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This pure-blood vampiric race is not immortal, but their lifespans are about five times that of mortals. This race has lived in a vast city beneath a small mountain range for thousands of years (the world outside is populated by humans) Living with them, though apart in a specific section of the city, is a colony of humans whose purpose is to act as sustenance for the vampiric race. These vampires cannot imbibe anything other than blood.

Though their city is vast, it is still a finite space and the antagonist of my story will eventually use this as a point of argument to back up his belief that they should spread from the mountain and take over human lands. He’s not going to win this argument, but I can’t help but acknowledge that he has a good point, so I had a thought that maybe the female pure-bloods usually only have one child during their lifetimes; two is not unheard of, but is extremely rare and usually kills the mother. I’d like to have a well thought out reason for this, don’t really want to just hand wave it, so can anyone lead me in the right direction? Many thanks, in advance.

In addition, though, there are a rare few newborn vampires mixed in with population, my focus for this question isn’t really on them. My focus is the pure-bloods, descendants of several generations. The newborns are notably different from the pure-bloods, they crave blood after being turned, but do not experience the true pain of thirst that blooded and pure bloods do until after their first drop of blood. Also, before their first drop of blood, these newborns can sustain themselves on human food and drink, though only very small amounts of the former, but after, their bodies immediately reject it. I was thinking that perhaps I would make the newborns’ birthrates higher, so maybe the low birthrate of the pure-bloods has to do with how long they’ve sustained themselves on blood?

Update

Also, female newborns would give birth to pure-bloods if their partner was also a vampire.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monty Wild Oct 30 '19 at 0:13
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Blood, by its very nature, is an oxidant. As such, it's the exact opposite of an anti-oxidant. This may sound obvious, but thinking through the implications a little deeper makes one realise that there are some serious differences to nutrition going on in a vampire's physiology by comparison to a normal human.

In point of fact, this is a paradox in how vampires and their physiologies work; on the one hand you have creatures with incredible recuperative and healing powers, which implies over-oxidisation and a higher metabolism, but then they live extended life spans, which implies under-oxidisation and a lower metabolism. For the purposes of this question however it is important to note that anti-oxidants are supposed to remove free-radicals from the body, reducing the amount of damage done to it by left over oxygen artefacts generated by normal metabolic processes.

It was thought for some time that anti-oxidants actually improved fertility in women and increased their chances of bringing live babies to term, but recent scientific studies have largely discredited this. But, it's important to note that these scientific studies focused on normal women with normal metabolisms; there were no female vampires with their unique metabolic processes introduced into the studies that have been done on the subject.

As such, we really don't know what drinking blood as a sole source of energy and nutrition is going to do to a female vampire in terms of her fertility or ability to bring a baby to term, but here is a bit of supposition for you;

For one thing, vampires are likely to have an excess of free-radicals in their system because their source of nutrition is designed from the ground up to bind oxygen to itself. As such, without a nutritional source of anti-oxidants to clean up the excess free radicals generated by the metabolism, these are likely to do a lot of damage to the body over time and may well be the reason why your vampires are not eternal.

Secondly, to live as long as they do, vampires, particularly given the existing issue of no anti-oxidants in their diet, may well have a lowered metabolism. This would explain their ability to survive on only small amounts of blood, and their extended lifespan, but would be contrary to their abilities of super speed or accelerated healing. Let's assume for now however that our vampires have a large number of free-radicals in their system AND a lowered metabolism.

The practical upshot of this is that even if vampire fertility was similar to that of humans, the capacity of the body to bring a baby to term is diminished. At best, vampire gestation rates would increase by the same factor as lifespan (say x5, or nearly 4 years) but likely even longer, and the free-radicals mean there's even more chance of random damage to the body in such a way that something can go wrong with the pregnancy. The birth itself would also be precarious, because the amount of energy required to deliver the baby would be massive by comparison to the amount of energy that the mother has available to her for the process. There would be no margin for error.

