Creature Feature: Fight Or Flight Response
Most people are aware of this process in some fashion. Fight-or-flight, or fight-flight-freeze-or-fawn comes from a phsyiological response to a perceived threat or attack. It involves a number of physiological responses within the body in response to danger. This enables the animal to fight or flee.

It goes by other terms, as some animals do not respond by fighting or fleeing, but instead freeze, faint or fawn. So the more neutral terminology is “hyperarousal” or the “acute stress response” as the outcome varies among different species. So let’s take a look at the physiology behind it.
Autonomic Nervous System
The nervous system of animals has different components, and the one we’re interested in here is the autonomic nervous system. This system controls various bodily functions, usually unconsciously, in other words without you having to think about it. This includes things like the heart rate, digestion, respiratory rate, pupil dilation and other functions.
The autonomic nervous system is divided into the sympathetic and parasympathetic nervous system. The parasympathetic nervous system is the calm part, reducing hert rate and stimulating the digestive system: known as the “rest and digest” response. The vagus nerve is an important part of this system.
The sympathetic nervous system is the fight-or-flight part, and is stimulated by a perceived threat. It is activated by signals from the hypothalamus, in the brain, and a number of things happen. The heart rate increases, blood pressure and respiratory rate, improving blood flow to the muscles, heart and brain.
This allows a higher output from the heart to give the body more energy, causing release of glucose (sugar) from the liver to meet increased energy demands. Breathing rate is increased to supply extra oxygen to the body. Even things like blood clotting are increased to help prevent loss if an injury is sustained. The pupils dilate to let in more light and improve vision. Trembling comes about from increased muscle tension to give speed and strength.
Adrenaline Rush
The sympathetic nervous system also causes the release of hormones known as catecholamines. These include adrenaline and noradrenaline. You may hear epinephrine or norepinephrine – these are alternative names for adrenaline and noradrenaline. That’s what is in an Epi-pen used to treat acute allergic reactions.
This system is rapid, but begins in the brain. The amygdala triggers the hypothalamus, which then activates the pituitary gland. This gland is responsible for controlling most of the hormones of the body, the endocrine system. The pituitary gland releases ACTH – adrenocroticotropic hormone, which acts on the adrenal cortex. So you can see where the name comes from.
The adrenal cortex is part of the adrenal glands, a pair of small glands that are found just around the kidneys. The adrenal glands then release adrenaline and noradrenaline which acts upon many parts of the body to further promote the fight-or-flight responses. It increases blood flow to the muscles, boosts cardiac output and blood sugar levels.
The adrenal glands also produce and release cortisol, a glucocorticoid hormone that has a number of effects designed to help the body cope with stress. Cortisol increases blood pressure, blood glucose levels and acts as an immune suppressant. This increases the amount of energy available to the body, especially the muscles.
Hysterical Strength and Adrenaline Junkies
For humans, the fight-or-flight response is highly effective when faced with a bear, an axe-murderer or a meeting with your boss. That’s the issue with this system. It works great for animals and certainly ensured our species continues to exist. It is a primal system, helpful for dealing with predators or actual threats to our lives.
Indeed, there are situations where people have committed impossible feats due to the power of adrenaline and the sympathetic nervous system. ‘Hysterical strength’ is a display of extreme physical strength in life or death situations. There are many stories of people lifting cars and vehicles to safe people pinned underneath them.
Of course, humans being humans, we can find joy and pleasure in all kinds of things. That includes adrenaline, and you have almost certainly heard of the term ‘adrenaline junkie’. Adrenaline can activate the brains reward system such as releasing dopamine. Dopamine is a neurotransmitter (chemical messenger) that makes us feel good and increases motivation.
For a lot of people, they enjoy this kind of thing. Tame versions include enjoying rollercoasters, watching horror movies or playing scary games. Some people do more extreme things like risky sports and committing crimes to get a rush of adrenaline.
Feeling Anxious?
The dark side to this is anxiety and anxiety disorders. The fight-or-flight system is great for physical dangers like predatory animals, but it doesn’t discriminate between a tiger and a job interview. It’s normal to feel a bit worried about stuff like that, and a little bit of anxiety can help keep you focused. Things like interviews, exams, meeting a potential romantic partner for the first time.
The problem is this system, when active for a long time, can become damaging to the body and mind. People with anxiety disorders are in fight-or-flight mode all the time, or a lot of the time. Panic attacks, stomach churning and gastrointestinal upsets, dizziness, heart palpitations (increased heart rate), changes to toilet needs and habits, sweating and restlessness, insomnia and feeling a sense of impending doom. It can be very scary, people experiencing a panic attack can worry they’re having a heart attack.
Cortisol, the stress hormone can have an impact on mental health as it impacts emotional processing and cognitive functions. A lot of people in the ‘wellness’ industry seem to be blaming cortisol for everything. It is a hormone that can be problematic in excessive amounts, but as always I advise you take the advice of your own doctor. At least a qualified medical person, rather than some random people on the internet.
Playing Possum
All kinds of responses are seen in the animal world when the acute stress response comes into play. The way they react will depend on the species, the threat, the environment and other factors.
A lot of small prey species will try to run if they have the opportunity to do so but will fight if they have no escape route available to them. Some animals will freeze to avoid being spotted: rabbits are known for this, as are deer. The classic ‘deer in the headlights’ cliché comes from this.
Tonic immobility is another strategy. Rabbits will do this when put on their backs, and they will lie perfectly still and appear dead. Do not do this to a pet rabbit, as an owner or as a veterinary professional. This is very stressful for the rabbit and they will still feel pain and fear in this position.
‘Playing possum’ or playing dead is the same thing. The Virginia opossum being the most famous example of this, but many other species do this including snakes, frogs, birds and insects. Usually this strategy is to avoid predators but some species do seem to use it to attract prey or as part of mating behaviours.

As discussed before, changing colour is a strategy used to hide or evade predators and thus can be affected by the fight-or-flight response. It’s a complicated system but almost all animals have some form of it. Almost all bodily functions are part of the ‘survival of the species’ needs: living and reproducing.
Physiology of Fear
I know physiology can be a complex topic to get your head around. The body has so many systems, checks and balances and hormones. So many hormones! Physiology was one of the hardest subjects I studied at university, and one of the toughest exams I’ve ever taken. Except for pharmacology which is physiology times ten.
Hopefully this helps explain it all a bit easier for you. When considering different animals and their behaviours, knowing the how and why can be useful. When an animal is likely to flee, or try to fight. What animals will freeze or play dead. Even things like tail autotomy – where animals drop an appendage like their tail to distract a predator. This is seen in several lizard species, salamanders and tuataras. It is very weird to see a disembodied tail keep moving for a while: common pets such as iguanas and leopard geckos can do this.
If you are creating your own creatures, then it’s worth thinking about how they would respond to danger. Are they aggressive and likely to fight, or will they flee and try to hide? For creatures that already exist, either in real life or in accepted cryptozoology themes, remember that unusual behaviour could break the suspension of disbelief.
Thanks for reading. Next month, we’ll be discussing some medical terminology and grammar of plurals, so if you want to see more, you can subscribe below. Follow me on my social media channels for more content and behind the scenes stuff.
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