A quick point before we go any further: throughout this article and the next, I’m going to be using the term “vampire.” However, what we’re going to talk about can be applied to werewolves, ghouls, or just about anything else that eats people or at least kills them on a regular basis for some reason.
In order to adapt these principles you might not have to do anything more than change the names I’m using so that instead of vampires I’m talking about “draegathi wurm-walkers” or whatever you have in your story. Or you might have to make a few more calculations on your own. If you want to figure out what’s going on with werewolves, for example, then you might need to figure out how many people are killed specifically during the full moon, or how many calories a werewolf needs to consume in order to offset the massive energy expenditure that comes from shapeshifting— perhaps a werewolf who doesn’t eat a few people every month, or the equivalent, will starve to death.
In order to start figuring out how many vampires, goblins, or wendigo wizards your world can support, you have to figure out (1) how many people are disappearing or being murdered to begin with and (2) the dietary needs of your monsters. The first one isn’t as necessary if the story isn’t set in our world but the figures are still good to look at. As one poster on /tg/ stated, “There’s a point where it goes from ‘business as usual’ to ‘Oh there’s a lot of crazy out there’ to ‘Holy shit, please not me tonight please not me tonight…’”
How Many People Go Missing Each Year?
There are a few ways that you can go about determining the population level of your vampires. If they kill in the process of feeding, then one thing that you might want to do is take a look at Missing Persons reports and work backward from there.
From Crime Library we learn that 2,300 missing person reports are filed every day. However, most of these are eventually resolved. Todd Matthews, from the National Missing and Unidentified Persons System, described how there were 661,000 cases filed in 2012, but 659,000 were resolved one way or another (and “very quickly,” Matthews adds), leaving 2,079 cases unresolved. Half of the juvenile disappearances, going back to Crime Library, involve runaways and another quarter are abductions by family members. “Only about 100 missing-child reports each year fit the profile of a stereotypical abduction by a stranger or vague acquaintance,” to quote from Crime Library. This doesn’t tell us very much about the statistics for adults, who comprise 44.6-56.1% of all missing persons (depending on whether you include “juveniles between the ages of 18 and 20”).
Narrowing the numbers down any more from here is pretty difficult. Let me know if you can find statistics on the number of people who are found actually alive, rather than just dead, because I haven’t been able to. If we want to take the 2,079 as a rough estimate, however, assuming that some of those are actually conventional kidnappings but also that some discovered corpses are vampire-related, then the next step is deciding how often vampires have to feed. Feeding once a week will allow a population of about forty vampires, while feeding daily means that the United States can support about five vampires.
You can add unsolved murders to boost the population. According to an article from 8 News Now, up to 6,000 murders go unsolved each year, about 47% of all homicide cases. Keep in mind, however, that “cities with a large number of drug or gang-related murders tend to also have lower solve rates,” which means that you can’t pin all the deaths on vampires.
Because I don’t have the numbers on how many solved missing persons cases turn up a dead body, I can’t give a firm upper bound on the number of vampires that you could possibly have for any given rate of feeding. I hope, though, that this will give you some sort of idea.
How Nutritious Is Blood?
Coming up with a random number for how often a vampire has to feed can be unsatisfactory, however. You might have to do it if, say, your vampires feed off “death energies” or something like that, but if you’re willing to deal in calories then you can attach your numbers to something concrete.
Liang-Hai Sie, a former intensive care physician, calculated that there were 9 calories per ounce, basing this on a calculation of 6-8 grams of protein per deciliter. David Mark goes on to give a more complete accounting. You can check out his post “Calories in Human Blood” but what it comes down to is that a 500 ml donation of blood holds “roughly 450 calories,” although it actually comes down to 425 calories in female blood and 460 in male blood. Diabetics would have a slightly higher count, with more sugar in the blood, but the amount is so negligible that they are unlikely to be sought out preferentially. Males, on the other hand, clearly would be.