Vampires with lower metabolisms are essentially working with bodies designed for marathons, whereas delivering a baby requires a sprint; a relatively short, sharp, but massive expenditure of energy to safely bring the baby into the world.

Now, sure; we're already working with dichotomies like accelerated healing and it's possible that the body can switch between these two modes almost at will; increasing metabolism (and thirst) under times when it needs to heal at an accelerated rate and then ratcheting back down once the healing is done. Same would go for the delivery of the baby, but in my view each of these transitions between metabolic rates would incur a cost, meaning that warrior vampires, as well as female pregnant ones, only have so many times they can make that transition and so bringing more than a single baby to term requires an extraordinary constitution, access to additional blood supplies, luck, and probably all three.

Put simply, your female vampires run cool most of the time, need to run hot when bringing a baby to term, and switching between the two carries both a price and a risk that will eventually kill the mother, and probably the baby.

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    $\begingroup$ nutrition alone would limit birth rates, but this answer goes above and beyond that! $\endgroup$ – V. Sim Oct 25 '19 at 1:51
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    $\begingroup$ @TimBII Great answer! Really short version: it takes a lot of blood to gestate a baby. With their finite resources, perhaps they just evolved to survive within their environment's limitations. Having more than one baby would jeopardize that population's survival, which would develop into a cultural taboo. $\endgroup$ – Suncat2000 Oct 25 '19 at 12:16
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    $\begingroup$ @Tim B II Yes, I was assuming that female newborns would give birth to pure-bloods if their partner was also a vampire. Should I add this point to my post? $\endgroup$ – P. M. B. Oct 26 '19 at 2:56
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    $\begingroup$ @P.M.B. Yes, that's a good idea as it helps clarify the disparity between birth rates and population sustainment. Thanks. $\endgroup$ – Tim B II Oct 26 '19 at 4:38
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    $\begingroup$ @Tim B II Also, I was actually thinking of toning down things like super strength and healing. Of course, they will be stronger than humans and heal faster (concerning the latter, I was thinking a slower version of Wolverine’s regeneration. Like a cut would take a full day to heal, a bad wound, a few days to a week) Your point that they would be designed for marathons helps with this. You’ve given me a lot to think on, in a good way :) $\endgroup$ – P. M. B. Oct 26 '19 at 5:51
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Well let's be perfectly honest here and say that what determines the birth rate of any species is so deep into our genome that we are not even certain in real life. If you just say, "Female vampires can only give birth once every 5 years," no biologist on earth will question that. Unless your story is taking place in a genomics laboratory, you're never going to be asked to prove anything about birth rate to the audience. So go ahead and hand-wave it. Literally everyone else does, even real biologists.

OK, but I should add.. the "evolutionary adaptation" explanation you might want to use is very simply because, they sleep all day and have a really poor calcium intake.

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Tim's answer was great, but there's just one small thing I'd like to add to that. In the world we know, it seems in a great many cases that predators and creatures good at evading predators have fewer children the further up the food chain we go. With what little I know of vampires, they're at the top, possibly with some other fantasy critters to go with them. Add that to Tim's genius answer, and you have an even smaller number of newborns.

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There is one point here that is pretty problematic.

1 Child per vampire couple, from a species perspective, is ultimately suicidal. From that perspective you are pretty much halving the population every generation since the one child is all there is to replace 2 adults down the road. That does not make any sense at all from a purely biological standpoint.

Taking care of that is fairly easy though. Say, for example, that it takes 20 years for the female vampire body to become fertile again. You can explain this in a whole host of ways, it doesn't really matter. Maybe the pregnancy is very tough on the female vamp and it takes that long for everything to heal. Then you add an additional condition. By the time the female has gotten to her third fertility window, she is also very close to menopause. That's where you get a third pregnancy being very risky. This could allow for a population that might very nearly become stable, as the next generation can replace the previous. It's very close to a precipitous balance that way. Lose a few females in a war situation and the population would be very slow to recover.

You guy's case for the purebloods to expand in to the outside world could be easily thwarted by the fact that the existing population is just barely maintaining equilibrium. It would not be expanding, creating high population pressure.

