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Undoubtedly, one of mythology's most iconic creatures is the centaur, a human being with his or her waist glued to the torso of a horse. For this post, we are avoiding the question of how evolutionarily feasible such a creature would be because if we were to talk about that, we'd be here forever! For now, let's look at my proposals on how it would look in real life because biology is never as clear-cut as mythology.

enter image description here

Let's start with the elephant in the room--traditional centaurs are portrayed as having two torsos. Why? Just...WHY? No other animal on Earth has two separate torsos in one body simply because one is good enough. My proposal is that the transition between human and horse is in the midriff. That way, we can have two separate sternums fused into one. That still raises the question as to which organs will be in which ribcage. Here is my proposal:

Human

  • Brain
  • Eyes (in all mythological humanoids--not just centaurs, but also satyrs, fauns, mers, angels, giants, elves and dwarves--the eyes have four types of cone cells, resulting in tetrachromacy.)
  • Tongue
  • Esophagus
  • Lungs (in all mythological humanoids, the lungs make up 15% of the total body volume, rather than the typical mammalian 7%.)

Horse

  • Stomach (all four of them)
  • Kidneys
  • Heart (if it were human, we'd diagnose him with cardiomegaly)
  • Liver
  • Intestines

The next issue is what would a centaur eat? Would it eat grass, like regular horses? I say no and here are two reasons why not:

  1. Grass isn't ideally nutritious, which is why some grazers spend the most part of the day mowing the lawns.
  2. Grass grows low, far out of reach for the centaur's augmented torso.

Instead, let's put those hands to good use--leaves, fruit, nuts, eggs and even insects.

Traditional pictures put centaurs using modern horses. While I say we keep that traditional image, I propose that this not be the be-all-end-all. Some tribes would keep the ancestral three-toed design.

Classical image often portrayed centaurs as being sexually unrelenting. Fortunately, biology can justify that behavior--just look up musth, a bi-annual condition in which a bull elephant's testosterone skyrockets, making him excessively aggressive.

This next issue is for anyone who wants to write a romance fantasy between a human and a centaur. Traditional centaurs are simply too tall for interbreeding to be merely as burdensome as regular breeding. My proposal is this--make the average centaur four to six feet tall. Which means structuring the centaur less like a thoroughbred and more like a pony or, even better, an Icelandic horse (the only one to have a fifth gait, ambling, which made this horse the ideal traveling companion during the Middle Ages).

(If you want to use the traditional thoroughbred, though, then go right ahead, but I'll warn you this--labor pains will reach critical.)

The final issue that needs addressing is color.

enter image description here

As you can see, the human half and the horse half are never in pigmentary sync. In nature, you have to be one color scheme or the other. So here's my proposal--the centaur's human half will be of either Caucasian, Asian or Polynesian design, but both halves will be the same color, be it:

  • Bay
  • Chestnut
  • Grey
  • Black
  • Pinto
  • Appaloosa
  • Palomino
  • Roan
  • Smoky black
  • Silver dapple
  • Smoky cream
  • White

Just as both halves will be the same color, both halves will grow the same coat in the winter. It's less clear-cut and more believable that way.

Are any of my proposals listed above sound, or have I created some unintentional side effects to the centaur body?

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    $\begingroup$ You'd want heart and lungs of a horse...You'd need a human-esque stomach with the size of a horse, if not bigger and a bigger liver, etc to process all of it. $\endgroup$ – Durakken Sep 18 '16 at 15:48
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    $\begingroup$ There's an entire paper on centaur anatomy. $\endgroup$ – Aify Sep 18 '16 at 17:21
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    $\begingroup$ Horses and their relatives have only one stomach. Cows and their relatives have four. $\endgroup$ – Monty Wild Sep 20 '16 at 2:33
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    $\begingroup$ I was just pointing out that you appear to have been confusing equine and bovine digestive systems. Horses have only one stomach, lots of teeth, and a big caecum (the appendix in a human). Cows have four stomachs, fewer teeth and a smaller caecum. Horses can eat rich food without consequence but cows cannot. Cows swallow their food quickly then regurgitate and chew it more thoroughly later, while horses do all their chewing in one go. If a centaur was a non-ruminant, it would need a lot more teeth than is traditionally depicted, and if a ruminant, it would have a more restricted diet. $\endgroup$ – Monty Wild Sep 20 '16 at 3:51
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    $\begingroup$ I don't think a centaur would be a herbivore or surviving off those little nibbles - they have the speed and stamina of a horse, and the intelligence and agility (esp. opposable thumbs) of a human as well as being renowned archers. Surely they would make perfect hunters! $\endgroup$ – colmde May 9 '17 at 13:18
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Seems quite reasonable. I would recommend a few changes/additions, both in keeping with tradition and to improve the model.

