Why Some Primate Moms Carry Their Babies After They Die

Mothers in some non-human primate species may express grief over the loss of a baby by carrying their infants around with them for months, a new study finds.

Researchers have been divided about whether primates and other animals are aware of death and experience grief. But these new findings suggest that primates are able to have an awareness of death.

“The field of comparative thanatology, that specifically wants to address these questions, is relatively new. However, scientists have been speculating for some time about primates' and other animals' awareness of death,” study co-author Alecia Carter, a lecturer in evolutionary anthropology in the Department of Anthropology at University College London, tells Treehugger.

“There have been some suggestive studies addressing grief in animals, too, and new progress in neurobiology that behavioural scientists are starting to catch up on now.”

Thanatology is the scientific study of death and the psychological mechanisms used to cope with it.

For their work, researchers studied 409 cases of maternal responses to their infants’ deaths in 50 primate species. They compiled data from 126 different studies on primate behavior to analyze a behavior known as “infant corpse carrying.”

The results were published in the journal Proceedings of the Royal Society B.

Carter says she first saw the behavior years ago and it made an impression on her.

“I was so shocked the first time I witnessed a baboon carrying a dead infant over a decade ago, but I was told that this was a common behaviour, so at the time I didn't pursue it further,” she says.

Her research became progressively more focused on cognition.

“In 2017 I watched individuals who were not the mother responding to an infant's corpse in baboons, and this made me even more curious about mothers' motivations after reading the literature.”

Species and Age Matters

Researchers found that 80% of the species they studied performed corpse-carrying behavior. Although the behavior was well-distributed, it was most common in great apes and Old World monkeys. These species carried their infants after death longer than any other.

Some primate species that diverged a long time ago—like lemurs—didn’t carry their infants after death. Instead, they showed grief in other ways, such as visiting the body and calling out to the infant.

Other factors also were found to have an impact on how likely they were to carry their babies after death.

“Whether a mother will carry her infant or not depends on how the infant died and the age of the mother,” Carter says. “[Mothers of] infants that die of traumatic causes, such as being killed by another group member or in an accident, are less likely to carry the infant's corpse. Older mothers are also less likely to carry.”

The length of time mothers carried their babies’ bodies depended on the strength of their bond, which usually was determined by the age they were when they died. Mothers carried infants longer when they died at very young ages, while there was a significant drop when the babies reached about half weaning age.

Processing Death and Grief

The authors say that their results suggest that primates might need to learn about and process death in similar ways that humans do.

“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,” Carter says. “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.”

Cater points out that human mothers who have a stillborn baby are less likely to experience severe depression if they are able to hold the baby and 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.”

Researchers are working to understand why primate mothers carry their infants’ corpses.

“At this point in time, with the evidence we have, I suspect that a large part of it is the strong mother-infant bond in mammals and the long duration of dependency that primate infants (and some other mammals) have,” Carter says.

“Although it's still speculative, it seems that carrying behaviour can be compared to human grief, though we need more data to really know. Speaking of closure is difficult given that this could vary for people. But I do think that some mother primates need some time to sever the strong attachment they have to their infant.”

The study could have important ramifications in many areas, the researchers say

“These findings have implications for broader debates over animal cognition, the origins of grief and awareness of death, and, by extension, the ethical standing of animals in society,” Carter says.

“Should we treat primates differently if we know that they grieve for the loss of a closely bonded individual in a similar manner to the way we do? In practice, if primates are to be kept in zoos, our results suggest that corpses should not be immediately removed if mothers are to 'process' the loss.”