What is a Prion?
Links to Stuff
Population Dynamics of a Supernatural Human Predator
With special emphasis on its ecology in the United States specifically
(The classic examples being werewolves and vampires)
WARNING: This is a thought experiment of the most random, morbid, and potentially creepy kind. Do not read if you are easily offended, or if the callous discussion of human death bothers you (for the record, I'm not a murderer, just a scientist)
PART 0: Define Scope of Thought Experiment
A human-predating creature would have to be an intelligent, cunning, and wary creature, capable of pattern recognition and able to learn quickly, because the first revelation of the existence of its kind could provoke a technology-and-paranoia-fueled rampage by its erstwhile prey. In this case we will be dealing with something from a fantasy-horror film, where the predator has limitations and must move cautiously to avoid disruption of the activities of its kind. Anything natural attempting to fulfill this role would have suffered exposure long ago, and probably would have driven the course of human evolution, so some supernatural qualities will be assumed (in this case, taking human shape will be one of them, though by no means necessary in the general case).
I should also clarify I will be dealing with predictable and limited predators, rather than omnipotent predators that could rearrange the thoughts of humans or are impossible to detect even with a good set of IR goggles or a hearing aid amped up to setting 11. Such a thing would be outside the scope because either it would move with impunity, it would see no need to increase its population to carrying capacity, or it would wipe out the human race like the roaches we are.
With that, I begin.
PART 1: Define Dietary Needs
How much would one predator have to eat to stay healthy? Who/what would they eat? Could the predator subsist on nonhumans? I will address three cases that should hopefully be illustrative of a range of predators: Werewolves (obligate killers), Lone Vampires (obligate feeders), and Dominating Vampires (nonobligate feeders).
First, how much meat is on a human? The average American male between the ages of 20 and 74 was 191 lbs (between 1999 and 2002), while the average female was 164 lbs. A supernatural hunter might have preferences, but we can run our stats on a typical person of 180 lbs and quibble about the particular preferences later. A human male is 40-50% muscle by mass, making him up to 95 pounds of meat, while a female is 30-40% muscle, making her up to 65 pounds of muscle. Fat and organs may or may not constitute additional delicacies, but the large, accessible meat concentrations are probably the "best" part of humans (the muscles of the limbs and abdominal cavity, and of course the delicious and nutritious liver).
Given all this information, let us suppose that an adult provides on the average 60 pounds of food, mostly meat, for an undisturbed but non-tool-using predator (tools could hypothetically permit a far higher yield of food). The human body is approximately 7% blood by weight, meaning around 12.5 lbs of blood in a person, since that will matter too (and probably makes up a small portion of the food yield of a person).
How much sustenance is that? Cooked animal meat seems to run between 200 and 400 calories per 100 grams, but raw meat seems to have fewer calories (cooking burns off a lot of water and grease, reducing the mass more than calories) so let's suppose human meat is 150 calories per 100g (for a middle of the road approach). Fat turns out to have a lot more calories than muscle (probably 300 calories per 100g), but the percent body fat of Americans varies wildly (average percent is probably around 20%, though the internet is even more unreliable than usual on such sensitive topics). That makes 1.5*(48lbs * 454g/1lbs) + 3*(12lbs * 454g/1lb)= approx 50,000 calories. If you just went WTF! remember that most predators make a kill on a weekly basis and gorge themselves so they're probably playing with calorie values like this on a regular basis as well (though even a male lion doesn't eat an entire zebra all by itself). Take-home message: a single human, properly killed and thoroughly eaten fresh, could feed five people for a week. Or a pride of lions in one short sitting.
What about the blood? Well, blood is over 50% water, and the red blood cells aren't known for their high protein or DNA content, so living off blood is probably 100% supernatural (more on that later).
