A wild sloth gives birth, cleans her newborn and then does something that can surprise human observers: she eats the placenta. The behaviour is called placentophagy, and it is common across mammals. In sloths, however, it is rarely witnessed and scientists are still careful about explaining exactly why it happens.
A recent field observation by researchers from The Sloth Conservation Foundation involved a wild Hoffmann's two-fingered sloth that had just given birth. By the time the team arrived, the mother had eaten the placenta and was cleaning her baby. The moment offered a rare glimpse of a private part of sloth life.
There is no single proven answer to why mother sloths do this. Instead, several ideas fit what scientists know about placentophagy in other mammals and about the challenges of raising a newborn in a rainforest canopy.
First, what is the placenta?
The placenta is a temporary organ that develops during pregnancy. It connects the developing baby to the mother through the umbilical cord, allowing oxygen and nutrients to pass to the fetus while waste products move away. After the baby is born, the placenta and associated membranes are delivered as the afterbirth.
Eating some or all of that afterbirth is known as placentophagy. A major scientific review of the behaviour in human and non-human mammals describes it as widespread among mammalian mothers. The authors also stress that familiar explanations, such as hunger, nutrition or predator avoidance, do not neatly account for every species.
That caution matters. A behaviour can have more than one benefit, and an advantage seen in one mammal may not apply in exactly the same way to a sloth.
Could it remove evidence of the birth?
One leading idea is that consuming the placenta removes a strong-smelling sign that a vulnerable newborn has arrived. A sloth's usual survival strategy depends heavily on being difficult to notice. Slow movement, a life high in foliage and fur that can take on a greenish tint all help a sloth blend into the canopy.
The afterbirth is different. It contains blood and other fluids that may produce odours around the birth site. Eating it could reduce the scent clues available to predators. This sounds especially plausible for an animal that normally avoids drawing attention to itself.
It is still a hypothesis, not a fact proven by a controlled study in sloths. Dropping the placenta away from the tree might also remove it from the immediate area, while spending time eating it could keep the mother in one place. Nature is rarely as tidy as a single explanation.
Could it help replace nutrients?
Pregnancy, labour and caring for a newborn all require energy. The placenta contains protein, fat, iron and other biological material. Eating it may return some nutrients to the mother at a time when she has just completed the demanding work of giving birth.
This possibility feels particularly relevant to sloths because their energy budget is unusually tight. The Smithsonian's overview of two-toed sloths explains that they eat a plant-heavy diet and have a slow digestive system. A sloth cannot simply race off to find a large, energy-rich meal after labour.
Even so, the nutritional explanation should not be overstated. Research across mammals has not shown that ordinary hunger alone explains placentophagy. Mothers can show a very specific interest in the afterbirth around the time of delivery rather than a general appetite for meat.
Could it support recovery after labour?
Another line of research suggests that consuming placenta and amniotic fluid can influence pain responses in some mammals. Experimental work discussed in the scientific review found that substances in afterbirth can enhance the body's own opioid-mediated pain relief in laboratory animals.
That does not prove that a mother sloth eats the placenta to ease pain. Sloth-specific studies have not yet demonstrated this effect. It does show why scientists look beyond simple ideas about cleanliness or calories. The behaviour may interact with the mother's physiology during the short period around birth.
Cleaning the newborn is part of the same immediate sequence. Licking can remove membranes and fluids, stimulate the baby and strengthen contact between mother and infant. Some amniotic fluid may be consumed naturally during that care, whether or not the mother deliberately seeks it out.
A difficult behaviour to study
Wild sloth births are rarely observed from beginning to end. They happen in dense vegetation, often high above the ground, and responsible researchers avoid disturbing mothers at such a sensitive moment. That means many reports begin after the baby has arrived, when the placenta has already been eaten or has disappeared.
Small numbers of observations make it difficult to compare species, individuals and circumstances. Two-fingered and three-fingered sloths have different diets, activity patterns and evolutionary histories, so researchers should not assume that every species behaves identically.
There is also a difference between saying a behaviour has a possible benefit and saying the animal consciously understands that benefit. A mother sloth does not need to think, "I must hide this scent from a harpy eagle." Natural selection can favour a behaviour because mothers and babies that perform it are more likely to survive, even when the immediate trigger is taste, smell or hormonal change.
What this rare moment teaches us
Placentophagy is a reminder that sloths are not simply sleepy animals hanging in trees. Their lives include complex reproductive behaviour, tight energy decisions and careful newborn care. A mother must recover from birth while keeping a small, dependent baby secure against her body in a habitat full of physical and ecological risks.
The honest answer is that scientists do not yet know exactly why mother sloths eat the placenta. Removing scent clues, recovering nutrients and supporting post-birth comfort may all play a part. Future field observations could reveal whether one explanation matters more than the others.
For now, the best response is curiosity rather than disgust. What looks strange through human eyes is a widespread mammalian behaviour and possibly one more quiet adaptation helping a new sloth family through its first vulnerable hours together.
Sources and image credit
- The Sloth Conservation Foundation: Why Do Sloths Eat the Placenta After Giving Birth?
- Kristal, DiPirro and Thompson: Placentophagia in Humans and Nonhuman Mammals
- Smithsonian's National Zoo: Two-Toed Sloth
Image: The Sloth Conservation Foundation, from its article on placentophagy in sloths.