Animal emotions: Do fruit flies feel fear?

When you swat at a hovering fly, does it fear your hand?

Maybe not, but Caltech researchers say they have identified the “building blocks” of emotion in fruit flies.

While studying the neurological mechanisms behind emotion, professor David Anderson and colleagues found that flies exhibited fear-like reactions when exposed to a “shadowy overhead stimulus.” Their findings, which were published Thursday in Current Biology, could someday improve our understanding of human emotion.

Emotion is notoriously difficult to measure empirically. The scientific community has yet to settle on an exact definition of the word, much less parse the neurological complexity of human emotion. So Dr. Anderson and his colleagues focused on the humble fruit fly. Humans and flies aren’t exactly close relatives, but the simple neural systems of insects are easier to study. There are pitfalls to this approach, however.

“There are two difficulties with taking your own experiences and then saying that maybe these are happening in a fly,” co-author William Gibson said in a press release. “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.”

Luckily, there is a way to deconstruct emotion into something a little more universal. Previous research by Anderson suggests that each emotion can be broken down into simpler “emotion primitives.”

Upon hearing the sound of unexpected gunfire, most people react with fear. But there are several steps within that very basic emotional response. Upon hearing one shot, most people immediately experience a negative feeling, a primitive called valence. Generally, that feeling lasts for several minutes and causes people to act differently for that duration. That primitive is known as persistence. And you’re likely to be more frightened by the sound of 10 gunshots than by the sound of one – this primitive is called scalability.

Another primitive is known as context generalization: If you happened to be reading a good book when you heard the hypothetical gunfire, the resulting fear would take precedence, making you forget about your book temporarily.

As it turns out, fruit flies experience the same primitives humans do. Anderson and colleagues developed a device that would wave a dark paddle over their flies, and used a specialized program to track the flies' movements. Flies responded to the stimulus by jumping away, freezing completely, or entering an agitated state. The reactions were scalable, intensifying with each additional wave of the paddle. And hungry flies would leave their food temporarily, darting around for several moments before returning to finish their meal – textbook persistence and context generalization.

“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,” Anderson said. “It suggests that the flies’ response to the threat is richer and more complicated than a robotic-like avoidance reflex.”

Researchers are very careful not to suggest that flies have feelings like fear or happiness – that was never the goal of the study. But their findings do pave the way for new studies of emotion. By combining genetic techniques and neuro-imaging, Anderson and colleagues hope to link emotion primitives to specific regions of the fruit fly brain. From there, they can look for the same mechanisms in more complex organisms, like humans.