What determines how much human is enough? Well let's suppose that Exhibit A (Werewolf, Sasquach, Chupacabra, Amarok, other cryptids), the obligate predator, has a simple daily dietary calorie requirement. An extremely active 7-foot tall 500-pound human would need about 6000 calories a day to remain fit and hunting (assuming quite blithely that the caloric intake calculators from the internet can extrapolate that far). So a solitary hunting werewolf would need 50,000/6000 = one kill every 8 days or so. Once a week is a lot, but probably sustainable for a grown adult. It would be a lot more calories to also grow, feed young, etc, plus consider that energy storage is not so efficient that one meal will literally sustain someone for an entire week without even snacking. A pack of three grown werewolves would need to make a kill every 3 days, which would be a lot easier with three but would also present a more challenging demand on the population.
For the record, lionesses require about 11 pounds of meat a day, a number which is actually not too far off the 6000-calorie number out of the water (around 8.5 pounds). Lions can also eat up to 60 pounds of food in one sitting, which would be a meal for the week. This suggests that a human-targetting predator might indeed need a kill a week and an undisturbed couple of hours to eat the entire body, much as a lion would. A lion would need a human a week to live, a human predator could probably go with less but arguably not too much less.
The implications of burning calories to effect a transformation (say, from a human form into a wolf or wolf-human hybrid form) will not be discussed, as the caloric demand on a body of a supernatural activity is undefined in the literature.
As previously mentioned, a vampire is not assumed to require caloric intake from blood, as it's a bit low in real calories. Presumably, they need it either for entirely supernatural sustenance, or they are incapable of producing hemoglobin but require it. If the latter is the case, a Vampire will need to drink an amount of blood equal to the amount a human normally goes through. Since the average lifespan of a red blood cell is supposedly about 120 days, making the average age of blood in a person 60 days old (assuming they haven't donated blood or been drunk from recently), a vampire will need to drink an entire human's worth of blood (7% of 180 lbs is 12.5 lbs) every 60 days, which is 91 grams per day, and with a density of 1.06 that makes 86 cc's per day, or 1 liter every 12 days or so (yes I'm rounding a lot, once you start there's no point in stopping). In theory a vampire could drain a person bone-dry once every 60 days, but a vampire could also take sips (probably from junkies, criminals, and others unlikely to notice a brand new mystery scar) without killing anyone. 86 cc's is not a lot of blood, the actual blood loss would probably not be noticed at all (though the wounds might).
PART 2: Define Dietary Activity
The purpose of this paper has been to explore the biology of something that is preying on humans. The obligate predators from group A have already put themselves in a bit of a pickle by requiring a dead adult human a week. This problem is amplified by the fact that the easiest prey are the young, the old, and the poor, none of which are known for having meat on their bones. Regularly preying on fully grown adults, especially muscled and/or fat ones with more sustenance, would be very hard (see part 3 for more). One way to improve survival and reduce risk of discovery is to maintain farming.
Assuming these predators need meat, as opposed to human meat, allows for the large-scale farming of animal livestock. An adult cow can weigh over half a ton, but typically provides somewhere in the region of 600 pounds of good meat. At 10 times the volume of a person in meat, the whole domestication aspect, and the normality of the sight of caged cows, the cow would make an ideal farmed food. A werewolf could subsist on a single cow for almost three months, and killing 4-5 cows a year is quite manageable in a large population of cows. Only one breeding bull would be needed, but if a group A member is eating 5 cows a year, 5 cows need to be born a year, which means at least 5 replacement females in a herd per predator (probably more, possibly up to 10 to avoid being hosed when stuff happens).
If human meat is needed, a human farm is theoretically possible, but humans take a lot longer to grow up than cows (20 years versus 2). A human farm would also be a huge expense, especially if the humans in captivity have been taken as children and raised in innocence of their ultimate fate (getting humans to breed, while typically easy, may be very hard if they have been captured as adults and are largely focused on escape and/or prison rioting). Also, since a werewolf may eat 50 humans in a year, a population would need upwards of 1000 breeding females (breeding females not for food and having a baby every year) per werewolf. Feeding 5000 hungry people may be an easy job for Jesus, but for werewolves, the expense, even of bread and water, could be crippling to the project.
