The same pattern has been shown in wild mice and in fish living with high levels of predator threat. These neurochemical signals parallel those seen in rodent models of PTSD that researchers have long used to understand the syndrome in humans.

Despite the mounting evidence that a wide range of animals experience long-term impacts of extreme stress, many psychologists still see PTSD as a uniquely human problem. “PTSD is defined in terms of human responses,” says David Diamond, a neurobiologist at the University of South Florida. “There is no biological measure — you can’t get a blood test that says someone has PTSD. This is a psychological disease, and that’s why I call it a human disorder. Because a rat can’t tell you how it feels.”

Some researchers now disagree with this human-centric view of PTSD, however. “A lot of things are shared between humans and other mammals,” says Sarah Mathew, an evolutionary anthropologist at Arizona State University. This includes learning about and responding to danger, and avoiding situations that present life-threatening risks. Mathew believes that PTSD has deep evolutionary roots, and that some of its symptoms arise from adaptations — like a heightened state of alert — that allow individuals of many species, including our own, to manage danger.

This evolutionary perspective is beginning to change minds. Clinchy and Zanette have organized conferences on the ecology of fear and PTSD that bring together ecologists, psychiatrists and psychologists. “The psychiatrists and psychologists were talking about PTSD as maladaptive,” recalls Clinchy. “We were arguing that this is an adaptive behaviour, to show these extreme reactions in this particular context, because that increases your survival.”

Diamond came to agree. The brain of someone with PTSD, he says, “is not a damaged or dysfunctional brain, but an overprotective brain”.

“You’re talking about someone that has survived an attack on his or her life,” he adds. “So the hypervigilance, the inability to sleep, the persistent nightmares that cause the person to relive the trauma — this is part of an adaptive response gone awry.”

“There’s a stigma involved in PTSD, frequently,” says Zanette, “so people don’t seek treatment. But if patients can understand that their symptoms are perfectly normal, that there is an evolutionary function for their symptoms, this might relieve some of the stigma around it so that people might go and seek treatment.”

* This article originally appeared in Knowable Magazine, and is republished under a Creative Commons licence.

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