gingival bleeding smells good to us?

No, the sweet scent of blood makes these vampires the equivalent of a girl with the genuinely great smelling perfume. Or maybe the girl who just piles it on but the young men like it even though it's actually rather pungent.

Vampire women, in their turn, are thrilled with a husband who spontaneously nosebleeds during intimate times.

Obviously blood tastes good too, and bruising is beautiful and alluring. Unwise vampires that use Instagram too much self harm in order to look hot. Which is vapid, but true to life.

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It doesn't substantially affect their survival:

A genetic disease is self-limiting if it has a significant effect on survival and reproduction. If the disease is likely to kill you before you have kids, the frequency goes down.

Mutations constantly affect every living thing. A gene that has zero effect on survival gradually acquires more and more mutations. These mutations eventually will cause the gene to be unable to perform the task it was originally evolved to do. This is why fish in caves have their eyes slowly degenerate. The genes for the eyes gradually acquired mutations, but these mutations didn't effect survival.

Your vampires have a ready source of Von Willebrand factor - human blood. High prion resistance allows their bodies to incorporate a wide variety of proteins (especially those free-floating in the circulation) directly from food. They absorb the intact protein directly from their food and incorporate it into their circulatory system. So while they may still have some of the superficial effects of the disease, the external doses of vWF prevent the more profound symptoms and allow those with the disease to survive.

Advantages:

Advantages to having such a disease are not going to be direct. You need your blood, and it is harming you to lose it. But we can come up with some possible reasons that may or may not make sense in your setting.

  • Ritual: You vampire society has blood-letting rituals that demonstrate self-control, faithfulness, or subservience. In these rituals, the individuals who bleed for the longest are considered to have some virtue - self control since they haven't fed on blood recently and bleed longer, faithfulness because a long ritual bleed is associated with holiness, or subservience since a long bleed fed on by a lord leaves the lord pleased with their servant. All these might potentially favor the survival or improved reproductive opportunities for individuals (especially since the disadvantages can be treated by drinking blood).
  • Overdosing: High levels of vWF can be associated with an increased risk of strokes. Since your vampires absorb vWF and other clotting factors from their prey, they are at risk for strokes if feeding frequently. Individuals with Von Willebrand's disease are less able to clot, and are thus less vulnerable from strokes due to overfeeding.
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A terrible plague has recently swept through the population

A terrible, blood coagulating plauge has recently run through the Homo hematophagous population. Members of the population with Von Willebrand's disease, (or those who have one copy of the gene for it) are immune to the effects of this disease. For real world examples, see sickle cell gene carriers increased immunity to malaria.

This would provide a selective advantage that is likely to result in a considerably higher rate than in the base human population

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Clotting agent comes from the victims

enter image description here

Painting by Kam on Pinterest

10% prevalence of anything is no advantage for the species as a whole. More likely, the disease is simply less crippling for the vampires than for normal humans. This means it is not selected against.

Blood is not a good source of food for humans. Fortunately Vampires have a different metabolism. They recycle some of the components of the victims' blood into their own blood, instead of breaking them down and reassembling.

In particular they do not digest the von Willebrand clotting factor. They just move it from the stomach into their own bloodstream.

Even the other 90% of vampires don't produce a lot of clotting factor. They get it from their victims.

Not until the vampire goes a few weeks without drinking blood, does their von Willebrand disease become visible.

Historically this was not obvious since all vampires either drank blood regularly or died. It has become a problem in the modern age however, when vampires increased their population and moved to cities and started developing processed alternatives to blood.

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You could pattern it after an actual human disease such as sickle cell disease. The gene which causes it is recessive, so you only get the disease if both parents carry the gene. But if you only get it from one parent, you get protection against Malaria which is definitely a selective evolutionary advantage.

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They had a natural predator and used to live substantially shorter lives

A natural predator killed most of the population before/around reaching the age where VWB symptoms onset started to become a problem, and therefore there was never a natural selection reason for the population's DNA diversity to stray away from the genes.

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There is if you want one; not if you don't.

This is your world and all its rules, including selective evolutionary advantage, are up to you to first define and then explain.

If it helps a mammalian species from the Homo genus to have a high rate of von Willebrand disease, why not show that - or at worst, state it - and support it with details?

What Von Willebrand disease is matters how?

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