Halloween is right around the corner and whilst this spooky holiday brings a welcome chance to tip the fear-o-meter for us humans, not many other species seem to enjoy being afraid like we do—as far as we know.

Today’s version of Halloween—the one with costumes and trick-or-treating—is far different than the 2,000-year-old Celtic festival of Samhain it originated from, but one element remains unchanged. Fear is, and always has been, the name of the game.

Even in ancient times, autumn and the onset of winter were associated with death; a time when the souls of the dead look to reclaim a spot amongst the living. That means the living needed to try and fit in with the roaming ghosts, donning costumes and offering food in exchange for not being haunted. Humans are a pretty odd lot really, which accounts for some of this bizarre tradition’s persistence and growth over the centuries, but certainly not all of it. So what’s the motivation behind scaring ourselves?

While we might not all love the response, fear is a universally shared state. The ability to be afraid is after all, crucial to survival. That means that though each species in the animal kingdom handles being afraid a bit differently, a whole lot of the basics are incredibly similar.

‘We think a lot of the fundamental elements of the fear process are quite well conserved, or similar, amongst many different species,’ says Bo Li, a neuroscientist at the Cold Spring Harbour Laboratory in New York. ‘Though obviously over time adaptations or adjustments to these systems in species with more complex or developed brains have occurred.’

Because fear is something so common to the experience of being alive, researchers are realising there’s likely a lot we can learn about how humans handle fear by tracing the responses’ evolutionary history. Here are some of the knowns and unknowns of the fear-system, and a few instances of when the process goes haywire.

Brain, take the wheel…

Physiologically, fear responses are triggered when our sensory organs detect alarm signals in the environment and send a warning message to a part of our brain called the amygdala. Always on the lookout for trouble, the amygdala processes these signals and if it deems danger could be lurking, it stimulates the autonomic nervous system into flight or fight mode.

The kidneys produce hormones like adrenaline that rapidly impact the entire body, the result of which we’ve all experienced. The racing heartbeat as our muscular-organ ramps up action to deliver more oxygen to muscles that could be sparked into a sprint at any moment. Don’t forget the cold sweats too, as your body gets ready to fight or run away from whatever’s lurking in the shadows.

Luc Arnal studies speech processing, and in his hunt for the origin of vocalised language, he’s come to realise the scream may be this starting point.

‘It is the first vocal signal used by newborns across all culture, biologically relevant to our survival as long as we’ve existed as a species, and is an intensely primitive and innate form of communication,’ says Arnal.

Arnal has found that there’s something in the acoustic range employed by the scariest screams—between 30 and 150 Hz—that hits the human fear sweet-spot. Asking people to judge the fright factor of prerecorded shrieks of all varieties from Youtube clips, movies, and from volunteers in his lab, Arnal uncovered that the scariest screams exploit an attribute dubbed ‘roughness’, something not used in regular day to day speech. That make senses, explains Arnal, by using a different range than everyday speech, screams can easily be distinguished from other regular noises.

Looking at other distress signals, it turns out many artificial warning calls, like car and house alarms use this same rough frequency range already—their inventors stumbling upon the attribute by trial and error.

Next Arnal wants to check out baby screams, to see if they’re particularly ‘rough’, he also wants to look for a common scream-ancestor in the animal world.

‘My guess is we share this attribute not only with other mammals, but very likely also birds and other life forms we haven’t even considered,’ says Arnal.

‘It’s very rare to have such a widely shared innate process to explore—and exciting.’ This similarly isn’t random chance—it’s a testament to the importance of fear as a survival tool.

Evidence of animal fear

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Wild species have just as much reason, usually even more so than us humans, to have a fine-tuned fear response. But whether these responses are simply an appropriate reaction to a stimulus, or the reflection of an emotional state, is a question still very much up for debate.

It may be easy to tell when your pooch gets freaked out by fireworks, an unknown visitor, or being left alone—but how do you know if a snake is scared?

David Anderson and his lab-mates are curious about the puzzle pieces that fit together to craft emotions, studying how Drosophila melanogaster, the common fruitfly, responds to fearful stimuli.

‘We think a useful way to study the mechanisms underlying emotions are to look for their building blocks, or emotional primitives,’ says Anderson.

