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Imagine a creature that through congenital diseases and mutations starts slowly losing its digestive system as it becomes less and less efficient and smaller until it is vestigial and eventually completely lost.

How much of the digestive system can I remove and still enable the creature to "feed"?

Would the creature without a complete digestive system, or a completely gone one, still be able to extract nutrition from blood or pure glucose or processed minerals and amino acids? Either orally or through direct injections in veins?

As a base for the creature we will start from the most bland and primitive thus most adaptable mammal you can think of.

Preferably a human-like herbivore since humans have lower protein requirements per pound of flesh compared to other animals.

Something that can last a long time with straight up free running blood sugars and minerals from the blood of others.

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    $\begingroup$ Please do not substantively change your question after answers have been submitted. If you want to ask a different question from the one you perhaps mistakenly wrote, it's fine to make a new Question. $\endgroup$
    – g s
    Apr 10 at 15:54
  • $\begingroup$ If you want no digestive system at all, no, that is not possible for an independent organism, no matter how you get nutrients. The digestive system is necessary for processing and expelling waste. Cells must expel waste to maintain homeostasis, and the blood must transfer that waste outside of the body. Babies in the womb do it through the placenta and use the mother's digestive system to expel the waste, everything else uses its digestive system. $\endgroup$
    – g s
    Apr 10 at 16:04
  • $\begingroup$ @gs the last edit was before the first answer appeared to me, and it was a mispell correction....and I meant the bigger part of digestion like intestines and such $\endgroup$
    – Xenophile
    Apr 10 at 17:44
  • $\begingroup$ A digestive system that was hyperadapted to one very specific diet, something nutrient-dense could have "no stomach", but everything from nematodes to cows and us are still tubes. $\endgroup$
    – Nick T
    Apr 10 at 21:17
  • $\begingroup$ @NickT as g s points out, how do you get rid of waste? For example urea which is produced when you break down proteins? Though of course, OP only asks about the input part, not the output part. $\endgroup$
    – Michael
    Apr 11 at 8:42

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The simplest answer, though one that requires a certain amount of pseudoscience/magic is to have a creature that can suck blood, but not into its digestive system, but directly into it's bloodstream, essentially giving itself a blood transfusion at the cost of the host. It would require the creature to be an absolute Universal Recipient, being able to get transfusion from any kind of creature (or at least, a closely related, but more numerous mammal).

I would not start with a very primitive mammal, since those have simple, rugged, robust digestive systems that would have no reason to be replaced. I would start with an omnivorous hominid, which all have awkward digestive systems with plenty of limitations, and would benefit the most from an alternative.

Hypothetical design for the creature:

Its a mammal similar to coexisting, herbivore or omnivore species, that shares its blood characteristics. What makes the creature different from it's standard cousin is a certain set of mutations:

  • the blood vessels in the creature's canines and incisors are much thicker, and lead to a hole at the back of the tooth, with a valve at the end.

  • the tooth blood vessels extend into modified sinuses, which tissue can be engorged with blood when strong under-pressure in the sinuses is present (This is achieved by the creature sucking strongly at something, while both the nostrils and the blood vessels that lead from sinus tissue to the rest of the vascular system are constricted. Basically, the sinuses become a vacuum pump powered by the lungs).

  • once the sinus blood-sponge is fully filled with blood from the victim, the vascular constriction happens in reverse order (tooth vessels close, vessels behind sinuses open up,) and the sucked out blood goes into the creature's own bloodstream, at a much slower, gentler rate.

  • (optional) since getting rid of excess blood might be difficult for such creature, assume the fang-sinus vacuum pump can be used in reverse, either to sneeze blood out (basically controlled nosebleed/mouthbleed to discharge old, useless blood) or even pumped back into the victim. The alternative is the creature metabolizing the excess blood and pissing it out.

