PTSD: The evolutionary advantage

PTSD via Jesper Sehested

A third of people who survive traumatic events experience post-traumatic stress disorder (PTSD). PTSD survivors involuntarily and vividly relive their trauma through flashbacks and nightmares. To escape these painful reminders, they try to avoid any potential triggers, and they are constantly on edge. This has devastating consequences on their quality of life. Jacqui, a rape survivor and founder of PTSD UK, explains: “[My PTSD] left me with debilitating, exhausting, uncontrollable fear. … Fear of everything, and unable to do almost anything.”

For wild animals, life-threatening encounters with predators are a fact of life. All studied manta rays, for example, have multiple bite wounds from sharks. Until recently, the consequences of such encounters were thought to be rapid and short lived — the so-called freeze, fight or flight response. Once the danger had passed, the animal was thought to continue about its business unaffected. 


PTSD survivors involuntarily and vividly relive their trauma through flashbacks and nightmares.

A study recently published in Nature shows, for the first time, that exposure to predators has long-lasting effects on the brains of wild animals, demonstrating that they too, experience PTSD. 

At the University of Western Ontario’s Advanced Facility for Avian Research, Liana Zanette and her collaborators caught wild black-capped chickadees. For two days, the birds were exposed to either predator or non-predator vocalizations — to simulate a life-threatening situation — following which they were housed on the roof of the facility for seven days. 

The study yielded two crucial results. First, exposure to danger affected the birds’ subsequent behaviour in similar situations. After a week, the birds were exposed to ‘high zee’ alarm calls — the sounds other chickadees make to warn of impending danger. Birds that had been previously exposed to predator vocalizations were much more fearful than their peers. They were immobile and vigilant (the ‘freeze’ response to danger) for six times longer than the control birds. This showed that the birds had a long lasting, heightened sensitivity to danger in general, rather than specific predators. Previous experiments showed that this heightened sensitivity affects the survival of offspring. Because of the increased time they spend vigilant, adult birds have less time to search for food and feed their chicks. Like humans, their quality of life is affected by their experience. 

Second, changes to the birds’ brain chemistry were still measurable one week after the danger. The researchers measured the levels of ∆FosB, a well-established marker of long term neuronal activation in the amygdala and hippocampus of the birds. Broadly, these areas of the brain are involved in the acquisition and processing of emotional memories. In particular, the amygdala is a crucial component in the response to life-threatening, fear-provoking experiences. Birds previously exposed to predator sounds had much higher levels of ∆FosB than controls, showing that exposure to danger caused persisting neurobiological effects. 

“These results have important implications for biomedical researchers, mental health clinicians, and ecologists,” comments Zanette. “Our findings support both the notion that PTSD is not unnatural, and that long-lasting effects of predator-induced fear with likely effects on fecundity and survival, are the norm in nature.” 

Indeed, the symptoms of PTSD are consistent with a hyperactive rather than dysfunctional amygdala. In a review paper published in 2016, David Diamond and Phillip Zoldaz compare PTSD to a bull in a china shop: “A raging bull confined within a china shop can cause immense destruction as it attempts to escape, but we do not consider the bull or its behaviour to be dysfunctional.” The bull is simply trying to escape from a threatening environment. 

In other words, PTSD seems to be an evolutionarily conserved response to life-threatening danger. This makes intuitive sense: being on high alert for danger makes us more likely to survive such experiences in the future. However, it changes our personality and behaviour in the process.Timothy Fong, M.D, provides the testimony of a patient deployed to Afghanistan, who witnessed the death of a comrade: “At that point, all he cared about was getting his best friend and himself home alive, even if it meant killing others. His personality changed, he said, from that of a happy-go-lucky farm boy to a frightened, overprotective soldier.” 


“Our findings support both the notion that PTSD is not unnatural, and that long-lasting effects of predator-induced fear with likely effects on fecundity and survival, are the norm in nature.”


– Liana Zanette

Diamond and Zoldaz explain: “the misery caused by a hyperfunctional amygdala in PTSD is the cost of inheriting an evolutionarily primitive mechanism that considers survival more important than the quality of one’s life.” In fact, Zanette and her colleagues suggest that the only reason PTSD seems unnatural to us now is because as a species, we have progressively eliminated our predators.

By providing the evolutionary context for their symptoms, this study helps patients with PTSD understand their disorder, hopefully alleviating some of the stigma that surrounds it. Currently, the most effective treatment for PTSD is psychotherapy. This helps patients to process their experiences, recover a normal level of amygdala activity and, eventually, calm the bull in the china shop. After her last therapy session, Jacqui, the founder of PTSD UK, recounts feeling a sense of calm: “I walked out of the room – I wasn’t ecstatic, I wasn’t jumping for joy at getting rid of PTSD, I was just… well… me!”

Written by Helena Cornu and edited by Miles Martin.

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