Fruit flies exhibit fear and could even feel other emotions as well, according to a new study. The insects appear to be frightened of shadows of objects above them, such as fly swatters, researchers concluded.
Fear consists of several parts, which investigators label as "emotion primitives." An initial stimulus, such as a scream, can bring a feeling of fear to the person hearing it. Such a state can make other stimuli more apt to bring greater feelings of fear to the subject. Investigators found that shadows can bring about this same state of mind as that seen in humans who hear a frightening sound.
"No one will argue with you if you claim that flies have four fundamental drives just as humans do: feeding, fighting, fleeing, and mating. Taking the question a step further — whether flies that flee a stimulus are actually afraid of that stimulus — is much more difficult," said William T. Gibson from the California Institute of Technology (Caltech).
Fruit flies were recorded on video as they were exposed to shadows. Analysis revealed that the insects increased their speed when avoiding the shadows, and hopping became more pronounced. Some flies reacted to the perceived threat by freezing in place in much the same way as rodents. The effect was even seen in flies as they fed, suggesting the impulse was highly negative.
The flies also took some time to "calm down" and return to a food source after being scared away by a shadow from an overhead object. The more often they had experienced the stimulus, the longer it took for them to return to the scene for feeding.
Although mice are closer to humans biologically than fruit flies, the simple anatomy of the insects makes them an attractive subject for laboratory experiments. However, their neurological systems are so different from ours that behaviors that seem human, such as fleeing from a threat, may not have the same biological basis as it does in our own species.
"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," Gibson said.
Researchers hope their study of fruit flies can help biologists learn more about the chemical processes underlying the emotion of fear. Such an emotional state involves a complex interaction of neural circuitry, neuropeptides and brain chemistry.
Future research could examine the physical structure of the brains of fruit flies to see if biological functioning of the insects as threats occur is similar to that seen in people.
Study of the emotions of possible emotions in fruit flies was published in the journal Current Biology.
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