In the Niokolo Koba National Park (Senegal) lives a population of Guinea baboons that has been studied for more than ten years. On March 9, 2020, my colleagues and I left the camp in search of him like every morning at work. We knew that the baboons used to choose trees along the riverbank to spend the night, so we drove our car along the road that runs alongside the Gambia River. We find them in one of their favorite bedrooms, an area on the banks of a meander where the vegetation is not too dense and a large tree takes center stage in the scene. Most of the baboons had already descended from the canopy and were rested, playing or having their first drink of the morning. It seemed like a normal day in the field, but we were going to witness a very unusual behavior whose explanation is still not clear to science.
I noticed an intense stench that I instantly identified as the smell of death. I went to discuss it with the rest of the group, but a veteran colleague beat me to it: “Oh, no. Not again”. In front of her I recognized Taki, a rather older baboon, with an independent character and easy to identify because her tail is full of knots. In her arms she held a piece of dried skin that, if it weren’t for the hairs on the end of her tail still being visible, he wouldn’t have guessed that it was her daughter, or what was left of her.
I had heard of primates carrying their dead children for days, but seeing it firsthand was shocking. I watched Taki for a few minutes, wondering what made her continue to hold a clearly inert body. Did he understand that her daughter was dead? Was she showing signs of grief? At one point, I saw her pinch her fingers into dry skin, then bring them close to her mouth. I didn’t know for sure if she was grooming her corpse or if she was eating it. Moving across the ground, she dragged her skin without any care, leaving behind her the foul smell. At the beginning of the morning I was fascinated to see her, but after hours working with the stench, I just wanted Taki to leave her with her death to another part of her. Can such behavior have evolutionary advantages?
Primatologists have suggested many hypotheses to try to explain this behavior. The unconsciousness hypothesis proposes that mothers are unable to discriminate between pups that are unconscious or stunned, but can return to activity, and pups that are already irreversibly dead. Therefore, they continue to carry and care for them, thus avoiding the risk of abandoning a young prematurely. In line with this idea is the death learning hypothesis. Primates may not understand death intuitively at first, but after such experiences they become familiar with the concept. If this hypothesis is correct, one would expect that it was mainly the younger mothers who would continue to carry their dead children.
On the other hand, the hormonal hypothesis does not focus so much on what the mother can or cannot understand, but on the instincts that lead her to continue perpetuating the care of her offspring, even if she has died. After giving birth and during lactation, the mother’s endocrine system releases hormones, such as oxytocin, that promote maternal behaviors. These effects could last even after the death of the infant, thus delaying the moment in which she detaches from him. It could also be, as the maternal bond hypothesis suggests, that over time the mother has established such a strong emotional bond with her offspring that not even her death can destroy it.
Another hypothesis, that of grief management, is also about hormones. Just as humans need to mourn, there are primatologists who propose that, for other primate mothers, continuing to carry their young helps them cope with the stress of loss. If so, we might expect this behavior to lower blood levels of stress hormones such as glucocorticoids. Finally, the maternal learning hypothesis suggests that the cadaver allows mothers to practice and improve their care. After seeing how Taki was dragging the body of her daughter through all the grass and bushes of the Niokolo Koba Park, I only hope, for the sake of her next child, that this hypothesis is not correct.
All these hypotheses are perfectly testable, but for this, many data are needed, taken systematically, such as the age of the mother and of the offspring at the time of death or the different levels of hormones in the mother’s blood. Unfortunately, most of the records to date are descriptions of anecdotal cases like Taki’s, which is normal given the low frequency of this behavior. For example, no one in her right mind would consider a doctoral study that consisted of following a group of primates and waiting for a calf to die and her mother to continue carrying it. It would take a lifetime to collect enough information to present a thesis.
For this reason, studies such as the one recently published on the chimpanzees carrying dead young in Budongo Forest, in Uganda. Scientists have been taking data for 40 years and have managed to record a total of 191 births, of which 68 failed. On nine occasions, the mothers carried the carcass of their calf for several days, until the body began to decompose. In addition, in three cases they did so for more than two weeks, despite the fact that their calf was already fully mummified.
In none of these three episodes did the mothers show any type of attention or care towards the corpse. Therefore, the authors of the study find it unlikely that the unconsciousness hypothesis is correct, since they did not treat their offspring as if they were alive. They also rule out the learning-on-death hypothesis, because there were four mothers who repeated the behavior, including three of them carrying their young for longer the second time.
First of all, hormonal tests are still needed to have more conclusive answers, but this study has provided interesting data. The authors are fairly certain that the mothers are aware of the loss, but continue to have a strong attachment to their offspring’s body and may experience something akin to human grief. The good news is that even this they recover from, or at least this seems to be the case with Guinea baboons. Taki has had another daughter named Tanuki and right now both of them will be running around Niokolo Koba.
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