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Sanguivores, also known as hemophages, are animals that consume blood. They include obligate sanguivores, like vampire bats, or facultative sanguivores, like the vampire finches from the Galapagos islands.

In this question, I would like to focus on vertebrate obligate sanguivores--organisms with internal skeletons including a backbone that feed exclusively on blood.

For some animals, blood makes a good food because it is widely available. However, blood is very far from being nature's perfect food. It has nutritionally negligible amounts of fat and way too much salt, creating problems with energy storage and kidney function for most warm-blooded vertebrates.

As far as I know, the only Earth creatures that fit this description are vampire bats, which are represented by a handful of species. Vampire bats are very small. One that I saw mounted in a museum had about a five-inch wingspan.

It occurs to me that larger creatures would forgo feeding on blood in favor of fattier and more nutritious meat. Blood would make a poor substitute for the meat that a tiger is big and strong enough to bring down. So, it stands to reason that obligate blood feeders would be small animals.

However, on an imaginary world inhabited by immense creatures, as big as titanosaurs, a "small" obligate blood feeder might be as big as a human adult.

With all of these considerations in mind, could our human-sized obligate sanguivore realistically evolve on my imaginary world?

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    $\begingroup$ Users have been asking "how big can X get?" on this stack for the better part of ten years. Technically, this appears to be a duplicate of Largest possible bat. Please take a moment to search the Stack and see what answers already exist. Use "is:q" to focus only on the questions. $\endgroup$
    – JBH
    Commented Apr 15, 2023 at 3:56
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    $\begingroup$ @JBH How is he asking for the largest bat?? Just because the only terrestrial obligate sanguivore vertebrate is a bat doesn't mean he's after a bat at all, let alone the largest one. $\endgroup$
    – Loren Pechtel
    Commented Apr 15, 2023 at 4:25
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    $\begingroup$ JBH: The question is not a duplicate of "Largest possible bat," since that question has nothing to do with blood feeding and the largest possible sanguivore would not necessarily be capable of flight. $\endgroup$
    – James Grossmann
    Commented Apr 16, 2023 at 1:12
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    $\begingroup$ @JBH This isn't a case of square-cube, though--in this case the limit is almost certainly in being able to find and obtain food. It's limited by it's prey long before it's limited biologically. $\endgroup$
    – Loren Pechtel
    Commented Apr 17, 2023 at 19:11
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    $\begingroup$ @KEY_ABRADE Yes, an organism can use blood-letting to kill a prey. However, it is unlikely to evolve to only use the blood if the prey is killed. There are so many other parts of the prey body that provide far more stored energy that almost every predatory animal uses the rest of the body of their prey. The whole point of predatory behavior is to get the stored energy that the prey has. Evolution favors eating the whole body. $\endgroup$
    – David R
    Commented Apr 18, 2023 at 16:10

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Largest possible creature is tough to answer. However, let's consider a made up blood sucking creature of about lion size, Smilodon Vampiris

A lion needs about 8000-9000 calories a day

500ml of blood, a typical donation, contains about 450-500 calories, so a lion would need to consume 16 litres of blood per day

A human survives fine losing 500ml from a roughly 4000ml of potential blood - we could probably push this if we only needed most donors to survive, so roughly 1/8th, possibly 1/6th of the blood in a given creatures body

So, your vampire lion can feed off

  1. horses, if it kills them, at 54 litres, or 6.7 litres for safe donation
  2. multiple people in a day
  3. elephants, at 254 litres, or 31 for a safe donation

Note, this is not likely to be a totally linear relationship - at a guess, elephants might be able to lose less, proportionately, of their blood than humans. Certainly, fainting would be worse for them.

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  • $\begingroup$ There is still the concern that a large animal cannot effectively digest an all-blood diet. $\endgroup$
    – Daron
    Commented Apr 15, 2023 at 9:37
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    $\begingroup$ Blood contains an awful lot of iron, a awful lot salt, a lot of protein, precious little carbohydrates, and very little fat. The size of the purely haematophagous animal is limited by the amount of fat in its diet, even assuming that it somehow manages to deal with the iron poisoning, the sky-high sodium level, and the protein poisoning. This is the problem which must be addressed. $\endgroup$
    – AlexP
    Commented Apr 15, 2023 at 15:04
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    $\begingroup$ Presumably. S. vampiris has evolved a digestive system that can handle a blood diet. Also a diet rich in iron and salt. I would consider those as givens, since the OP recognises their impact. $\endgroup$
    – elemtilas
    Commented Apr 16, 2023 at 3:33

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