Insects aren't so different from humans. The majority of their behaviours are motivated by the same things: food, sex and survival. So when a looming shadow gets closer and closer, a fruit fly, well, flies away – just like any other animal with a natural history of being gobbled up by bigger, toothier creatures. Looming shadows are bad news, and if day suddenly turns into night, your best bet is to flee.

But when the Drosophila fruit fly flies away from a shadow, is it afraid? In its last few minutes of life, does a fly actually perceive a flyswatter as scary? That's the question that California Institute of Technology postdoctoral fellow William T. Gibson wanted to answer. 

There's a long tradition in psychology and neuroscience of avoiding anthropomorphisation: just because an animal behaves as if it is afraid doesn't mean it actually is. If you suggest that flies have the same basic drives that humans do, "no one will argue with you," said Gibson in a statement. "[But] taking the question a step further – whether flies that flee a stimulus are actually afraid of that stimulus – is much more difficult."

It would be simple if we could talk to flies and ask whether they're afraid, but since we obviously can't do that, the next best strategy is to break the complex emotion of fear into its building blocks, which Gibson refers to as "emotional primitives".

One primitive is persistence: an animal remains scared even after the fearful stimulus has gone away. For example, we might become afraid when we see a clown, but thanks to our increased heart rate and amped-up adrenaline, that feeling will continue long after the clown has disappeared.

A second primitive is scale: the more clowns pile out of a clown car, the more afraid you become.

Yet another primitive is generalisability: once the clowns have made you afraid, you're more likely to be startled even by harmless things.

Gibson's experiment was simple: a group of flies was placed in a small arena over which a paddle moved back and forth, casting a dark shadow below. The researchers recorded video of the experiment and carefully analysed the flies' behaviour. 

They reasoned that if the flies' actions were persistent, scalable and generalisable, then their response to the shadow might be similar, at least in some fundamental way, to our own response to a gaggle of scary clowns, or gunshots, or heights, or sloths doing "Grudge" impersonations – or anything else that makes us afraid. 

And that's just what the researchers found: it looked as if the flies were afraid (or, to be most conservative, their behaviour was fear-like). The "fearful" behaviour continued long after the shadow had disappeared, and each time it passed over the arena, the flies flew or hopped away faster and faster. When the shadow passed over hungry flies, they stopped eating, and each time it reappeared, it took them longer and longer to resume their feeding. In other words, the flies seemed to react to the shadow the same way a human reacts to a succession of scary clowns: they became more and more scared, and took longer and longer to calm down.

A well-fed fly might be best suited to running away from a threat since it knows it will easily find a meal once the danger passes. But a fly that doesn't know where or when it might find more food has to make an important decision: is it better to hide and risk losing out on an important meal, or is it better to keep eating and risk becoming someone else's dinner? That's why a fearful response in a hungry fly suggests there's a more complex cognitive process at work, not just a quick, impulsive reaction.

This study is the first time that researchers have subjected flies to a repeated negative stimulus, and in doing so, they seem to have confirmed that the flies' behaviour is more than a mindless reflex. If the finding holds up to continued scrutiny, then the humble fruit fly might be poised to reveal more about the biology that governs our emotions than even other primates can.

So what's next? The researchers plan to find out more about the genetics and neurobiology behind the insects' fear-like responses. Because so much is already known about Drosophila flies, these tiny insects might be able to help researchers finally work out the connections between biology, emotion and behaviour.

Top header image: Marcello Consolo, Flickr