Fruit flies flee from shadows

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The shadow of a predator overhead sends fruit flies into a tizzy. In response to an aerial threat, flies exhibit behaviors that echo the human state of fear, scientists report in the June 1 Current Biology.

Finding signs of a fearlike state in the fruit fly Drosophila melanogaster may allow scientists to better understand how the human brain creates emotions and how that process can go awry in fear and anxiety disorders.

“The Drosophila brain is so simple,” says neuroscientist Kay Tye of MIT. “If we can understand how fear and anxiety work in a Drosophila, it’s a great handle for us to understand it in more complex brains.”

In the study, scientists explored how fruit flies in a circular enclosure responded to ominous shadows passing overhead. Every so often, a mechanical paddle would swipe over the flies, creating a shadow and causing the insects to scurry away, jump or freeze in place. These immediate reactions were accompanied by more long-lasting effects, biophysicist William Gibson of Caltech and colleagues found.

The effect of the shadow lingered even after it had passed. The situation is akin to hearing a gunshot, Gibson says. A person would be jumpy long after the pop. The effect was also cumulative, meaning that 10 shadows were more alarming than one.

Even hungry flies would leave their food when spooked, the team found. The researchers placed a food patch in the middle of the enclosure to test whether the appeal of food could overcome the ominous shadow. As more shadows crossed the enclosure, more flies fled the food patch, Gibson says.  “And once they’re off the food patch, there’s a long delay before they come back.”

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SNACK STOPPER  Even hunger can’t keep fruit flies feeding when ominous shadows pass. Flies happily eat before a shadow passes (left panel) but after three (center) and five (right) shadows pass, the insects leave their food.  Credit: William T. Gibson and David J. Anderson

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Gibson emphasizes that these might be signs of fear, but the researchers aren’t claiming that the flies actually experience that emotion. “We don’t want to imply that they have feelings, because our work can’t address that,” he says.

But the results do show that flies exhibit behaviors that are consistent with some of the building blocks of fear, he says. “What we’ve done is we’ve decomposed the concept of an emotion into these emotion primitives, kind of like you can decompose a color into primary colors.” These building blocks might be similar to the ones humans use to construct emotions, he says.

Yet the issue of what to call the flies’ experience may be about semantics, Tye says. “The word ‘fear’ is a very loaded term,” she says. People spend a lot of time debating whether it’s possible to ever know whether an animal is experiencing a particular emotion. “And you can’t,” she says. But “in my opinion, you can’t even know if other humans are experiencing a similar emotional state to other humans. If you’re going to make that argument, how do you know that babies feel fear?”

While it may be impossible to know how animals feel, these behaviors offer clues that flies respond to threats in similar ways as people do, and that opens up exciting possibilities for more studies, Tye says. 

FLY FREEZE  A passing shadow makes a fly jump and then freeze for several seconds before fleeing. The second clip shows the same fly’s response to the shadow at half speed.. Credit: William T. Gibson