Exposing flies to a series of variously paced and spaced out overhead shadows, the team found the tiny insects displayed primitives that they think are analogous with the fear response.

‘What struck us was that the fly’s reaction to the overhead shadows was not a one-off, all or none response,’ says Anderson. ‘Instead their response grew with the intensity and spacing of the shadows we subjected them to.’

Feeding flies would disperse from a food source more immediately and avoid returning for a longer interval with more intense shadow-sequences. Freaked out fly-tricks like popcorning, or hopping, also increased with the more frequent shadows, and even persisted for several seconds after the shadows stopped.

‘This told us that the flies’ reaction was a cumulative response, one that could scale, endure and showed valence, attraction or avoidance behaviour—important components of emotion,’ he says.

If flies are keen to fear-coping mechanisms, it sure stands to reason the frightened antics of other critters are probably pretty calculated too. While some defence skills animals employ are fairly obvious, like a skunk’s stinky spray or a squid’s inky-discharge, others are a bit less intuitive to us humans.

Freezing in the face of danger allows some species to avoid detection, and gives them time to fully take in their surroundings for a potential escape route or seek the upper hand from the landscape in the event of a fight. While frozen, muscles in the eye relax and the pupils dilate to take it as much of the environment as possible.

An opossum playing dead. Photo by Stacy Barnett
An opossum playing dead. Photo by Stacy Barnett

Playing dead may not seem like the best defence strategy at first glance either, but animals like possums or opossums deter predators from eating them by robbing them of the glory of the kill. Most animals will avoid carcasses they come across, generally a good idea in case the critter died from unnatural or contagious diseases. With this same fact in mind, opossums under severe threat begin drooling and frothing with such ferocity that predators assume them sickly.

Plenty of animals try to stretch themselves out or puff themselves up to look bigger, like the aptly named puffer fish, gulping seawater when they sense danger. Other fish like blowfish use this method too. As do some toads, sucking in air to inflate themselves, and some species of lizards and snakes possess blow-up collars and hoods.

Other animals evade predators by losing a body part, like lizards which drop their tails when being followed, leaving the amputated appendage behind as a wriggling, writhing distraction as they flee. Dormice can pull the same stunt, but unlike their reptilian counterparts, can’t regenerate their tail once lost.

Hairy frogs are also extreme fear responders, pulling off eerily like Marvel’s Wolverine. When they sense danger, the frogs actually release bones that pierce through their toe-pads to serve as claws. Sea cucumbers can even change their material state to elude predators, transforming from solid to liquid and back in mere milliseconds.

Some animals that live in huge numbers become martyrs for their colonies. The Malaysian ant for example has huge poison glands and when they sense danger, tighten their abs propelling the poison through the ant’s head, which explodes in the process spraying the fatal fluid everywhere.

When things go wrong…

Li says he’s always been curious about how some people are tormented by fear, while others can sluff it off as an everyday encounter. ‘Some people are simply overcome and overwhelmed by the fear process, despite it being a routine behavioural response,’ he says.

Li and his team are intent on uncovering the mechanics of fear, using mice models to begin to map out the specifics of the neural process, like the individual neurones and brain regions involved in fear memory and behaviour. They think knowing these fundamental targets and trigger points could be key to treating fear based disorders like anxiety and PTSD—examples where the normal response is out of whack.

Right now, the team is looking for the fear-control circuit starter beyond the amygdala. Li explains that the amygdala has long been cited as the danger-processing centre, dating back to monkey studies in the 1930s, but that’s about all we know.

 ‘Many studies since have continued to tell us how important the amygldada was, but no one had explained how it causes and ultimately controls fear,’ he says. ‘ We wanted to know how it was processing, organising and eliciting the response.’

In 2013 the team pinpointed a specific region, the central amygdala, as the more precise fear-controller, and that another region, the Paraventricular Thalamus was involved in the signalling process. This past winter they confirmed this finding, showing that when the connection between the PT and amydala is harmed, so too is the fear memory process. They also found a specific fear-mediator, a growth hormone, called BDNF. Interestingly, the same hormone has been implicated in PTSD.

But despite such advances, a lot more research still needs to be done before we know completely how our biology controls feelings of fear and being afraid. And that’s perhaps the scariest thing; the fear of still not knowing.

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