If we assumed the blood-sucker is a humanoid, or at least a mammal just as complex, there would be some visible changes, like:

  • bloodshot eyes, with dark shades under them (this is from excess blood in the sinuses right under the eyes)
  • oversized canines and incisors, likely with a mild overbite
  • sinusoidal energy cycle; the blood sucker would be sluggish and low energy before feeding, not unlike an anemic person, and highly energized right after feeding. As a predator, it would be one that ambushes prey intermittently, and then rests, possibly even sleeps, for a long time, in a form of hibernation or torpor.
  • extended lifespan: in general, in nature, blood-sucking creatures live much longer than their regular counterparts. Vampire bats live much longer than most mammals their size. Leeches can live for up to 8 years. Blood-drinking mosquitoes live 5 times longer than nectar-eating ones etc. This is because blood diet requires changes optimized for energy saving, which in turn slow down metabolism and ageing.
  • low body-fat. As the blood itself can become energy storage, and the metabolism is designed to slow down dramatically, there is no need or gain for a lot of body fat. The creature might instead gorge itself on excess blood before hibernating.
  • wiry, fast-twitch musculature. The Blood-drinker must necessarily hunt as an ambush predator, because it cannot waste energy on persistence hunting. Therefore, its muscles must be optimized for sudden explosive strength, favoring minimal, but very dense and fast-contracting muscle mass, like that of mustelids, cats, snakes or bats.
  • extremely robust immune system: getting constant blood transfusions from other creatures would necessarily lead to countless infections, not to mention a risk of leukemia from the compounded blood contaminants. The bloodsucker's immune system must necessarily be very powerful at combating bacteria, viruses and cancer cells, as well as getting rid of metabolic trash from the consumed blood. A side effect of that is also increased lifespan (compared to its prey, and other predators).
  • overly wide mouth with powerful jaw muscles. Since the creature needs to bite and hold the prey with its maw, it has to be able to open its jaw wide, and then clamp it on the prey's neck, or another body part with enough force to not be thrown off if the prey fought back, but also to maintain vacuum seal on the wound. In a humanoid mammal, this would result in a triangular lower jaw with the aveolar front being pointy, and the mandibular hinges being overdeveloped. This would give the creature a V-shaped mouth, and a very angular face.
  • likely nocturnal, or semi-nocturnal. Since it is an ambush predator, it would benefit the most from attacking sleeping prey, or prey that is separated from its herd in the dark. There might be some minor adjustments to its night-vision, hearing, or even coloration to make it easier.
  • avoidance of sunlight, fire and heat in general. The bloodsucker needs to avoid sweating, panting or otherwise wasting moisture when not absolutely necessary, as this thickens the blood, making its transfusion diet significantly harder to do. Excessive heat can also cause the sucked-in blood to spoil or clot, which could be deadly. Bloodsuckers that had not hunted in a long time might appear to have wrinkled and dried skin, mouths, even occasionally lose earlobes and noses, as their body fights to preserve moisture between hunts, even at the cost of external organs. This also means that a bloodsucker would likely prefer moist, cool, stable temperature environments for sleep, like caves, burrows, or even submerging itself partially in water.
  • sunlight avoidance means the need for extreme pallor, possibly bordering on albinism, to get vitamin D from little sunlight they get occasionally. It is unlikely they could get the entirety of their vit D needs from blood itself, as most of it is stored in the liver, not free floating in the bloodstream.
  • the same mechanism that allows the creature to effectively suck blood into sinus sponges, would also allow it to suck in air through the sinuses themselves with great effectiveness. Coupled with its powerful jaws, suction-enabling tongue, and powerful lungs, it is likely the creature would have a very spectacular ability to emit sound: from ear-splitting roars, to near-supersonic screech, and even maybe sound mimicking to lure prey?
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Tapeworms do what you want, residing in the intestine of their host and using the host's digestion to process the food for them. They are able to feed with no stomach at all.

Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass is Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cestodaria. All tapeworms are endoparasites of vertebrates, living in the digestive tract or related ducts. Examples are the pork tapeworm (Taenia solium) with a human definitive host, and pigs as the secondary host, and Moniezia expansa, the definitive hosts of which are ruminants.

