This has been puzzling me, because if the human half needs human nutrients, then suckling from the human nipples makes sense. But the horse half needs different nutrients, so which set of nipples makes most sense?
The answer is clearly the "human" nipples. Horse necks evolved so that they could reach the teats while standing up. Human necks just don't have the necessary range of motion to be able to do this.
Did you know that horses can stand/walk around within minutes of being born?
This depends on a few things and with some considerations, baby centaurs would probably be best suited to nurse from the human teats, rather than the equine teats.
The first thing to consider is whether the offspring is full Centaur or not. Will your story contain a race of centaurs, complete with families, culture, cities, etc.; or will the story contain a small selection of centaurs or possibly just one, as a magical/mythical being?
If the centaur(s) in your story are mythical, then there is a high likelihood that any offspring of the Centaur's is half of something else too (i.e. human, horse, god, faun, minotaur, gorgon, etc.), in which case, the mythical nature of the scenario can allow you to forgo answering the question entirely via a number of interesting ways, the easiest being the spawn is born mature.
If the centaurs are a race and have an obvious biological component, then the offspring is likely not a mix breed and you must indeed answer this question.
Pros and cons of the human teats: A centaur baby will have a human head and a human mouth. An infant mouth and a mother's teat are very much like a lock and key in most mammals. The teat is correctly sized and adapts temporarily to each infant's nuanced differences. However, being half horse, we can consider that the infant supports itself much, much sooner than an human infant, so the mother would not be carrying around a baby for months, but rather weeks or even days. The human teat would be too far to reach easily.
Pros and cons of the equine teats: An equine teat is far too big for a human mouth, especially an infant's. Also, an equine teat would deliver far more flow than an infant would want. But, it is conveniently positioned; the infant could reach it quickly and easily without much burden to the mother.
Some have noted nutritional differences, but I think that's an odd distinction if we already assume these centaurs are a biologically natural species. If they are, then the milk is biologically suited for centaurs regardless of which teat it comes from.
So with this in mind, the human teat makes the most sense, but considering centaurs are neither human nor horse, there's no reason to assume that they behave or develop similarly. What makes the most sense to me is that Infant centaurs are weaned relatively quickly from the human teat. I would say about a week. They also develop rapidly, growing the human equivalent of four or five years in only a few months. The equine teats are never used and you could even say that they do not exist.
Another, albeit creepier, option that I thought of is that infant centaurs metamorphose into their adult shape. They could start as human infants, but then quickly or in an odd cocoon develop their horse half. Or they could do the opposite, which sounds more enticing to me because it could leave it mysterious why your centaurs have young foals nursing from their teats. Allow their culture a high integration with actual horses and no one will actually think that some foals become centaurs "spontaneously". This avoids all the problems with infants depending heavily on their mothers compared to foals.
Clearly they suckle from a centaur's nipples. Both sets.
Centaur females have both human and horse nipples. I feel silly saying it only because it seems like such common knowledge.
As with any other mammal, they might be able to suckle from another animal's nipples alone, but won't receive the complete nutrition they need. In this case suckling on either only the human nipples or only the horse nipples would not, as you point out, be complete nutrition.
Suckle is a bizarre word in that both the act of feeding the baby is suckling and the act of the baby eating is suckling. It could lead to some ambiguity.
Neither - Centaur nipples would be located elsewhere.
I'm making the assumption here that your centaurs are natural creatures that just happen to resemble both humans and horses. This doesn't apply to the traditional human-torso-replaces-horse's-neck, which has many other biological issues (as others have pointed out).
Given this assumption, centaurs would have evolved so that their nipples would be easy to reach for their colts without too much inconvenience for the mother. Mares can nurse their colts while they continue to graze and slowly walk around, and the colts' necks can turn to suck on a dangling nipple. Human women can nurse their babies while walking around and supporting the baby in one hand (or a sling, to free up both), and the baby's flat faces don't get in the way. So a centaur female should likewise be able to gently move around without disrupting the child's ability to nurse.*
Horse nipples don't work: As cimmanon's answer shows, a centaur would have to have a neck that's far longer and more flexible than a human's in order to get under its mother to nurse. This would make the "seeming like a human" part much harder.