Firstly, on coat color. I agree that the human hair and the horse hair should match, but skin tone really doesn't need to. My cat has black hair and white skin, so if she were bald from the ribs up, she'd be mismatched too, but its perfectly normal. Most animals don't have stripes or spots on their skin, despite have striped or spotted coats, so a centaur might have brown hair and white skin, or even white hair and black skin, although it would probably be more visually pleasing to keep the two colors similar. Additionally, I would think that the human's hair would extend farther down his spine, like the mane of a horse. Maybe not all the way to the second shoulders, but at least down past the first shoulders.

Secondly on internal organs-

Digestion: As mentioned in some of the comments, a centaur really can't survive on human-sized organs, so I recommend horse guts in the horse part. Big lungs, big heart, big stomach, but only one stomach. If a centaur is really a mix of human and horse, it should have a human-esque stomach, capable of processing meat. Classical centaurs are archers, presumably not shooting at grass and berries, so give them big, horse-sized human stomachs. This also solves the problem of low-nutrition grass. If centaurs were only eating berries and leaves, they would be too busy grazing to get anything done, like teaching Achilles and Jason to fight. Side note, if Chiron had spent a little less time foraging and a little more time teaching, Achilles might have known to wear boots into battle instead of sandals.

Heart: Definitely a big horse heart in the horse chest, but I think a smaller, back-up heart should be in the human chest. Most mythological creatures are pretty hard to kill, so it makes sense that if you spear the centaur's horse-heart, a secondary, emergency heart could take over. Of course, this heart couldn't keep the centaur up and active, but it might be enough to keep him alive for a while in a comma-like state. It would eventually give out, but if he got a horse-heart transplant or something, he might pull through. With that in mind, a second, smaller set of lungs should be in the human half as well. If nothing else, it would increase the oxygen uptake and blood flow of the centaur.

Brains: Of course, centaurs are assumed to have human intelligence, suggesting a human brain, but brains do a lot more than just support consciousness. People say humans only use 10% of our brains, which only accounts for conscious use. 100% of the brain is used, just not for thinking. A lot of it is devoted to little things like telling the heart to beat, the stomach to churn, and the lungs to fill. Not to mention processing nerve signals, handling balance, and regulating temperature. A mouse and a rabbit have similar conscious brains, but a rabbit still needs more gray matter because the rabbit has more body to manage. A horse is less intellectual than a man, but the horse still needs a bigger noggin because the horse is much bigger, so I advise a secondary brain at the juncture of human and horse. The lower brain should handle basic functions, the higher brain should handle consciousness and personality. Another way to do this might be to extend the brain down the spine, eliminating the troublesome second brain. Some animals already have brain extending down their brain stems (that's why, if you do it right, you can cut the head off a chicken and he'll still run around) so it's not an impossible idea.

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    $\begingroup$ If centaurs were built to eat grass, they'd just have to bow down and try to reach the blades from there. Why not something closer to arms' reach, like whatever you find on a tree or a bush? Besides, selecting human and horse organs is just too clear-cut. Name me one real-life vertebrate that has a backup heart. The large plant-eating dinosaurs, like Diplodocus and Stegosaurus, had long bodies but very small brains, but they went out fine without the need for a secondary brain. $\endgroup$ – JohnWDailey Sep 18 '16 at 23:19
  • $\begingroup$ Well sure, but name me one real-life vertebrate that has two torsos. I'm not saying that it would ever happen, I'm just saying that if I were making a centaur, that's what I'd do. $\endgroup$ – Aziri Sep 18 '16 at 23:23
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    $\begingroup$ Also, while my original answer and my first comment stand, there are creatures with more than one heart. There are even some people with more than one heart (albeit through surgery), so it's not that far out. If you're interested, check it out. mentalfloss.com/article/52337/3-creatures-more-one-heart $\endgroup$ – Aziri Sep 18 '16 at 23:28
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    $\begingroup$ @JohnWDailey Name a vertebrate that have six limbs. Really, having a backup heart isn't that much of a stretch if you are already messing up with evolution enough to the point of having those incredibly different limbs and body shapes mashed up together. Also, humans sometimes have an extra heart already - its a rare mutation, sure, but it happens! $\endgroup$ – T. Sar May 11 '17 at 20:50
  • $\begingroup$ @JohnWDailey Dinosaurs "went out fine without the need for a secondary brain." While I agree with you generally, I have to jokingly note that they didn't really fare so "fine" - seeing as how all of them are dead (and besides, their fossilized bones wouldn't keep the remnants of a second brain if they did have one). $\endgroup$ – Truth Nov 16 '18 at 22:10
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I'd just like to point this towards this excavation exhibit at University of Tennessee, Knoxville.

enter image description here

(This is a hoax, as detailed here. But quite well done, I thought. (I saw this exhibit once; it was set up in one of UT's libraries. The case is full scale, and quite realistic looking.)

