Darrell J. Pursiful

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Musings on the Ecology of Vampires

So I’ve been putting some ideas about vampires together for my next novel, tentatively titled Dead of Night. And as usual, I’ve gone way overboard with the worldbuilding. I’m curious how many vampires a given human population could support? The answer depends on how often they feed and how much they want to remain hidden from the world.

Murder Rates and the Masquerade

Let’s start with staying hidden. To do that, the vampire(s) would have to keep their kills below a certain threshold—or at least be diligent in hiding the bodies. Too many unexplained murders will raise alarms. So the first thing to consider is how many murders is too many, from the vampiric point of view?

To answer that question, we need to find the annual homicide rate in a given locale. In large US cities, the average is about 5.5 per 100,000, but there is great variation. In places like Baltimore or Memphis, the rate is much higher. In Provo, Utah it is much lower. But what matters isn’t how many murders take place in a given year but how many go unsolved. You can probably find a breakdown of this online, but the bottom line is that roughly one-third of all homicides remain unsolved in the US. Maybe we can arbitrarily say that a vampire population that feeds at only half of this rate—one-sixth of the local homicide rate—will be in an ideal situation to remain undetected.

Mind you, I’m not assuming that every last vampire kill gets investigated by the police. I expect most of the time vampires take pains to hide or destroy the bodies and perhaps use a number of other strategies to reduce their ecological footprint: drinking from living donors (willing or unwilling), feeding on animals, raiding blood banks, etc. Still, one-sixth of the local homicide rate at least gives me a place to start.

By the way, you can find crime statistics for the US at the FBI’s website. Here’s a link to the 2019 data arranged by metropolitan statistical area. Beware, however, that some metro areas did not report data this year. (Looking at you, New York and Chicago!) If you back a few years, though, you’ll find what you need.

This is going to be a very small number: smaller than seems to be the case in most vampire-related fiction. So we can imagine that most of the time, a vampire population exceed this ideal. The more vampires, the more likely one of them is going to make a mistake and break the masquerade beyond repair. My guess is that 25 times the ideal threshold is the point at which the jig is up. I have no convincing reason for this number; it just suits the needs of the story I’m spinning! It does, however, seem to produce reasonable and fictionally satisfying results.

With those two numbers in mind, we can describe a number of different scenarios for vampiric activity:

Ironclad Masquerade. At this threshold, the masquerade is virtually impenetrable. This represents the absolute safety threshold, where the vampire’s yearly kill rate falls below half of the total number of unsolved murders in a given area.

Strong Masquerade. This threshold is arbitrarily set about 3 times the ironclad threshold (cube root of 25). The masquerade is robust enough to handle an occasional misstep without completely unraveling. Spikes in vampiric activity can still upset the balance, however. Any alpha vampires in the area will take forceful measures to preserve the masquerade.

Moderate Masquerade. This threshold is arbitrarily set at about 8.5 times the ironclad threshold (square of the cube root of 25). The masquerade generally holds as long as everyone agrees not to do anything foolish. Mistakes are inevitably made, though, and there may be repercussions, especially if the vampires don’t have powerful connections, etc. Law enforcement officers and others in authority may know that something is amiss even if they don’t know what. Outsiders at best scratch the surface of the truth.

Weak Masquerade: This threshold is arbitrarily set at 25 times the ironclad threshold. At this level, the presence of vampires is definitely felt. Many locals are in denial about what is going on, but the authorities know that something is amiss and strongly suspect the paranormal. This is the situation that seems to be depicted in Sunnydale, California in Buffy the Vampire Slayer (though the Sunnydale numbers are beyond explanation!)

[Aside: Brian Thomas, a real-live ecological scientist has actually run the numbers for the Buffyverse and finds that ~36,000 humans [Sunnydale’s population is 38,500] per 18 vampires is actually a sustainable figure. His analysis doesn’t account for secrecy, however. On the African savanna, the antelopes know all about the dangers of lions; in Sunnydale, people may whisper about the creatures that go bump in the night, but few people know anything for sure.]

Broken Masquerade: Once the vampire population rises over 25 times the ironclad threshold, people definitely know that something sinister is afoot. This predator-to-prey ratio might describe, for example, a remote village that is actively being terrorized by one or more vampires, such as the peasants around Dracula’s castle in Transylvania.

Feeding Strategies

So now we can describe the thresholds of secrecy that a vampire population might cross. But how quickly do they cross them? Folklore and fiction imagine vampires feeding at different rates. We won’t know how many vampires can exist within a given “masquerade” scenario without considering how much blood they need to consume. Once again, we can describe a number of different scenarios based on what I call the VPV rate: the number of victims per vampire per year.

Parasitic Feeders (VPV <1). If a vampire needs a pint of blood per month, it can cultivate about three willing donors and none of them need die. (A healthy adult can safely give a pint of blood every two to three months.) More people means the vampire can live in secrecy in a larger population. At this feeding rate, even a very small population can sustain a single vampire. Whether or not it is discovered is more a matter of the creature’s intelligence than population size.

Light Feeders (VPV 2.5). Some vampires feed only once every four to six months and fall into a state of torpor between feedings. A single light-feeding vampire will be responsible for two or three mysterious deaths or missing persons per year, possibly an inconsequential loss in a large enough community.

Cold-blooded predators such as snakes or crocodiles often feed at this rate, and it is likely the same for at least some vampires. Note, however, that a vampire that is particularly active will feed more often and thus potentially draw more attention to itself.

Most vampire types that live in remote places and prey on lone travelers feed at this rate, and those that find themselves in sparsely populated areas can subsist at this level, at least in the short term.

Average Feeders (VPV 12). This rate splits the difference between “reptilian” model of the light feeders and the “mammalian” model of the heavy feeders described below, assuming vampires must feed about once per month. If more than one kind of vampire exist in a given locale, this rate is probably a fair estimate of the overall VPV level.

Most vampires that don’t fall into torpor between feedings but rather remain active all the time probably feed at this rate. It’s not a bad estimation for “generic” vampires.

Heavy Feeders (VPV 52). This rate assumes a vampire has to feed once per week or thereabouts. Large warm-blooded predators such as lions usually feed at this rate, gorging themselves and then resting between kills. Only the most voracious vampires feed at this rate.

Example: The Vampires of Detroit

Here’s how this might work out in practice. Since I grew up in southeastern Michigan, let’s use it the Detroit metro area as an example. According to the FBI data linked above, greater Detroit has a population of 4.32 million people and a homicide rate of 8.4 per 100,000. In absolute numbers, that’s nearly 363 homicides per year.

This lets us calculate an ironclad masquerade threshold at one-sixth of that number or about 60 vampire kills. Remember: this number represents half the number of unsolved homicides in the area.

Now we have to compare that number to the feeding rate of our vampires. Let’s keep it simple and say the “average” VPV of 12 is accurate. If so, then we can predict how many vampires will living in the greater Detroit area at the various population thresholds described above:

Up to 6 vampires can exist here with an ironclad masquerade
7–17 vampires can exist here with a strong masquerade
18–51 vampires can exist here with a moderate masquerade
52–151 vampires can exist here with a weak masquerade
152 or more vampires can exist here with a broken masquerade

If I wanted, at this point I could fiddle around with the VPV to fine-tune my results. For example, I could raise or lower the VPV to better reflect how vampires work in my (or anyone else’s) fictional world.

Anyway, does this sound reasonable to you? How would your own favorite vampire fiction mesh with my analysis? What variables might you shift?

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