Vampires would probably have better luck with farms. Cows produce a lot of blood and would be easily farmed, along with any other large domesticated animal. The movie Blade Trinity briefly explored the thought of capturing humans and keeping them comatose while feeding them intravenously and draining their blood regularly. One person per vampire would be the minimum requirement, though more would be better and probably safer. Since covens of vampires are often portrayed as being civilized, rich, and ruthless, there is little reason to believe this approach would not work with modern technology. There is no reason to believe it would be the only approach, however, as vampires could easily be considered as fractious, flaky hunt-lovers unwilling to put the effort into farming their blood.
PART 3: Define Ecological Pressures
Aside from the previously stated issue of requiring total societal concealment, there are many ecological pressures on feeding on humans.
For group A, the pressure is extreme. Committing and getting away with murder on a weekly basis is quite difficult. If the CSI TV series are to be believed, it always gets figured out and caught sooner or later. So where are these werewolves going to be (assuming they have not moved to Iowa and started farming cows)?
First, they would have to mask their feedings behind the crime rate of their location. This necessitates a certain population density. While random murders out in central Nevada might be a lot harder to investigate, the National Guard will get called in when there's two killings a week for months straight. Those sorts of things just don't go unnoticed among lower population areas where everyone knows everyone else.
Second, they would also have to be closer to cities because the best targets, homeless people and criminals, tend to also congregate in cities.
I took a data set from the FBI (via wikipedia) and sorted the cities by per capita murders, but many of the winners were small cities with unusually high numbers of murders (say 8). Such cases probably get more attention, and so I created a new ranking, the number of murders multiplied by the logarithm of the population. The winners are cities with per capital murder rates between .01% and .05%, but these translate into hundreds of murders (New York, the winner with the adjusted rank, had 496 murders in 2007). If a population of werewolves wanted to live somewhere, New York itself could probably support a family of 5 (assuming 50 kills a year each), assuming half of all murders were werewolf-instigated. Still a somewhat small number, assuming these werewolves wanted to keep a permanently low profile.
The carrying capacity of predation of New York city is, of course, much higher than 5. Conceivably, an entire pack of 20 werewolves could live exclusively off humans, pushing the murder rate up to 1000/year, double what it is now, and this is ignoring the propensity for humans to kill each other. While not every murder would be reported as a murder, with most probably merely registering as a disappearance or abduction, people simply not coming home tends to provoke manhunts and Amber alerts. Cities would seem even more lawless than they are now, even if these werewolves were performing a public service by only killing criminals, prostitutes, the homeless, and each other. Additionally, only a city with a population about 100,000 could realistically support one or more predators, since a murder rate higher than 0.05% seems to attract a lot of attention. New Orleans's murder rate is nearly twice that, but the city has still not recovered from Hurricane Katrina, and it is the only city above 100,000 to have any number near that.
If we suppose every werewolf in the US plays by the rules, killing 50 humans a year and making them untraceable but also not alerting anyone besides a few FBI wonks, then there are about 300 cities that could support one or more predators. By assuming one werewolf per 95,000 people, the total carrying capacity of US cities is 788, though this neglects the suburbs that ring many large cities and could provide additional food. It is easy enough to imagine that a city like Santa Clara, CA which can support one werewolf with its population of approx. 100,000, could probably also support such a werewolf's child or visitor based on the available suburbs around (though callous murders in the suburbs do attract more attention as well). It also estimates a carrying capacity of 86 werewolves in New York, which would make it a horrible nest of murder and villainy in the public perception.
Additionally, a werewolf or three could subsist on humans throughout rural areas by taking road trips, but they would need to change vans frequently, as the FBI would quickly catch on to reports of grisly murders occurring across state lines. This road tripping would probably not be a good lifestyle choice, making it more of a vacation or business trip than a means of breaking through the carrying capacity limits.
Werewolves could also theoretically live on the edge of large state parks where wolves have been reported, preying on lone hikers and leaving their corpses as a wolf or bear would. This might make a more stable lifestyle, though there would be no room for competition and there are probably not enough tourists in a year to anywhere, even Yellowstone, to make 50 disappearances a year seem normal.
Of course, this is only for the US, where the standard of living is high and the murder rates are low. Mexico, for example, is much more difficult territory to live in, and Mexico City has at least 20 million inhabitants (carrying capacity of 200 or even an order of magnitude higher if abduction and murder is not properly investigated). Of course, if werewolves were real, right now they'd all be over in Iraq feeding as much as they want.
