The Evolution of Stress: How Predator-induced Fear Might Explain the Origin of PTSD
On the hunt: a cheetah chases a gazelle. PTSD-like changes may occur in the brains and behavior of free-living wild animals that regularly experience life-threatening predator encounters
A former soldier, when she falls asleep, finds herself back on the sandy roads of the battlefield. She can see the yellow dust hanging in the air, hear the hoarse rattle of machine guns. When she awakes, it is no better; a sense of panic and claustrophobia overwhelms her. She does not know the trigger that set it off; perhaps it is the clatter of firecrackers from last week’s celebrations or the shrill daytime noises she hears from her neighbor’s apartment. It is as though she has brought the battlefield back home with her.
Trauma has a curious way of sticking to the mind. When organisms are faced with life-threatening danger in their environment, a chain of rapidly occurring reactions mobilizes the body’s resources to deal with the threat. These reactions – termed the ‘fight-or-flight response’ – serve an important biological function as they enable us to cope efficiently with fleeting danger. For example, an encounter with a growling dog on your morning jog would lead to increased blood flow to your muscles, providing you with extra strength and speed to help you flee. However, for some individuals, the traumatic experience may prompt longer-lasting psychological effects, which can lead to the development of post-traumatic stress disorder or PTSD.
Described initially in response to American soldiers returning home from the Vietnam war, PTSD is a psychiatric disorder that can occur after a person has been exposed to a traumatic event such as warfare, child abuse, sexual assault, or other life-threatening incidents. Although most people who experience such events recover from them, people with PTSD continue to relive this trauma through flashbacks, nightmares, emotional numbness, and general psychological distress. In recent years, there have been converging interests in the disciplines of biomedical science and ecology in an effort to establish whether the changes that appear in the brain and behavior of people with PTSD also occur in wild animals in response to predator-induced fear. From a research perspective, describing PTSD in wild animals may show that the disorder is not an ‘unnatural’ or ‘maladaptive’ dysfunction in humans, but rather the modern-day cost of inheriting an evolutionarily primitive mechanism meant to promote survival and reproduction.
A recent study published in Scientific Reports conducted by Michael Clinchy and colleagues at Western University tested whether exposing wild birds to the sounds of their predators would induce symptoms similar to PTSD in humans. Thus, wild birds were exposed to natural cues of predator danger (predator vocalizations) of varying intensities for up to 2 days. As an enduring effect on the behavior of birds can affect survival, these animals were then housed outdoors and allowed to flock for the next 7 days following the predator cue exposure to determine if the effects were measurable after a week of natural experiences. The study found that birds previously exposed to predator-cues showed an enduring memory of fear (a critical component of PTSD diagnosis), making them more alert to subsequent encounters with predator danger than unexposed birds.
Remarkably, this heightened alertness to predator danger was manifested in the persistent activation of neurons in two important brain regions: the hippocampus, which plays a central role in learning and memory, and the amygdala, which is involved in emotional responses such as fear, anxiety, and aggression. A major behavioral consequence of ‘over-activation’ in these brain regions is that exposed wild birds were better able to remember the predator sounds and become more alert to subsequent ones. Importantly, this long-lasting memory of fear and heightened sensitivity to threatening cues represent behavioral changes similar to those experienced by people with PTSD.
These findings present significant implications for free-living wild animals; an increased sensitivity to predators is known to impair parental care and reduce offspring survival. As free-living wild animals regularly experience life-threatening predator encounters (nearly 32% of adult female giraffes bear scars from lions, 25% of harbor porpoises bear claw marks from seals, and all manta rays bear multiple bite wounds from sharks), the long-lasting biological and behavioral effects that occur following exposure to predator cues may suggest that wild animals carry more than just physical scars when they escape predation.
However, it is worth noting that prioritizing hypervigilance to predators at the cost of poorer parental care and reduced quality of life may be more beneficial for the long-term evolutionary success of an organism. For example, if a wild animal fails to avoid predation, this could entail the death of most or all of its offspring, as well as a complete incapacity to produce any more. On the other hand, the emergence of hypervigilance and other PTSD-like symptoms in the parent in response to predator-cues may be less evolutionarily costly than the alternative, despite potentially leading to reduced offspring survival and reproductive success.
As humans now mostly live without significant risk of predation, the costs of maintaining this evolutionary mechanism far outweigh the benefits. In other words, evolution did not foresee that we would one day live in largely safe environments, unburdened by the constant fear of predation. Consequently, although PTSD symptoms of heightened alertness and a long-lasting memory of fear may have benefitted ancient human populations vulnerable to attacks by leopards, crocodiles, and other predatory threats in their immediate environment, today the appearance of such symptoms in humans largely entails a reduced quality of life.
Nonetheless, the knowledge that PTSD is an evolved response to threatening events could provide important therapeutic benefits for humans. Given the troubling statistic that 9.2 percent of Canadians will suffer from PTSD in their lifetimes, evidence that PTSD-like symptoms are the cost of inheriting a primitive evolutionary mechanism is at the heart of recent psychotherapeutic efforts to treat the disorder. Compassion-focused therapy, a branch of psychotherapy that aims to alleviate the sufferer’s shame by centering PTSD symptoms in the context of its evolutionary functions, has been shown to help promote mental and emotional healing in people with PTSD. The empirical linkage between animal model studies of PTSD and ecology may also help generate more discourse and interdisciplinary research between the fields of ecology and biomedical sciences. Given that we now live in highly anomalous environments free of predation risk, such a research field could help reveal the evolutionary origins of other diseases that occur in humans but are as a result of evolutionarily adaptive responses related to our primitive past.
Original research article
Zanette, L.Y., Hobbs, E.C., Witterick, L.E. et al. (2019). Predator-induced fear causes PTSD-like changes in the brains and behaviour of wild animals. Scientific Reports 9, 11474.