Fruit Flies: Do They Feel Emotion?

With summer right around the corner, it'll soon be time for a good old-fashioned picnic. The only thing that might ruin your good time is a pesky fruit fly, which can be shooed away with the simple wave of your hand. But have you ever thought about what that fruit fly could be thinking, for example, if it's actually afraid as it flees the scene? Well, researchers now suggest that fruit flies may be able to feel emotion.

At least, that's according to a new study published in the journal Current Biology, which details how a fly's response to a shadowy overhead stimulus might be analogous to a negative emotional state such as fear - a realization that could one day help us better understand the neural circuitry involved in human emotion.

Although you might think that mice would be more suitable for studying human emotion, since they are closer to humans on the evolutionary family tree, insects actually provide valuable information also. In this case, the fruit fly has a much simpler neurological system that is easier to study. Nonetheless, studying emotions in insects or any other animal can also be tricky.

When it comes to studying emotion in animals, anthropomorphosis is common.

"There are two difficulties with taking your own experiences and then saying that maybe these are happening in a fly. First, a fly's brain is very different from yours, and second, a fly's evolutionary history is so different from yours that even if you could prove beyond any doubt that flies have emotions, those emotions probably wouldn't be the same ones that you have," postdoctoral scholar William T. Gibson, first author of the paper, said in a statement. "For these reasons, in our study, we wanted to take an objective approach."

To achieve this goal, Gibson and his colleagues broke down the idea of emotion into basic building blocks - like the way the color orange can be broken down into two colors: red and yellow. This way, they operated under the assumption that "emotions are a type of internal brain state with certain general properties that can exist independently of subjective, conscious feelings, which can only be studied in humans," explained co-author David Anderson.

"That means," he said, "we can study such brain states in animal models like flies or mice without worrying about whether they have 'feelings' or not. We use the behaviors that express those states as a readout."

"And if we can show that fruit flies display all of these separate but necessary primitives, we then may be able to make the argument that they also have an emotion, like fear," Gibson added.  (Scroll to read on...)

The researchers observed the insects in the presence of a fear-inducing stimulus - an apparatus that would pass a dark paddle over the flies' habitat. Using a software program to track the flies' movements, the team found that the insects displayed all five primitives attributed to the emotion of fear.

For example, when the paddle passed overhead, the flies would either freeze, or jump away from the stimulus, or enter a state of elevated arousal. What's more, repeated exposure to the stimulus produced an increase in each response, suggesting that their fear heightened each time.

"For us, that's a big step beyond just casually intuiting that a fly fleeing a visual threat must be 'afraid,' based on our anthropomorphic assumptions. It suggests that the flies' response to the threat is richer and more complicated than a robotic-like avoidance reflex," Anderson explained.

"Our work can get at questions about mechanism and questions about the functional properties of emotion states, but we cannot get at the question of whether or not flies have feelings," Gibson was quick to point out.

Nonetheless, the researchers hope to use their new technique to gauge whether emotion primitives exist in higher organisms, such as mice or even humans.

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