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The capacity to grieve for dying loved ones was long considered a uniquely human ability. But now, researchers from Japan have shown that humans are less unique in this respect than once thought.

In a study in Primates, researchers from the University of Osaka showed that Japanese macaques exhibit human-like responses to the deaths of their adult companions.

Japanese macaques live in large and highly complex social groups, and their social behaviors have been studied for decades. Despite this, observations of the macaques' responses to death have been hard to come by, except for mothers who carried their dead infants for a while.

Researchers at The University of Osaka had several unprecedented opportunities to make detailed observations of the reactions of individual free-ranging macaques to their dying and dead companions. They showed that some macaques tended to remain near—and even repeatedly groom—the corpses of individuals with whom they had spent much quality time while alive.

"It's incredibly rare to document these kinds of behaviors in wild animals," says lead author of the study, Masayuki Nakamichi. "By tracking which individuals were near the deceased and recording their behaviors, we were able to show that their reactions to death were significantly influenced by social bonds."

Obvious injury or infestation seemed to be a strong deterrent to physical contact, whereas more subtle signs of decline were less likely to affect social interactions. In particular, many macaques clearly avoided the presence of maggots in both dying and dead individuals. However, the researchers recorded one case where social bonds triumphed over this aversion—one alpha female not only remained near her dying grooming partner, but also picked maggots off his back and ate them.

"The fact that some individuals remained near deceased companions they had close relationships with suggests that these macaques have emotional capacities that resemble human compassion," says Kazunori Yamada, co-author of the study. "It shows that strong social bonds can affect the behavior of macaques, even after death."

Whether social bonds affect the interactions with dying and dead companions in other non-human primates remains uncertain.

"More observations of death-related events in more species will be needed to fully understand the emotional responses of non-human primates to the dead," says Nakamichi.