What happens if you lose this or that amount of blood? A Class I hemorrhage (up to 15% blood loss) doesn’t do much to you. According to “The body’s response to blood loss,” from the journal Vox Sanguinis, “Minimal physiological changes occur at this level. A patient may exhibit mild anxiety, but heart rate, blood pressure and peripheral circulation largely remain unchanged. Urine output is only slightly decreased. The body can compensate well for this degree of haemorrhage. This situation is mimicked by a blood donation.”
In a Class II (15-30% blood loss), visible symptoms include paleness in the skin, coolness in the extremities, an increase in heart rate, and a decrease in blood pressure. Simply replacing the lost blood with a saline solution will alleviate this. In a Class III (30-40% blood loss) a transfusion is necessary, and in a Class IV hemorrhage “the situation is immediately life-threatening and within 15 min a mortality rate of 50% is to be expected.”
It seems unlikely that a vampire would regularly feed enough to cause a Class III but go no further, since the end result would be the same. Most likely, vampires either limit themselves to inflicting no more than 30% blood loss or go all the way. An average male weighing 150 to 180 pounds holds 1.2 to 1.5 gallons of blood, which translates to 9.085-11.356 servings of 500 ml each. This means that a single adult male can provide between 4,179 and 5,223.76 calories from his blood. Draining 500 mil from the same individual, as mentioned before, would 460 calories, and draining 250 mil (in order to make symptoms almost unnoticeable) would provide 230 calories.
However: What if vampires did more than drink your blood? For the most extreme example, let’s take the idea I had awhile back of vampires who fed by liquefying their victims’ insides like spiders, leaving behind only the skin (and maybe bones, I’m not sure yet). According to the research of James Cole, who was interested in shedding light on prehistoric cannibalism, a body weighing 64.9 kg (143 pounds) contains 81,500 calories. In his breakdown Cole states that the skin provides 8,294 of these calories (which means that my spider vampires wouldn’t have access to that). Looking elsewhere on the web garners estimates of between 77,000 and 110,000 calories. These are laypersons drawing calculations from the armchair, though, and sometimes they start by taking the caloric value for other species, so we should be safe in taking Cole’s conclusions over any others.
As we find in working from missing person reports, there are some vague areas. Cole could not get precise breakdowns on some areas of the body, the paper tells me nothing about how many calories my spider vampires would get if they dissolved the bones too, and the numbers are for a 143lb male, not one that is 170lb or 200lb or what-have-you.
How Many Calories Does a Vampire Burn?
According to the Dietary Guidelines for Americans 2010 report, a moderately active female aged 19-30 needs 2,000-2,200 calories per day. A male of the same age range and activity level needs 2,600-2,800 calories. If they are sedentary, these figures work out to 1,800-2,000 and 2,400-2,600 calories respectively. Assuming a moderately active vampire and averaging the numbers, we find that a vampire needs 2,400 calories per day. We will call this the standard living type when we come back to it.
What if vampires, as might befit the undead, don’t need to burn calories simply to stay alive but only expend energy in order to move? This seems plausible enough, if you want to do it that way. If it is true, then maybe we should subtract the numbers for the sedentary lifestyle from those for the moderately active lifestyle and keep the difference in order to get a rough idea of how many calories would be expended from moving around. In that case we go down to 200 calories per day. If we substitute the average for active lifestyles (2,700 calories, from 2,400 for females and 3,000 for males) then we get 500 calories. We’ll call these the standard dead type and active dead type respectively.
Before we move on, though: wolves burn up to 70% more calories than other predators of their size. Whether vampires are unusually active (for example, in stories where they are exceptionally fast) or have some other reason for burning additional calories, let’s use this as the basis for a kind of vampire that burns more energy that we would expect: the wolf type burns 4,080 calories.
Next month we’ll go into what these numbers mean for feeding behavior, maximum sustainable populations, and so on.
Links to some of the material mentioned above:
· Tinyurl.com/bloodcalories: “Calories in Human Blood” by Mark David
· Tinyurl.com/mablood: “The body’s response to blood loss” by M. A. Garrioch.
· Tinyurl.com/cannibalessay: “Prehistoric Cannibalism: An Act of Nutritional Necessity or a Result of Socio-Cultural Conditions?” by James Cole.
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