For most readers, I assume, the practice this week’s OpenQuestion refers to is entirely out of the question – and the simplest answer to it seems to be: “it’s just plain revolting!” Biologically speaking however, placentophagy appears to entail some notable benefits: Amongst others, it is said to increase the interaction between mother and infant, to offer pain relief for the delivering mother, to foster her caretaking instinct towards the infant by stimulating brain opioid circuits, and to prevent postnatal pseudopregnancy. Not surprisingly (?), practically all nonhuman mammals ingest the afterbirth after delivering the infant. Surprisingly (?) humans – at least most of them/us – don’t, and there is no reliable evidence that we have ever done so routinely. Considering that giving birth to a child comes with additional problems specific to humans (i.e. postpartum depression), and some of the effects of placentophagy could be evolutionarily beneficial (?). So why do humans not eat the placenta after childbirth?
In addressing this question scientifically, there are several problems. First of all, it is not even clear why nonhuman mammals ingest afterbirth. There are several hypotheses as to why they do (including the ones mentioned above), but none of them seems to apply to mammals in general and there are always several alternative explanations and critical exceptions: For example, the function of placentophagy is assumed to lie in keeping the nest sanitary (but non-nesting species that could easily walk away from the site of birth eat it too), avoiding the smell of blood as to not attract predators (but some species sit there for hours while eating it), replenishing nutrients and hormones lost during birth (but humans also need salt and certain nutrients after giving birth, and some species reject other meats and similar substances that could also provide those things). Assessing the possible effects placentophagy might have on humans experimentally is even more complicated, since there is no adequate animal model for human post-birth behavior. And because there is no “natural” behavior to compare in humans there is no way to differentiate negative findings from null results.
So if we cannot be sure about the physiological functions of placentophagy (or its omission) we may turn to possible social functions. Anthropological or sociological reasoning offer perspectives here. An obvious explanation would be the one we started with: a culturally (i.e. specifically human) acquired disgust towards eating afterbirth. Indeed, there are taboos concerning placentophagy in some anthropologically documented cultures. For example, placentophagy is considered what it apparently is: something animals do, and something which – for that reason – humans don’t do. Similarly, ingesting human tissue and blood might also be considered cannibalism and therefore rejected on a moral basis. This way, the rejection of placentophagy could work as a distinguishing mechanism and a way of humans’ self-assurance. Interestingly, taboos are usually formed against behaviors that are recognized as likely or at least possible. However, there is no anthropological evidence for routine placentophagy in humans, although there have been accounts of substitute practices like the ritual burying of the placenta at the roots of a tree and then symbolically ingesting it by eating the tree’s fruit. But if ingesting the real thing has positive physiological effects, such behavior would be evolutionarily detrimental. Or wouldn’t it? Just the contrary may also be the case: As, evolutionarily speaking, humans have compensated for their lack of natural instincts by developing social structures that enable them to share knowledge, not eating the placenta may insofar be evolutionary beneficial, as it can promote social bonding: the increased rate of pain experienced during birth requires and invokes more assistance and support by other humans during delivery. This explanation, of course, builds upon the assumption that the pain-soothing effects of afterbirth ingestion observed in nonhumans apply to humans as well – which we cannot prove.
And finally: what are we to make of recent movements that propagate placentophagy in modern western society (such as http://placentabenefits.info/medicinal.asp?). Human restraint from placentophagy appears to be a complex issue, and it is not even clear where and how to begin or carry on with (what kind of) research.