Humans who have been mauled by big cats or suffered other huge injuries often report that it isn’t painful at the time, because adrenaline has an anaesthetic effect. The pain comes about 20 minutes later – but by then the prey animal is dead.

This makes sense from an evolutionary perspective. You don’t want to be handicapped or distracted by pain while you are fighting for your life, but if you survive then the pain kicks in to tell you to keep still while your injuries heal.

Reason 1: self-anesthesia.

Pain is the signalling response of our nervous system that allows us to avoid damaging ourselves or aggravating the existing trauma. And like most other functions in our body, it is subject to intricate control so that it always works in the best interest of the organism.

So, certain hormones in our body, such as endorphin and adrenaline, can stiffen the perception of pain. Naturally, animals have evolved to release them when feeling pain is unnecessary or detrimental.

You probably know what sex is: a very vigorous activity that could easily aggravate one’s traumas. However, it’s well-known that sexual arousal suppresses the pain, and most animals, including humans, will gladly engage in sex despite having minor traumas. Apparently, leaving offspring is more important than letting one’s leg heal properly, so evolution has ensured that the hormones are released during sexual arousal.

This example is illustrative of the overall trend: in every situation when getting a minor trauma seems evolutionary favourable to the whatever alternative, you would expect the anesthetic hormones to be released.

As Claire Jordan has explained, feeling pain while being attacked by a predator is evolutionary detrimental: getting away from the predator at the cost of aggravating the wounds is better than being eaten, and feeling acute pain then would only be a distraction. Not to mention that unbearable pain could literally kill you, and thus only facilitate the predator’s “job”.

Reason 2: acclimation AKA long-term self-anesthesia.

I have a kickboxer friend who says that being hit used to hurt the most during his first year of training, and now, after seven years of doing kickboxing, he barely notices being punched.

Our bodies acclimate to ignore repeated stimuli, including pain: the receptors become less sensitive, and the brain learns to process the signals in less “alarming” manner. The reason is the same as with hormone release: if an animal is being subject to the same kind of damage throughout its life, then avoiding it is probably impossible, and pain is a distraction.

Unlike humans, wild animals live in generally more hostile environment, and frequently get bruises and minor injuries. Thus, an average wild animal, especially the one that faces predators on regular basis, would’ve acclimated to ignore a good deal of pain. That’s why our ancestors that lived thousands of years before the rise of modern human behaviour were likely less sensitive to pain — despite being the same species as us.

To us, being hit is an exception; to them, it was a rule.

Reason 3: adaptation AKA evolutionary self-anesthesia.

Some species, due to their lifestyle, experience more damage than others, to the extent that the individuals with lower pain threshold are less likely to survive because of being constantly distracted by pain.

In this case, a sort-of acclimation on the scale of entire species occurs, when the traits that help to ignore pain become selected for and hard-wired into the DNA. It’s called adaptation in evolutionary biology.

For example, birds on average have way higher pain threshold than mammals: because they frequently bruise and cut themselves when flying through branches and cannot afford to be distracted by pain in mid-air. One of my fellow ornithologist even claims that some birds don’t react to syringe injections or even minor surgeries without anesthesia.

Reason 4: different pain response.

Finally, the most interesting point that people often ignore.

What you need to realise is that “screaming in agony” is neither the definition of feeling pain nor some universal pain clue. Screaming is merely a particular behaviour that, for evolutionary reasons, was attached to pain in some species.

When people are hurt, they scream. They wave hands. They flinch. But they don’t do it because those are the embodiment of agony — but because those make sense. Flinching, waving hands and screaming is helpful when being attacked because:

• It may scare the predator away.
• It makes harder for the predator to target a vital spot.
• It may accidentally hurt the predator.
• It alerts other members of the pack.

We don’t freak out when being attacked because “that’s what pain is supposed to look like” — we freak out because our ancestors who did so had higher chances of survival.

In many animals, a response similar to ours makes sense. But not in all animals.

What do you think you should’ve done if you were a tortoise under attack? Or a leaf-tailed gecko? Or a stone fish? Or a hedgehog?

Those animals don’t scream when being hurt. Neither they run away. Sit low and tight is what they do, because it makes sense.

No, it doesn’t mean that they don’t feel pain — it just means that different neural pathways got attached to pain centers in their brains. Whenever a tortoise is hurt, just like you irresistibly and reflexively want to scream and get away from the source of pain, it irresistibly wants to pull its head and limbs inside and sit still. Sitting still and doing nothing is its way of screaming in agony, and the more it hurts, the more still it will be, same as the louder a human will scream.

Never judge an animal’s subjective perceptions by its behavioural response: it’s very misleading and just plain wrong. A lot of animals might do entirely different things when being hurt, sometimes the opposite to what we would do.