Speculation on the subject of the dietary requirements of vampires must first deal with what the blood is actually being used for. Some fiction, including Elrod's novels, seems to assume that it is for circulatory purposes. I am inclined to doubt this, in view of the general agreement that hose who walk by night need not breathe. If the lungs are not being ventilated, the purpose of blood circulation to the tissues seems questionable at best.
Assuming that blood is being digested seems much more reasonable. It raises, however, energetic issues. Blood is an awkward material to make a living from. Vampire bats do this, but they have very little safety margin. A bat that cannot feed on one night stands a strong chance of starving to death before the next night. They also consume large fluid volumes relative to their own body size.
Assuming first a digestive system operating after the pattern of living animals, breaking down complex organic molecules to carbon dioxide and water, one can do an interesting theoretical investigation of the energy balances involved.
The recommended daily minimum caloric intake for a young adult male is asserted by the relevant government agencies to be in the neighborhood of 2800 nutritionists calories (kcals to a physicist or biologist) per day. This is alleged to be sufficient for minimal maintenance, not heavy labor. Like most such figures, the adult in question is assumed to be the stereotypical 70 kilograms, North American.
A mammal digesting protein obtains about 4.8 kcals from one gram (dry weight!) of protein. The constituents of blood are almost entirely proteinaceous, so it is convenient to use this value. To obtain 2800 kcals, then, necessitates the consumption of 583 grams dry protein per day.
I have been unable to obtain a lumped value for the total dry matter present in a given volume of mammalian blood, but a convenient reference tome (Altman and Dittmer; Biology Data Book) asserts that the concentration of hemoglobin in human blood is 150 dry grams per litre. Yes, litre. Because human blood cells do not accomplish much except hemoglobin packaging, it is not unreasonable to take the 150 g per litre figure as the total protein content.
The previous two paragraphs imply, then, that 583/150 =3.9 liters of blood are required to meet human-level metabolic requirements.
I have a considerable distaste, for the above calculation. At most generous estimate, a second and indubitably human adult will have a total blood volume approximating 10% of his body weight .. let us say seven kilograms. Seven litres, then. Mammals cannot survive the catastrophic loss of more than about 30-40 percent of their total blood volume, that being a maximum of 2.8 liters from the above 7 liter scenario. The loss of 20% of blood volume is almost invariably survivable, meaning that a loss of 1.4 litres would not be fatal, though the impact would be substantial and unpleasant. All this suggest that IF mammalian energetic constrain the obligately nocturnal readers of this list, they must be feeding on three people a night. (The annual number of unsolved murders in the country, high as it is, is not high enough to support the obvious possibility.)
A more reasonable (but not, I think, the most pleasing) suggestion would be that the energetic model to examine is not that of mammals, but of reptiles. The profligate habit of maintaining an elevated body temperature imposes a tremendous cost on those of us silly enough to do so. As a very rough approximation, a reptile has energy demands one tenth those of a bird or mammal of the same body size. This certainly brings the dietary requirements back to a more manageable level.
The shortcomings of this outlook are, of course, the same as the shortcomings of reptiles. Chemical reactions, including biochemical ones, proceed much more slowly at low temperatures. If the masters of the night are thus constrained, any of them trying to survive at high altitudes have a problem. They had best be exceedingly stealthy, for they couldn't outrun a toddler at 35 degrees Fahrenheit. Given the abundance of vampire legends from northern countries, this limitation seems unlikely. I do acknowledge the fondness of Ricean vampires for southerly climates, but they do not seem to suffer in extreme cold.
Both the mammalian and reptilian models suffer also from the fundamental assumptions arising from the metabolism of living cells. Neither mammals nor reptiles function in the absence of oxygen (and I need not be reminded about hibernating turtles - turtles cheat). Yet a near-universal agreement seems to hold: the undead need not breathe. We are not discussing organisms small enough to make simple diffusion a viable option, so I don't believe that any energetic system which demands oxygen as a terminal electron acceptor will suffice to explain the phenomenon.
Yet there is, perhaps, a better way. What of a much more fundamental energetic option?
Energy (in joules) = mass (in kilograms) * 2.997 exp 8 m/sYsquared ..
One gram of material, regardless of water content, yields 8.98x10-to-the-thirteenth joules of energy. This works out to about 2x10-to-the-tenth kcals. Obviously, only a minute fraction of the blood is used for this purpose, or the problem would be one of disposing of energy rather than acquiring it. If this suggestion has any validity, most of the ingested material must be used for structural maintenance and repair of the body, or excreted in some manner. I am inclined to suggest gaseous loss, primarily in the form of carbon dioxide, water, and nitrogen.
I will not propose a mechanism for this, as no living animal has yet worked out a way to pull it off at bio-compatible temperatures. Clearly, though, this out to be a matter of great interest to the cold-fusion crowd.
Summary
- A human-size organism functioning like a mammal needs to feed heavily and frequently for even minimal survival on blood.
- A human-sized organism functioning like a reptile would get by energetically, at the cost of putting up with typical reptilian shortcomings.
- Direct conversion of matter to energy (E=mc2) would resolve these problems handily.
The point of all this is not to pretend that the hunters of the night cannot exist - blatant folly in view of the number of them reading this - but to suggest that the biology of magic is a largely unexplored field.
As with more mundane human pathogens, the first major issue in addressing this problem is the establishment of a model system suitable for investigation. Thus I have a query for the, shall we say, more senior readers of this list .. Is your condition transmissible to small rodents, and if so, what containment procedures are sufficient to manage them? Vampiric rats might present certain unusual hazards for university animal maintenance personnel.
The overall problem of approaching vampirism biologically, as this possibly-myopic entity perceives it, is that of finding an invader capable of rebuilding its human host to do one of two things. It must either produce more energy from limited materials than living organisms can manage by chemical means (the cold fusion suggestion) or it must conserve energy so dramatically -- without compromising the strength and speed associated with the condition -- that the scant energy available from blood will suffice.
These problems invite speculation that something other than a living vampiric disease/parasite is involved. Time permitting, and if the net humors a pedantic peccary's speculations, I'll expound on the possible perter-biological underpinnings of contagious vampirism (referred to in "Night Out" as metabolic aberration) in a later post.
Author: Javelina