Do dolphins feel grief?

When an adult striped dolphin emerged from the Mediterranean Sea in 2016 pushing, nudging, and circling the carcass of its dead female companion for more than an hour, a nearby boat of scientists fell silent. Afterward, the students aboard said they were certain the dolphin was grieving. But was this grief or some other response? In a new study, researchers are attempting to get to the bottom of a mystery that has plagued behavioral biologists for 50 years.

Grief, in humans at least, is a reaction to the permanent severing of a strong social or family bond. Although chimpanzees, baboons, and elephants are thought to experience the complex emotion, scientists don’t yet know enough about it in other animals. There are dozens of photos and YouTube videos of grieflike behavior in dolphins: Some mothers have been seen carrying their dead infants in their mouths or on their backs for a week or longer, even as the body decomposes; a couple adult males have also been seen holding dead calves in their mouths.

In the new study, cetacean biologist Giovanni Bearzi of Dolphin Biology and Conservation in Pordenone, Italy, and his colleagues at other institutions analyzed 78 scientific reports from 1970 to 2016 of these kinds of displays—which they labeled “postmortem-attentive behavior.” They found that just 20 of 88 cetacean (dolphin and whale) species engage in them. Of those, most were dolphins from the Sousa and Tursiops genera. Just one was a baleen whale—a humpback.

The scientists also found a correlation between grieflike displays and the cetaceans’ brain size and complexity; dolphins, which live in more structured social groups, generally have larger, more complex brains than baleen whales do. Though the correlation might simply reflect the fact that most studies focused on dolphins, it still suggests grieflike behavior may evolve only in animals with large, complex brains and societies, the researchers report this month in Zoology .

But is it possible for researchers to prove that any of the dolphins or whales are actually grieving? Jane Goodall and others have largely proved that chimpanzees grieve by collecting detailed accounts of death events. For instance, one young chimpanzee unable to cope with the death of his mother in Tanzania’s Gombe Stream National Park grew lethargic, refused food left by researchers, fell sick, and died 1 month later. Other scientists have identified grief in female baboons by analyzing their stress hormone levels before and after losing a close companion or infant.

But no such detailed records exist for cetaceans. So Bearzi and his colleagues say that, no matter what we may think these animals are feeling, the question of grief—and of their understanding of death—remains open.

“They are being appropriately cautious,” says Lori Marino, a marine mammal biologist at The Kimmela Center for Animal Advocacy in Kanab, Utah, who has studied cetacean neurology and self-awareness. Richard Connor, a cetacean biologist at the University of Massachusetts in Dartmouth, calls the study “interesting,” but adds that, from an evolutionary standpoint, “there’s no reason to think grief would be restricted to humans.”

The next steps may prove challenging. Bearzi and his colleagues say that when other scientists find dolphins and whales with their dead, they should put hydrophones in the water to record their calls and use drones to collect blowhole spray to analyze their hormones later.

But that might not help in all cases. One male oceanic dolphin, for example, was seen carrying a dead calf, accompanied by two female dolphins off the coast of Hawaii. No one knows whether the male killed the calf, which was thought to be the infant of the younger female. But by holding the body, he ensured the females stayed with him—a smart tactic, says Bearzi, should one of the females become ready to mate. In other cases, an animal might not be grieving but trying to determine why their companion isn’t reacting. That may have been the case with the event witnessed by Bearzi and his students.

At times, the living dolphin placed its chin on the carcass and pressed down on it. At the same time, it looked down at the body—as if watching for a response. But the adult was alone with the carcass for more than an hour, a dangerous move for a species that relies on its large pods for safety.

“Besides filming and observing, I didn’t know what to do as a scientist,” Bearzi says. “Maybe [additional] data will give us a better understanding about what is in their minds and if they feel grief. The bottom line now is: We do not know.”