Nearly every mammal will consume the placenta after it is born. There are four main scientific theories that attempt to describe the causes of placentophagia, but none of them adequately explain this phenomenon in all of the cases in which it manifests.
The first theory is that there is a sudden shift toward carnivorousness upon giving birth, causing a confirmed herbivore to suddenly crave meat. Basically, since the placenta is readily available, the mother consumes it. In testing this hypothesis, a variety of meats were presented to herbivores before, during, and after birth; all of the meats were refused, while the placenta was enthusiastically consumed.
Another theory along the same line is that the mother just happens to be hungry after all that hard labor, so she eats the placenta. While many species do refuse food and drink during labor, it is not true of all species. Even animals that consume food and drink throughout the duration of labor will still consume their placenta.
Some researchers have hypothesized that a new mother has a sudden and specific hunger for placenta, presumably as a reaction to childbirth. Although there may be physiological components to the compulsion to consume the placenta, it has been observed that a large number of mammals will consume placenta from another animal, even if they have not yet delivered their own young. This would imply that the unique physiological state present at birth is not necessary for placentophagia to occur.
It's not to clean the nest site!
The last, and most commonly cited hypothesis for placentophagia, is that mothers consume their placenta to clean the nest site and keep the offspring safe from predators. While a seemingly sound theory on the surface, there are many reasons to refute this popularly held belief. For one, unchallenged predators consume their placentas, even though they are not likely to fear that predators could harm their young. Additionally, even in species in which the baby is able to get up and walk away from the birth site, the mother will stay until the entire placenta is consumed. Certain primates that deliver their young high in a tree will not let the placenta drop to the ground, but will instead consume it, even taking an hour or two to do so. And, lastly, birth is a messy process. Yet there is no effort to clean up any of the blood and other fluids that exist on the ground of the birth site, which would also presumably attract predators.
An evolutionary purpose
There must be some other evolutionary cause for placentophagia. A very interesting adaptive theory is that consuming placenta may actually affect the mother's immune system, by suppressing her body's inclination to create antibodies as a response to antigens present in the baby's blood. As an example, women who are negative for the Rh antigen can have difficulty supporting a subsequent pregnancy if her first baby is positive for the Rh antigen. Her body can create anti-Rh antibodies, which then attempt to fight off the next pregnancy if the next baby is Rh-positive, mistakenly recognizing it as a threat. Placentophagia may actually cause a suppression of this response, allowing her to have successful subsequent pregnancies. Human women who are Rh-negative are often encouraged to get a shot of Rhogam, a vaccine that blocks the creation of high levels of these antibodies. Mammals may have adapted their own antidote over thousands of generations, simply by practicing placentophagy.
This is an exciting area of research, and we will hopefully learn much more about the benefits of placentophagy in the future. Perhaps we will even discover that Nature has had a cure for this antibody issue all along.
Summary of Mark B. Kristal's research paper, Placentophagia: A Biobehavioral Enigma: Neuroscience & Biobehavioral Reviews, Vol. 4, pp. 141--150.
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