In addition, the Vamps would pretty much be Apex Predators, and will have to exist in an equilibrium with the prey species. If the human population is growing, the Vamp population will grow along with it, within the bounds of whatever fertility limits you put on them. If you get too many vamps, the prey species will whither. This could explain why there is a stigma against turned vampires. Turned vampires are competing for the food source.

Anyway, just things to think about.

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    $\begingroup$ I’ve been thinking on the issues with population sustainment that you and others brought up and wondered if making the gender ratio 3:1 in favor of women would help, like an evolutionary adaptation. I really don’t want to change the birthrate, because it goes along with the origin story of the race in a way; I might make second children being carried to term slightly less rare (depending on care), though I want the risk to the mother to remain the same. Do you think making the population predominantly female would help? $\endgroup$ – P. M. B. Oct 30 '19 at 2:22
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    $\begingroup$ @P.M.B. Only slightly. From a biological standpoint a species has to be able to replace itself from one generation to the next. it takes 2 to make a child, and therefore any given pairing should make 2. Anything else from a species perspective is suicide. $\endgroup$ – Paul TIKI -Monica come Home Oct 30 '19 at 13:34
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Here is a completely different approach. What is it about the bite of a vampire that turns a non vampire? Let us imagine that there is a chemical generated in their fangs that deposits a retrovirus into the blood stream of the victim, transforming their genome and reforming their body.

So what would happen if a vampire bit another vampire? Were that retrovirus to deposit into the blood of the victim vampire, coming as it does with a slightly different DNA or RNA sequence it might also inject into the genome of the victim vampire, but doing so on top of the changes that already made him/her a vampire, overwhelming the genome, and the normal process of transforming the victim into a vampire would be deadly were he/she already a vampire.

Now how does this relate to children? Imagine a female vampire breast feeding her infant. Neonatal vampire milk would have to constitute mainly blood products to sustain the pure blood child. And since this was the case it would be extremely hard to give the baby vampire nutrition in any other way.

Initially, he would have no teeth, but the fangs would quickly grow, and so if she was not EXTREMELY careful she would be in danger of her child biting her and injecting her with the toxic retrovirus.

Consequently, for a female vampire, every time she nursed her child she would be putting her life at serious risk. Her natural maternal instincts would drive her to take the risk, as human females have for millennia, however, the much higher risk of death would severely deplete the rate of childbearing.

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Well the answer to why species have certain birth rates is very complicated for real species.

One answer why the birthrate would be low is because of energy loss to metabolism in ecosystems.

There is a loss of energy at every stage of the trophic level ("food chain" level) approximately 10% of the energy remains at each stage. Essentially this is why there are so few big animals in the wild. There is only so much grass or other plant material in a given ecosystem. Then the herbivores (primary consumers) eat that and there can only be so many herbivores per ecosystem or the grass would run out and there would be a kill off. Then the secondary consumers eat the herbivores and there is only 10% of 10% of the original energy left. Tertiary consumers eat secondary consumers and they only get ~10% of 10% of 10% of the original energy input (sun energy usually). For this reason Quaternary consumers basically don't exist. A Quaternary consumer would have to be something that would eat an animal that eats animals that eats herbivores which eat grass (an example would be an animal that only eats lions or eagles or another tertiary consumer. How would it survive? In a given ecosystem there may be 100 lions and if this animal ate 1 a day or even a few days and the lions only gave birth once a year it would not be sustainable) there are good diagrams that explain this if you google trophic levels. So the lesson here is that long lifespans and low birthrates are the only way that Vampires could have possibly existed.

All species given enough time get tuned to their environment by evolution and natural selection or they are exterminated. Vampires would have evolved alongside humans as humans were required to survive. If at any point they over consumed all vampires would die. Instead they would have evolved to be tuned according to the rates of humans being born and consumed.

There are, in your case, the emergent property of social species which can affect birth rates though societal norms which is a whole different can of worms.

there are many many other reasons that it would have to be that way but I think those would eliminate most concerns.

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