What’s fight-or-flight response?

When faced with a threat, most animals can defend themselves or run away – fight or flight. Both actions are potentially reckless because they involve movement, which would reveal a prey’s position to nearby predators, so many creatures will freeze first. But contrary to popular belief, that behaviour isn’t driven by being too frightened to move.


The fight or flight response is a response to acute (short-term) stress triggered by a threat to survival, typically a predator, and prepares the body for immediate fighting or fleeing.

What happens to the body during the fight or flight response?

Fight-or-flight is best understood in mammals. The brain sends signals telling the adrenal glands to secrete hormones such as epinephrine (popularly known as adrenaline) to cause changes to physiology, such as making the heart pump harder so the blood delivers oxygen and fuel faster.

Epinephrine prompts the liver and other organs to convert complex food molecules, such as carbohydrates or fat, into simple sugars that muscle cells can use directly to power speed and strength.

Why do animals freeze before the fight-or-flight response kicks in?

Because a threat might not be real. If prey jumped at everything that moved, they would waste valuable energy! Freezing allows an animal to gather evidence of possible danger while avoiding detection in case it needs to safely escape.

Freezing is an innate response, meaning that every member of a species is born with an ability to freeze at the sight, sound or smell of a potential predator.

What happens while they’re frozen?

The response is a ‘fixed reaction pattern’ that, like a reflex, often leads to automatic muscular activity. But the two kinds of reaction differ in their speed and duration. Reflexes are influenced by the intensity of a stimulus – the louder the noise, the more rapid the reflex, for example. Incredibly, in rodents, the flinch-like auditory ‘startle response’ is triggered in under one-hundredth of a second.

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By contrast, the freeze response isn’t determined by a stimulus’ intensity and freezing behaviour lasts anywhere from seconds to 30 minutes. During that time, the brain will process information from separate neural circuits and release neurotransmitters such as acetylcholine and norepinephrine – molecules that have an opposite effect to epinephrine, keeping an animal calm by slowing its breathing and heart rate while also sharpening its mind for rapid decision-making if the sensory inputs confirm that a threat is indeed genuine.

Is freezing caused by fright?

No. Although neural circuits for freeze and fright are located in the same brain region, the amygdala, they’re distinct responses. The idea that a rabbit or deer in the headlights is paralysed by fear is a common misconception – prey are often hit because the animal is blinded by brightness and an oncoming car ‘attacks’ at an unnatural pace compared to, say, a fox or wolf.

Unlike innate responses, fear is programmed by experience. An animal learns to be afraid following physical pain or mental trauma, which means the brain must load past events into memory and contrast them with present information before it can forecast future outcomes. Such (conscious and unconscious) deliberation is too slow for life-or-death scenarios! That said, emotions do modify reactions through learning: just as physiologist Ivan Pavlov conditioned dogs to salivate when a bell was rung, survival circuits can be trained to respond to threats.

Okay, why so many F-words?

Like comic-book writers who name their characters Clark Kent or Peter Parker, biologists love alliteration! Adding to the list, some animals faint or feign death to trick predators that prefer a fresh kill. And besides words beginning with F, creatures such as chameleons and cuttlefish can change colour to conceal themselves through camouflage.

Immobile organisms can’t run from danger, obviously, but many are still able to respond in different ways. After an insect starts eating a plant, the injury triggers a release of glutamate in the plant (a neurotransmitter in animals), prompting cells to produce molecules that are toxic, taste unpleasant or repair damage – basically a fight-or-fix response.


Main image © Getty Images


JV ChamaryScience communicator

JV Chamary is an award-winning journalist with a PhD in evolutionary biology. He writes 'The Big Question' column for BBC Wildlife, and spent several years as the features editor on BBC Science Focus.