Human nipples don't work: If the mother has to bend over, it's the same problem as with horses. If she picks up the child, she has to be strong enough to hold it for long periods of time, and presumably it grows similarly to a colt (and so gets heavy).
Centaurs would need outward-facing nipples relatively low to the ground. Outward rather than downward, to support flat(human)-faced offspring. Low to the ground to support independently mobile four-legged offspring. Where would these nipples be placed? I see two possibilities, depending on how much you want to stretch the resemblance to humans and horses, although both are kindof weird.
One option, which borrows more from the horse side, is to have the nipples on either side of the horse torso, probably on the lower side. By putting them on the sides, you don't require the children to contort nearly as much, while still maintaining the general behavior of horses. I don't know how much supportive tissue (breasts/udders/etc) would be necessary, but it would almost certainly be something that grew out during pregnancy and got out of the mother's way after weaning.
The other option is to have more human-like nipples, but have them located approximately where a human crotch would be. This would provide the same general posture for the children as with the other option, but would require less deviation from a classic horse torso. The children would stand in front of the mother to nurse, which would make it awkward for her to move, but she would be able to reach them with her arms while they nurse (which a horse can do with her neck, but a centaur wouldn't be able to do with the side-nipple option). This has the bonus of not having sensitive parts of anatomy competing with harnesses, and (if society demands it) can be covered by a wrap similar to a human loincloth or skirt.
* Disclaimer: I have little-to-no experience with babies or colts. I may be totally off-base about nursing behavior. I'm just going based on what little I think I know combined with what makes logical sense to me.
This is complicated. Depending on choice baby centaur will either die or die.
But let's dig a tad deeper.
Most notable entries are:
- Vitamin C - There is none in horse milk. Humans lost ability to produce it so it must be acquired from outside (via mother in this case, but some fruits had to be sacrificed). Lack of Vit. C is very bad for humans, so by rule of thumb, even worse for baby. (Hint: do not google for "scurvy babies"). Theoretically horse part could supply Vit. C to human part if blood circulation would be shared.
- Calcium - Horse have four times more! Hypercalcaemia sounds scary, but so does Hypocalcaemia... ARGH!
Using two highly different milks would be very problematic (yeah, like anything is simple in realistic centaur design), slight imbalance in diet wold result in crippled baby (calf? calfby? balf?). Realism comes with evolution. Both two-milk and single-milk centaurs would die off without modern civilization for died supplement.
So yeah, evolution...
- Some crazy drunk god creates centaurs. I bet he was one of horny Greek ones.
- High rate of death among infants due to dietary problems.
- Evolution! Mares whose human milks are more similar in content have more adult babies to pass their balanced genes.
- Evolution! Horse part can provide more milk, and is easy access for hungry balfs. Human breasts slowly became evolutionary artifact.
- Success! Horse part provide perfect milk for balfs!
So about those human breasts... I do not like idea of flat-chested centaurs, but evolution can save us again! How can useless, but surely eye-catching, human breasts be of any evolutionary advantage? Ask Indian peafowl, useless body part works fine for them (which made Darwin sick).
- Big human breasts = healthy, well fed, mare! Best for breeding.
- Please wait... Breeding takes a while...
- Human breasts role degrade from baby feeding to sexual selection.
- Milk flow from human part stops, horse part keeps providing modified mixture.
All ends well after few dozen thousands years... Better pray to god of evolution.
Short Answer: Human Nipples
A centaur is no more 1/2 human 1/2 horse than a platypus is 1/2 duck, 1/2 otter. The centaur assuming it evolved that way would produce milk that nourishes its offspring completely.
Now the question is which teat the newborns will go after. Now if we assume the horse part does what foals do it will be up and walking almost immediately. At that point the human part will be sticking up in the air and their faces will be positioned quite well for suckling at the human teat. Would you want to bend over and try to turn your head around backwards to suck the nipples on the underside of a horse? Probably not, especially if there is a convenient source of food in a much more comfortable position.