If werewolves were to maintain a population long-term, only the most major cities would be suitable for courtship and mating, unless the population was well below capacity (which in turn would impact success of finding a mate at all). Werewolves would be more likely to immigrate to the US from undeveloped countries without censuses or rigorous law enforcement. Large swaths of Africa, chaotic regions of Eastern Europe and Southeast Asia, and unstable regions of South America could probably house thousands of werewolves or werehyenas as a base population, acting as a source population to other regions of the world.
Vampires from group B (Lone Hunters, not to say obligately solitary but rather not especially social) would have a much easier time in the USA. Based on blood volume, a vampire would only need to make a kill every 2 months or so if it had the ability to drain its victim dry (if it could only utilize the blood until the heart stopped, as in the Anne Rice film, a compensatory increase in the number of victims must be applied, for example a kill every month). 6 murders per year is much easier to conceal than 50, with an end result that not only can the same city support an order of magnitude more vampires than werewolves, but a killing vampire is able to live in many more locations than a werewolf. If a vampire needs only 10,000 people in an area to keep its murders off the front page newspapers, then the carrying capacity of the US is approximately 15,000. However, most of these smaller cities have murder rates much closer to 0.01% or less (10,000 people is not very many people). Such a vampire would probably require up to 50,000 people in an area to cover its tracks, bringing the number of killing vampires to a more realistic 2,000. It is still many more vampires than werewolves.
Lone Vampires must still make a kill every time they feed, unless they are keeping humans as chattel. Human chattel is a much more functional prospect for vampires than werewolves. For starters, the vampires would not need the humans to breed, and could repeatedly drain them without killing them. Letting live humans go without killing them, as previously mentioned, would only work if the humans were fed upon in small amounts while they slept. This would increase the population maximum somewhat in crowded cities, feeding off people for whom scar tallying may not be the highest priority and do not live behind a lot of security, but the increase would not be very drastic, since every encounter is a risk of detection, and minimizing this risk is probably the best route. Also, with many stories of vampires every bite is a kill or turning, removing this aspect from the equation.
The third type of predator I mentioned, Dominating Vampires, are the sort you see in Blade, than can enslave humans to their will, either with supernatural power or through societal power. The latter is naturally limited to how effectively you can get people to believe that being fed upon is the right thing to do, and is wildly open-ended. The former is as limited as the power and the will of the vampires involved, but could potentially remove all barriers for population. If vampires can feed off their thralls/familiars/slaves, they need only one or two for subsistence existing (unless their supernatural powers are powered by blood, necessitating a larger stock). This can effectively raise the cap of vampire population to the point where the other potential limiting factors become involved, including territoriality, rate of acquisition, discovery evasion, and any residual morality in the individual newly turned vampires. Theoretically, the entirety of the USA could become vampires and their thralls (a condition which is referred to at my alma mater as "Happyville loses"), with 100 million vampires and 200 million thralls to go with them.
Any fiction involving Dominating Vampires where most of the US is free and ignorant human is demonstrating one of the limiting effects on the vampire population. These limiting factors may include but are not limited to: a maximum number of thralls a vampire could hold, territoriality, general lack of interest in procreating, high standard for thralling as well as turning, high lethality of feeding, and a need to maintain secrecy. The last could easily be resolved by turning key members of the FBI and police, but these are among the most trained, wary, and often intelligent of humans, and no easy prey. If turning is highly lethal, a rash of FBI officer killings will attract the attention not only of the FBI, but of the CIA, NSA, secret service, and major newspapers. Insertion into the US government machine would be playing with fire unless the vampires have the most favorable powers available.
PART 4: Conclusion
In conclusion, I have investigated the dietary needs of both a predator that eats human flesh and vampires that feed on human blood, either for hemoglobin replacement or for supernatural sustenance. I have also briefly outlined alternate routes if human products are not mandatory consumption. Lastly I have illustrated the ecological pressures on a population of human predators, finding that feeding on human flesh is unsustainable here in the US, though it is more applicable in lower-world countries.