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An addition to @Aziri's answer

Anatomy

Stomach (all four of them)

Horses don't have four stomachs, I would recommend a single, hybrid human/horse stomach because then they can occasionally munch on grass, and can digest meat= the ultimate omnivore.

Heart (if it were human, we'd diagnose him with cardiomegaly)

An addition to help that single heart, borrow from giraffes and place some additional helping pumps to the human part's arteries.

Lungs (in all mythological humanoids, the lungs make up 15% of the total body volume, rather than the typical mammalian 7%.)

When I'm referring to human lungs, I'm really just referencing to air sacs in their place.

Structure it in an avian manner, where the movement of the front legs compressing and decompressing the air sack is analogous to how the bird's wing muscle compresses and decompresses the air sack. We separate the horse lung into two parts, a not compressing part, where the gas exchange occurs, and two air sacs, one type placed in front the main part and into a part of the human lungs, the other (from which, the air leaves the creature) is solely in the human lungs, that has a diaphragram. This ensures, that the air that passes into the lungs is always oxygen rich.

Also, apparently everyone overlooked the tiny little factor in this question, that horses are obligate nasal breathers. So use the human mouth breathing again, but don't forget to give them some sort of a phlegm that can protect the throat from getting sore.

Human lungs and the long windpipe can be used to filter and heat the air with lesser resistance, which is a limiting factor in horses, but we can mess around with it freely, avoiding the dead space (not the necromorphs) issue.

Instead, let's put those hands to good use -- leaves, fruit, nuts, eggs and even insects.

And meat. They were damn good hunters in mythology.

Traditional pictures put centaurs using modern horses. While I say we keep that traditional image, I propose that this not be the be-all-end-all. Some tribes would keep the ancestral three-toed design.

If they live in the forest, why not to use deer lower bodies instead?

There's also the spine,

Solution: Snake, more precisely: it's segmented spine.
Snake


Physiology

This next issue is for anyone who wants to write a romance fantasy between a human and a centaur. Traditional centaurs are simply too tall for interbreeding to be merely as burdensome as regular breeding.

There's another problem with this, click the link to see it, if you dare: MR. HANDS (spoiler: he died, reasons: perforated intestine)

By the way: if we're here, I would give centaurs a longer gestation period, so the human and the horse half will be equivalently developed.

make the average centaur four to six feet tall. Which means structuring the centaur less like a thoroughbred and more like a pony or, even better, an Icelandic horse (the only one to have a fifth gait, ambling, which made this horse the ideal traveling companion during the Middle Ages).

That's not true, any breed of horse can learn gaits and imagine if those horses want to learn the moves. It's way easier, and an extra layer of narrative fluff if you want to detail what type of movement they use in various situations.


Additional side effects:

I would also give a thicker skin and bones to the human body to explain the +1 natural armor, and the longbow usage, we've seen in D&D.

This would require stronger bones in the forelegs, but I think it's easy to come up with a solution for that.


Summary:

Powerful, fast and tough, centaurs have the upper half of a human. However this part, aside from the air sacs (protected by the ribcage (this is, why centaurs have two.)), the windpipe and the esophagus, which two are both protected by a cartilage structure which is similar in segmentation to the spine.

Thus the human part is very resilient and flexible as it mostly contains muscle bone, connective tissue, and the myoglobin reserves.

The horse part is optimized for longer endurance runs. But the myoglobin reserves, the avian lung structuring, and the hybrid nasal and orofacial breathing also enables them to perform in a sprint just as well.

Their communities are mostly hunter-gatherers, occasionally farmers and a small plus: They can digest the other parts of the wheat too.

Their numbers and low, mainly because of the long gestation periods, and can only increase slowly.

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    $\begingroup$ «Also, I'm a genetic engineer,» are you really, or is that some kind of in-character answer? (How about filling in your profile?) $\endgroup$ – JDługosz May 9 '17 at 9:43
  • $\begingroup$ @JDługosz Obvious trolling. $\endgroup$ – Mephistopheles May 9 '17 at 9:44
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    $\begingroup$ Not obvious, thus I asked. $\endgroup$ – JDługosz May 9 '17 at 9:46
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Mephistopheles May 9 '17 at 14:25
  • $\begingroup$ That was not a good summary. $\endgroup$ – JohnWDailey May 11 '17 at 22:30

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