Because it is very implausible for a newborn whose spine is curved in the way that a centaur's must be, and who can't possibly have more developed Motor skills than that prime example of mammalian precocialness that is a horse foal, to bend down /curl around to reach the udder. Assuming the centaur Baby is the same size in relation to the mother, as a horse foal, the Standing offspring would not quite reach the nipples at birth and either require the mother to kneel, or use a Kind of step stool. And then they would be tall enough to comfortably reach the breast after a few weeks, and would know when to wean when they grow so tall they would have to bend down... again with the spine
Also, suckling from the upper Body is just cuddly and nice and allows for eye contact and hugs, so I feel that would be more appropriate for a species that has both eyes in front and participates in somewhat human-like social interactions (If that's what you're going for).
Are centaurs altricial or precocial? If they are altricial (as humans are), then they are born basically as helpless as human babies. If they are precocial, like horses, then they should develop certain abilities much more quickly than humans: in particular, they should be able to walk within a few hours after birth.
Horse-like nursing is logistically much easier, if the babies are capable of it: the mother stands, the babies stand, and everything works out. But in order for this to work, centaurs would have to be precocial: a horse-like trait, rather than a human one. This is certainly doable, but the consequences can be far-reaching: much of human culture is organized around the logistics of raising altricial young. Precocial centaurs would not face this issue, and might organize their culture along very different lines, making them seem more alien to humans. This might be desirable for your setting, or it might not.
If centaurs are altricial, however, then human-like nursing is the only real option. If the babies can't stand, then they can't nurse like a horse. This is not to say that carrying a centaur baby in one's arms is a particularly easy task: centaur babies would be larger and heavier than human babies, and their body shape would make the task more awkward. But this may be all that is practical, and you've got to do what you've got to do.
All of the above assumes evolution by natural selection, or something very like it. If they were created relatively recently by some artificial process, then the answer to your question probably reflects their creators. Human-like creators would likely engineer a reproductive process as human-like as possible, including nursing, because that's what they relate to. Horse-like creators would instead probably favor a horse-like process.
There are a lot of good answers here, but they are missing something very obvious - humans have nipples because they need them to feed babies. They have two nipples, because humans very rarely have more than two babies at a time.
So ask yourself: If the baby centaur suckles from the horse part, then why does the human part have nipples?
So, I can think of two cases:
Centaurs evolved, therefore only have a single set of functioning nipples, and the babies suckle from those. Stick them wherever you want.
Centaurs were magically created, and the babies suckle from whichever nipples the wizard/god/plot that created them told them to.
Given that this is worldbuilding, the answer to your question will become obvious once you answer this one: What purpose in your world do the nipples server? Are they a necessary plot point to the story/game/whatever? Is this something that will actually be relevant to anything, or can you just hand-wave it?
I think that both are viable. Why not make it so that it depends on the culture of the centaurs?
For instance one culture of centaurs likes to upbring their babies by giving them the human teats while another culture likes to give them the horse teats. And yet another one that combines both according to the maturity of the offspring!!
That would be pretty interesting, even develop it further by making certain behaviors according to what milk they received (although theoretically both milks would be the same assuming they are biological beings)
The human nipples
I'll assume that the human breasts provide the right nutrition because I think of them as one complete organism, rather than most of a horse and much of a human. After all, if they really were part of a horse and part of a human then, depending on the genitalia involved in mating, wouldn't you either get a human baby or a colt instead of a little centaur?
The human nipples are better suited for a human baby's mouth.
And while it could well be that a baby centaur couldn't reach the mother's nipples while both were standing, centaurs are tool users and don't have to worry about running from wolves at a moment's notice. The mother can lay down to suckle her child.
Here is yet another argument for the human nipples: arms, horses don't have them. Horses nurse standing which present problems for centaurs because of necks. (Cows, goats, and sheep also nurse standing.) As a comparison cats dogs and pigs nurse with the mother lying on her side. Horses on their side is a sign of a sick animal. Humans and primates nurse holding their young. This solves the height and position problems and works well with latch issues.
The only remaining problem is how many nipples does a centaur have? Would any be on the udder? Would there be an udder? If there was an udder I would argue for it being placed forwarward so that it could be more easily reached by the mother's hand, but I think the better solution is that if the centaur had more than two nipples that it would be best if each was on it's own breast to allow ease of reach and to prevent/minimize being front heavy given the increased demand for milk due to larger infant body mass.