In many primate species, mothers have been observed carrying their dead infants – often long after death. The largest study yet of this behavior indicates non-human primates have a partial awareness of others’ deaths, but factors such as the strength of the mother-infant bond also influence behavior.
Whether we bury our dead or burn them, funeral practices are a universal feature of human culture and our grieving process. Anthropologists are keen to understand their origins, and in the journal Royal Society B: Biological Sciences, primatologists provide some answers. The researchers looked at 409 reports of mothers responding to the loss of an infant, spread over 50 species.
The heart-rending behavior of primate mothers carrying deceased infants (infant corpse carrying, or ICC) is the most commonly reported response to the death of a member of the same species. As the paper notes, ICC appears to be evolutionary maladaptive, since it interferes with foraging for food or escaping predators.
Last year, researchers reported that ICC can last up to ten days in Namibian chacma baboons. Sometimes, however, this carrying period is much shorter – as little as an hour.
One explanation for ICC is that primate mothers don’t always realize their child is dead. However, the researchers found the baboons treat infant corpses differently from live young, casting doubt on the idea.
Lead author of last year's study, Dr Alecia Carter of University College London, is co-author of this new paper on ICC, seeking patterns in previously published cases across all non-human primate species.
“Our study indicates that primates may be able to learn about death in similar ways to humans: it might take experience to understand that death results in a long-lasting ‘cessation of function’, which is one of the concepts of death that humans have,” Dr Carter said in a statement. “What we don’t know, and maybe will never know, is whether primates can understand that death is universal, that all animals – including themselves – will die.”
ICC was reported in 80 percent of the species studied. Our nearest relatives, the Great Apes, along with Old World monkeys are more likely to carry corpses, and do so for longer, the paper reports. ICC was not seen among lemurs, on the other hand, but they will return to check on a dead body and call to it.
Infants were carried for longer if they died at ages when the mother-child bond is considered strongest, and younger mothers are more likely to carry their young.
Perhaps the most intriguing finding is that ICC is greater when infants were premature or died from illness rather than accident or infanticide, despite the dangers if the infant’s disease was infectious.
“Through experience with death and external cues, primate mothers may gain better awareness of death and therefore ‘decide’ not to carry their dead infant with them, even if they may still experience loss-related emotions,” said co-author Elisa Fernández Fueyo. When an infant dies from internal causes, or the mother is young and naive, the mother may be uncertain of their death. This implies they are aware in other cases.
The data contradicts the hypothesis that ICC correlates with carrying young, rather than "parking" them (leaving the infant somewhere safe).
“Our study also has implications for what we know about how grief is processed among non-human primates,” Carter said “It’s known that human mothers who experience a stillbirth and are able to hold their baby are less likely to experience severe depression, as they have an opportunity to express their bond. Some primate mothers may also need the same time to deal with their loss, showing how strong and important maternal bonds are for primates, and mammals more generally.”