It would be logical from the point of view of energy to eat placenta; I have no explanations at hand why humans do not do it, probably it is a cultural heritage

(memetic evolution)

Thanks peter, I have read what little there is and it seems that placenta is consistently favoured over any other meat (even of higher energetic value) offered to newly delivered mammalian mothers. Is there something special about placenta?

Thanks James. Some have discounted the "nest cleaning" idea on the grounds that even top predators exhibit this behaviour (e.g. Lions, other big cats) But I' m not convinced about that as a pack of hyenas is a bit rough even for a big cat in good condition. I had no inkling of the reptile evidence - can you please point me to a ref?Many hanks. Al

Our lab group is starting to get interested in this subject. To date, probably the best reviews are by Mark Kristal (1980. Neuroscience and Biobehavioral Reviews 4:141-150. and 2012. Ecology of Food Nutrition. 51:177-197). Most recent papers suggest only humans, cetaceans, and cammelidae (actually I've only found references to Llama) do not eat the placenta. However, this appears to be much more variable than has been reported (e.g., domesticated goats occasionally eat some of the placenta but this varies from 10 to 40% of births meaning the majority of mothers do not). Really the only good data are for lab rats and lab mice where placentophagy appears to occur in all births with mothers eaten the entire placenta.

The idea that this is a 'high quality food' is commonly suggested but there are real problems with that. The main one is that virgins of both sexes seem to be literally repulsed by it. Second, new mothers are often very eager to eat the placenta but as Alan mentioned, may avoid other organ meat. In fact, this attraction appears to be short term. Finally, it seems odd that many herbivores should suddenly desire to eat a placenta.

Removing a potential predator attractant makes some sense to me but obviously that might not apply to all cases as James said. Immediate removal of the placenta might reduce disease risk as I've seen some evidence cows, especially the newborn, suffer higher fungal infections when the placenta is not removed (unfortunately I've lost that reference).

There is mixed evidence that mother's who eat the placenta produce more milk than those prevented from doing so. There is also some evidence that the content of the milk changes. However, these are all very old studies (mostly pre-WW II).

Kristal's main hypothesis is that the placenta acts as an analgesic although no one knows what chemical is doing this. He presents some good evidence for this effect in rodents. If this were generally true, I would expect placentophagy to be more common in species that have difficult births (like humans) or multiple births. The goats mentioned above are more likely to eat the placenta if they have twins than with single births. I've been looking for more data to back this up but haven't found much.

Basically, what we have found is that this is a very understudied problem.