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In "The Thousand and One Nights", the ghouls were vile tricksters and ravenous flesh eaters. They kidnapped victims and lured lustful men to their doom by taking the guise of beautiful women. Sometimes they even snuck into storerooms and munched on dates. The Arabic texts apparently did not identify them as grave robbers who dined on the dead. So I guess they won't turn down a living meal if the situation presents itself.

A panda's diet is 99% comprised of bamboo. But bamboo doesn't have a lot of nutritional value, so pandas have to eat 26 to 83 pounds of bamboo a day. I'm not an expert on biology but does that mean ghouls need to eat a lot of humans if their digestive system can only breaks down one type of food?

Pandas are from the animal order Carnivora, though, and so they occasionally will eat small rodents. I imagine this is true for ghouls too.

My question is: if ghouls are able to derive nutrients from flesh other than humans, why would they prefer to hunt and feed exclusively on humans? From my point of view, the ghouls risk being exposed if a lot of humans go missing.

Prions

It's known that there are certain diseases that are caused by prions (misfolded proteins, largely specific to the species) and target the same species. (Some, such as BSE, a.k.a. "Mad Cow Disease" can affect other species though)

When a ghoul eats a human, the prions, instead of causing illness, are used like a catalyst or vitamin - they are necessary for the ghoul to remain healthy, instead of turning into a shambling corpse. (Think scurvy, Vitamin D deficiency, pernicious anemia, or even diabetes)

(Also, prions are often found in high concentrations in the brain, so the answer also applies to zombies)

Humans are currently one of the most numerous large animals on Earth, and furthermore tend to concentrate in dense groups. For a creature that lives in cities and is able to kill humans easily, effectively and without significant danger of retaliation, humans will be the best source of meat. This would not have been a viable niche in prehistoric times, but it is now. People go missing all the time, especially in big cities.

Maybe in the far future, in a dystopian setting where population is high, food is scarce, and the value of individual lives are low, a subgroup of urban humans might adopt cannibalism as a major source of nutrition and evolve into a new species, growing better at snatching people off the street. Or perhaps they have been living alongside us in secret, having split off sometime since the growth of civilization.

If a species is eating one specific type of food, their digestive systems have evolved to become adapted to extract the most nourishment from it and to be resistant to eventual poisons. This also likely causes their tasting buds to "like" that type of food, thus driving them instinctively towards the food they are best adapted to. One distinctive feature might be the nourishment human brains yield to them.

Plus they have the skill sets required, as described in the question, to hunt for humans, so why should they forsake their nature to go against it? Morality could be a reason, however Nietzsche may disagree:

He tells us that “man needs to supplement reality by an ideal world of
his own creation.” That is, we are compelled by our biological natures
to see the world through moral lenses, judging it in terms of good and
bad, although the world is neither in itself.

Which could mean that ghouls would not see killing and eating humans as amoral, just as we don't see much amorality in eating cattle even though we could resort to eat stuff which is already dead or about to die like fallen fruit or animal corpses. Why should ghouls eat things which are less nutritious and tasty to them, which may be the things allowing them to reach their peak healthiness?

Another factor could be danger. However, that's something all species have to deal with anyway, especially carnivores. What if they are better off if continuing to take that risk, because doing what humans do simply does not cut it well enough (at least not up to ~2 centuries ago), or if they are intellectually-culturally unable to even consider that? It's not like humans consistently enact the most moral and best systems imaginable, so why would you expect them to step over their shadows in such regards - even if they are capable of thinking as humans?

There are plenty of arguments against this notion, but relations based on violence and coercion (parasitic or exploitative) usually do not argue in fair terms, nor do they benefit from doing so. The predator will always seek for things that makes his life easier, or whatever is needed to adapt. It is likely it will never even come to that consideration.

I may have a, let's say, dark undertone in this, but I guess you are good to go with ghouls preying only on humans, not only on a biological standpoint, but also on social, moral and intercultural. Maybe it can assist as fuel for explanation from the side of ghouls.

Humans are yummy

They prefer the taste of human flesh over the flesh of rodents that crawl around everywhere. Especially when munching on humans that are well off and can therefore afford to be clean(er) most of the time compared with other animals. That would also explain certain preferences when it comes to humans - the more well off someone is, the higher the chance that they will be eaten.

The strategy to disguise as women to find some men would also fit when those men were rich and therefore cleaner, making them more yummy. More work for more tasty food. The same could of course be said about dining on female humans, but maybe it's just easier to lure men into their doom and they don't really distinguish between male and female humans - humans are all tasty.

In a dire situation where the ghouls have to fight for their survival or it's too risky to feed any more on humans they might try to feed on something else, but they simply prefer the taste of human flesh.

Ghouls are traditionally a supernatural species, their lust for human flesh is, in large part at least, tied to this. In order to survive they need to consume not only food but also "life essence" for want of a better term. Normal foods have very little essence so they can sustain a ghoul physically for a time but they don't work for long. Humans aren't the most physically pleasant or nourishing food but they have essence to spare, the fresher the better. Only a steady diet of human flesh can actually satisfy a ghoul's complete food needs.

1. Meat is energy dense and very digestible. Bamboo is neither. If all you eat is one kind of meat, that is fine. There is not really an analogy between the diets of ghouls and pandas.




2. Hunting is dangerous; hunting humans is dangerous for sure. Eating dead things you find is much less dangerous. In places where there is a high population density of humans there will be a steady supply of people who have died of natural causes. You might need to dig up the meat if people bury their dead. In places where dead bodies are put in rivers or undergo "sky burial" it would be easier for scavengers - which is likely the fate of the corpses disposed of in these ways.

The analogy is the jackal. Jackals occur from Africa thru the middle east to India and are probably the origin of ghouls - I bet a jackal would be thrilled to eat a bunch of dates too. Jackals will definitely dig up a grave and eat the contents. They are tricky and wily like foxes. But jackals do not hunt humans because they are little and weak and would get their jackally asses kicked. I might imagine that a moribund human might be helped with his transition by an especially brave jackal but more likely it would just sit and wait things out. So too ghouls.