However it's hard to get a mammal do that: reproduction and parental caring can hardly happen in the limited space granted by the intestines, plus it would lead to competition for scarce resources and would make finding a new host problematic.

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  • $\begingroup$ note that the question is (now) not just asking about removing the stomach, but about (parts of) the digestive system more generally $\endgroup$
    – Tristan
    Apr 10 at 15:38
  • $\begingroup$ @Tristan always been like that $\endgroup$
    – Xenophile
    Apr 10 at 17:44
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    $\begingroup$ @Xenophile the original question was less clear, and asked how much of the "digestive stomach" it could lose $\endgroup$
    – Tristan
    Apr 10 at 18:13
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A human is human-like and adaptable, and can live just fine without a stomach. See: National Cancer Institute.

You are a high risk for multiple vitamin and mineral deficiencies after total gastrectomy. This includes deficiencies in iron, vitamin B-12, thiamine, folate, zinc, calcium, fat-soluble vitamins A, D, E and K and other micronutrients. [...] You must take [a specially formulated "Bariatric"] multivitamin and calcium citrate every day to avoid problems associated with deficiencies, such as anemia, hair loss and low bone density.

Edit: And since we want a procreating genotype with this disorder, yes, people without stomachs can have successful pregnancies. National Cancer Institute, pdf link.

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    $\begingroup$ mostly-biped and wingless is good enough for me to qualify as man-like $\endgroup$
    – Xenophile
    Apr 10 at 6:02
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    $\begingroup$ Are you sure that humans are human-like? I've met some people... $\endgroup$ Apr 10 at 15:16
  • $\begingroup$ note that the question is (now) not just asking about removing the stomach, but about (parts of) the digestive system more generally $\endgroup$
    – Tristan
    Apr 10 at 15:38
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Why is this creature is losing its digestive system? This may be because doing without provides more benefits than doing with.

I lived next door to someone who had almost all of his large intestine removed because of cancer. He had to drink regularly and take vitamin tablets. The large intestine re-absorbs water from the gut contents. In the wild, every pool of water may be contaminated, and every drink carries a risk. If you have a good diet and tap water, then you can almost do without.

People who suffer from peptic ulcers and acid reflux take Omeprazole, or similar products that inhibit the production of stomach acid. This is another thing that helps us survive in the wild, but is not needed when we are eating prepared food. Many people have taken omeprazole for decades without suffering side effects. I have taken it briefly, but gave it up when the reflux stopped.

Nature does not dump a part of the body because we aren't using it. We have wisdom teeth which may have replaced teeth lost in our youth, but now do more harm than good. I have never had wisdom teeth. We have an appendix, which has a function we can live without, but would have killed me if it was not removed. We have mast cells which would protect us against internal parasites which those of us who have clean food and drink never meet, yet can cause many allergies.

What would cause a creature to lose much of its digestive system? A dependable source of food that does not need sophisticated processing would seem to be key. Pandas and koalas have very simple diets, and yet they have all the same stuff on the inside. The panda does not have much large intestine compared to us, but it definitely has one. However, suppose you were a smart creature that could guarantee your diet, and do genetic engineering. You might try and get rid of some of the complications that we do not often use, and avoid the risks that come with them in later life, when we are unlikely to beget offspring, and natural selection has little say.

I don't think this will happen to 'the most bland and primative' mammal. It may happen to us.

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  • $\begingroup$ I mentioned dissease as the reason in my question...same reason some people are born sick and some are born healthy....but the sick can reproduce and thus drive their sickness to the next generation $\endgroup$
    – Xenophile
    Apr 11 at 14:21
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If you're not stuck on sticking with mammals (or have access to a "telepod"), I seem to recall Cronenberg's The Fly, halfway through his transformation, vomiting digestive enzymes onto his food.

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A person can live without a stomach, because only a part of substances is digested and assimilated in the stomach. But you still have to go on a diet.

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