Researchers have observed it in the wild many times: Primates from baboons to apes carrying around their infants after they've died. This happens regularly across non-primate species as well, but what's not clear is the motivation or reason behind it.

In the largest analysis of this kind of behavior in primates so far, a new study suggests that this infant corpse carrying (ICC) could be part of the grieving process, as far as we can tell. For example, there are links between the strength of the mother-infant bond and how long the carrying behavior continues.

The research looked at a total of 126 previous studies, covering 409 cases of primate mothers responding to the death of an infant. Of the 50 primate species included in the analysis, 80 percent of them showed some form of ICC behavior.

"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," says anthropologist Alecia Carter, from University College London (UCL) in the UK.

"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."

While it's difficult to ascertain whether or not primates actually understand that their infants have died, the research showed that younger mothers were more likely to carry dead infants and that traumatic deaths – such as accidents or infanticides – were less likely to lead to ICC.

ICC was observed most often in great apes and Old World monkeys, the two species that also carried their infants for the longest time after death, on average. Infants that died at a younger age – so assumed to have a stronger bond with their mothers – were carried for the longest.

Other factors taken into consideration, such as climate, didn't seem to have an influence, and the carrying of dead infants was by no means universal across the 409 cases. Some primates, such as lemurs, don't show ICC behavior but do sometimes return to the corpse and keep up mother-infant contact calls.

"Our study also shows that, 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," says biological anthropologist Elisa Fernández Fueyo from UCL.

As the researchers point out, our shared evolutionary history means that primate social bonds are likely to be similar to ours, but further study will be required to understand more about precisely what's going on here.

It's possible that early humans treated infant deaths in the same way as we see primates handling them here and that the rituals around death that we have in the modern day evolved from that point.

Now the team is looking for other links between humans and primates when it comes to thanatological behaviors. To that end, they've launched a website called ThanatoBase for researchers to record many more observations of how primates act around death – and how they might be feeling about it.

"Our study also has implications for what we know about how grief is processed among non-human primates," says Carter. "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."

The research has been published in Proceedings of the Royal Society B.