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(A version of this story appears in CBS Atlanta)

African elephants are suffering from mass killings that took place decades ago, says a new study.

While they may appear normal to the average human, the experts say these elephants show clear signs of social disorders, similar to post-traumatic stress.

Wildlife officials often used culling as a management tool from the 1960s to the 1990s.

The managers worried that too many elephants would overrun and destroy the preserve.

Park officials would then send young male survivors to other facilities.

“Some of these elephants ended up in Pilanesberg National Park,” in South Africa’s North West Province where part of the new study was carried out, says Graeme Shannon, a behavioral ecologist at Colorado State University, Fort Collins, and the lead author of the new study.

The researchers compared the reactions of these elephants and families of elephants that had not been through culling to different social threats.

They drove a Land Rover about 100 yards away from a family group and broadcast an unfamiliar elephant’s deep-throated greeting call for 10 to 20 seconds.

When the researchers played the calls to the control elephants, almost every family responded appropriately to the rumbles of an older, unfamiliar female.

They usually grouped together to form a wall against the intruder.

But he Pilanesberg elephants appeared to act randomly, like they didn’t know how to respond.

In one extreme instance, a family left the area at once, traveling more than a kilometer before they came to a halt—but they did so in response to the call of an elephant they all knew. “Yet when they heard the call of the older, strange female, they did nothing at all; they just stayed completely relaxed,” Shannon says.

“You might think because of their history that they were just more accepting of strangers,” McComb says. “But it wasn’t that. They simply failed at picking out the calls of older, socially dominant animals.”

The scientists theorize that because the Pilanesberg elephants grew up as orphans, they never had the opportunity to learn elephant socialization from adults.

And that can cause problems for the elephant population because, like humans, they depend on social cues to facilitate reproduction.

The team’s findings may also apply to primates and some other species.

“It is a groundbreaking study, because it is the first to demonstrate, experimentally, a direct connection between the effects of culling and specific psychosocial harms,” says Lori Marino, a neuroscientist and expert on dolphin behavior at Emory University in Atlanta, who was not involved with the research.

Richard Connor, a biologist at the University of Massachusetts, Dartmouth, says, “it is difficult to not conclude that the legal killing or illegal poaching of elephants is not only inhumane, it is barbaric.”