A male fruit fly (drosophila melanogaster) may look simple, but its small brain can do complex things, possibly even including feel emotions. [Photograph by André Karwath under CCo Share-alike license]
Emotions are fundamental to our lives, and yet we know very little about how they arise in the brain.  This is mostly because we don’t have the luxury of asking our model organisms, usually monkeys or rodents, how they feel. Instead, researchers studying emotions must rely on signs in the behavior of the animal, such as its facial expression or how much it moves. But who’s to say that we know what to look for to see any particular “emotion,” especially in animals that don’t closely resemble humans?

A recent publication in Current Biology uses a new idea, called “emotional primitives,” to explore whether fruit flies can exhibit the hallmarks of an emotional state. Emotional primitives do not have to resemble the emotional behavior of humans; instead, they follow a set of rules which the authors claim should describe all behaviors arising from emotional states. In the paper, they use this concept to ask whether flies exhibit a fear-like state in response to a visual threat. What they find is that flies will either quickly move or begin hopping rapidly after exposure to the stimulus, and these behaviors increase with the intensity of the threat. Their response also lasts much longer than the threat itself, with the flies continuing to hop or run around for 10 to 20 seconds after the threat is gone. Most compellingly, starved flies will flee from food when threatened, and are reluctant to return until nearly a minute has passed since the threat.

Is this an example of an emotional response? Only a small fraction of flies show the hopping behavior, and the increase in the intensity of movement to a stronger stimulus is relatively small. However, the behavior’s persistence is clearly not due to a reflex, lending their hypothesis some credibility. Additionally, this study is one of the first attempts at developing a paradigm that allows us to study and talk about emotions in a scientifically rigorous way, and deserves praise for that. Model organisms are our best chance at understanding the biology behind emotions, and if we don’t have a reliable tool for discussing and evaluating emotions in these animals, we have no hope of making sense of feelings in ourselves.

Managing Correspondent: Stephen Thornquist

Original paper: Behavioral Responses to a Repetitive Visual Threat Stimulus Express a Persistent State of Defensive Arousal in Drosophila

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