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Since blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without great effort, it looks like hematophage animals would be above carnivores in the food chain.

Animal tissue (meat) has much higher energy content than plants but still needs to be broken down in proteins. Breaking down those protein through digestion costs some energy.

Blood already contains those broken down nutrients and it would seem that it s even more energy efficient than meat.

On our planet, only small animals are hematophage. I wonder if it s because it would be too difficult for a larger animal to constantly hunt bigger animals and drink their blood.

Therefore I m wondering if a human sized animal could entirely be hematophage.

Also would it be possible to modify our DNA to make our digestive system be able to digest blood? That would turn us into vampires!

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  • $\begingroup$ Human sized hematophage == vampire? $\endgroup$ – Mołot Nov 21 '16 at 15:48
  • $\begingroup$ Just for reference: en.wikipedia.org/wiki/Blood_as_food . Our actual digestive system can deal with cooked blood as well as with cooked meat. It seems that your imaginary world is not far away from our real world. $\endgroup$ – Pere Nov 23 '16 at 0:06
  • $\begingroup$ While blood may be highly "nutritious" (minus the human bodies inability to digest large quantities), the hematophage would have to remain in place "harvesting" for long periods of time in order to feed. This would make them highly susceptible to being attacked by a carnivore. A hematophage from living organisms would therefore NOT be an apex predator, unless it killed its prey first, in which case it's no different than an omnivore that digests blood. $\endgroup$ – Isaac Kotlicky Nov 23 '16 at 12:10
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I recently saw an interview with Neil Degrasse Tyson where he mention the plausibility of adding every metabolic advantage that exist in the animal kingdom to humans. We all stem from the same evolutionary tree, so we are "compatible" in a genetic level. If there are enzymes and intestinal processes that allow animals to digest blood, then it should be possible to add them to humans. The problem would be for a grown individual to acquire enough blood to feed each day. And that blood to have all the nutrients needed in sufficient quantities, otherwise the vampire would suffer calcium insufficiencies and such. It would make more sense for a humanoid to modify their metabolism to digest as much kinds of food as possible. That way some days you'll only need to extract a litter of blood from a cow if you don't want to kill it for it's meat yet.

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    $\begingroup$ I'd never considered the fact that Vampires could get rickets and scurvy from feasting on humans with a bad diet. That's hilarious. $\endgroup$ – Ieuan Stanley Nov 21 '16 at 16:42
  • $\begingroup$ Not only that, the blood itself doesn't usually carry all the nutrients all the time. The vampire would need to feed on it's victim shortly after it has eaten one of its meals, other wise the blood would already have taken most of the calcium and vitamins to the bones and organs. Commons vampire maladies would be scurvy, osteoporosis and inherited high cholesterol. $\endgroup$ – ThreeLifes Nov 23 '16 at 0:07
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Blood may be more efficient to digest than meat, but it is also in lower supply, which is a problem especially for the required calorie intake.

I wrote a rather detailed answer here explaining how humans could be biologically forced to vampiristic behavior (basically, by disenabling them from absorbing iron from plants and making them allergic to meat which, contradictory as it sounds, is possible (see link) thus forcing them to get it from blood. They still have to complement their diet with plants to get calories and vitamin C which blood doesn't give them enough of).

The reason why only small animals are hematophage is probably, as mentioned above, that there simply isn't enough blood to hunt for bigger animals.

In regards to the ability to digest blood we have to look at every component singularly. Our digestive system can already absorb all the nutrients in it, but not absorb any of its proteins (which makes it impossible to absorb hemoglobin, like the question I linked proposed). The ability to do so would require very big changes to our digestive system and would be impossible to achieve by genetic manipulation, if that's what you asked.

What can be done to facilitate a blood-based diet is developing more efficient mechanisms to dispose of iron and sodium, of which there is a relatively high amount in blood if they really have a blood-only diet, which would require consumptions of more than 3 liters of blood every day.

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  • $\begingroup$ The assertion that blood does not contain calories seems a bit odd to me. I searched around a bit and what I found (though not reputable sources) seems to contradict that. Where did you gat that information from? The caloric counts mentioned seem to be between 750 calories per liter (calf's blood) and around 900 calories per liter (human blood). $\endgroup$ – Second to Last Unicorn Nov 21 '16 at 16:38
  • $\begingroup$ @SecondtoLastUnicorn You're right, that's not quite accurate. What I meant to say is that it's impractical to have one's only calorie intake be human blood because one would be forced to drink upwards of 3 liters of blood per day. It's explained better on the answer I linked. I'll edit this one to remove the inaccuracy. $\endgroup$ – Annonymus Nov 21 '16 at 18:15
  • $\begingroup$ Thanks for the answer! I just kept wondering "well how do ticks and leeches get along then?". $\endgroup$ – Second to Last Unicorn Nov 21 '16 at 19:37
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NO

Blood is actually one of the least nutritious components of the entire body. It's 92% water by volume, with only 8% composed of "stuff". By contrast meat is only 75% water by volume. In contrast to meat blood also lacks high-density nutrient components such as fat.

As an example, consider one of the only hematophagous vertebrates, the vampire bat. Blood is such a poor food source that vampire bats have to feed every other night to avoid starvation, and if they fail to find a cow or a capybara to feed on they have specialized behaviors where they beg for food from another member of their flock to avoid starving to death. If they don't feed within 48 hours they die. On top of that, blood is so poor in nutrients vampire bats cannot physically get enough blood into their bodies to sustain themselves. They have to eat half their weight in blood every night to survive. In order to do this they urinate all over themselves as they feed, trying to remove as much liquid water from their meal as possible before they take off.

This is bad enough, but now scale this up to the size of a human being. Someone actually crunched the numbers and found that a human-sized vampire would need 55 liters of blood a day, or in other words 10 peoples worth of blood per day, in order to get enough energy to survive. Notably, this link's value is probably wrong, because it doesn't take into account the square-cube law and doesn't take into account the fact that basal metabolic rate scales to the 0.75 power of body mass. The actual value is likely higher. The human metabolic rate is also twice what is predicted based on body mass, likely due to our large, energy-hungry brains. I have no idea if vampire bat metabolism is high or low relative to their body size, though there is some evidence they have adaptations to fasting.

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Due to how nutrient-poor blood is, being mostly water, I believe that such an adaptation would require a world with more megafauna. If we go with blood that has a glucose concentration of 1.30 g/L, and glucose has an specific energy of 15.6 kJ/g, that means that each liter of blood contains 20.2 kJ/L, or 4.83 kcal/L. Thus, the bloodsucker would need 414 L of blood a day, which is equivalent to 82.8 human bodies' worth of blood. Assuming that the megafauna's adults have a blood concentration identical to human blood, they would need about 414 L of blood each for the bloodsuckers to survive via draining one completely dry a day.

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  • $\begingroup$ An elephant only has 450 L of blood in its entire body. You would need huge numbers of megafauna. $\endgroup$ – user2352714 Jan 18 at 5:35
  • $\begingroup$ Yeah, and that can work by making the planet larger than Earth, and superhabitable, with conditions that support the evolution of megafauna. And probably these creatures have a higher glucose concentration in their blood, so that a lower amount of blood will sustain the bloodsucker for a day. $\endgroup$ – TysonDennis Jan 18 at 15:01

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