Flies are afraid, they experience fear, and thus have emotions – probably

Do flies have emotions? Scientists have found they display the typical signs of fear when exposed to an overhead shadow. Their response to visual threats include the hallmark elements of what humans consider as fear. If this is the case, they could help us determine what emotions are made of.

This is what scientists from Caltech (California Institute of Technology) and the Howard Hughes Medical Institute reported in the academic journal Current Biology (citation below) after carrying out a study on fruit flies.

David J. Anderson, Seymour Benzer Professor of Biology; Investigator, Howard Hughes Medical Institute, and colleagues said their findings in fruit flies are a step toward dissecting the fundamental neurochemistry, neuropeptides, and neural circuitry underlying emotion states, including fear.

Will flies help us find out what fear is made of? (Image: Caltech)

Study first author, William T. Gibson, a Caltech postdoctoral fellow, said:

“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.”

To ask the question in a different and more straightforward way, the scientists broke down fear into its fundamental building blocks, which they call ‘emotion primitives’.

Fear is persistent

First, fear is persistent, Dr. Gibson says. If you hear the sound of a gunshot, the sensation of fear it provokes will continue for some time.

There are also degrees of fear – the more gun blasts you hear, the more frightened you will become.

Not only is fear generalizable across different contexts, it is also trans-situational. Once you are in a state of fear, you are more likely to respond fearfully to other triggers such as a loud knock on the door, a clang of a pan, or a sudden loud clap of thunder.

Flies’ fear response similar to humans’

The question the researchers asked was: “In terms of these building blocks of emotions, does a fly’s response to shadows resemble our response to the sound of a gun?” According to their study findings “Very much so.”

That was the conclusion the scientists came to after enclosing the fruit flies in an arena were they repeatedly say an overhead shadow.

Experimental apparatus, including the walking arena for the flies, and the shadow paddle. (Image: Caltech)

In collaboration with Caltech’s Pietro Perona’s computer vision group, Prof. Anderson and colleagues carefully analyzed video recordings of the flies’ behaviors, which showed that shadows promoted scalable and persistent increases in the flies’ hopping and speeding.

As rodents often do as a defensive fear response, the flies sometimes froze in place. Fear even forced hungry flies to leave a food source, suggesting that the experience was generally unpleasant and generalized from one context to another.

It took a while before the flies that had been exposed to overhead shadows returned to their food, suggesting that their internal, defensive state gradually decayed.

Various levels and duration of fear

Interestingly, how long it took the flies to ‘calm down’ and return to the food was directly related to the number of shadow exposures – the more shadows they saw, the longer it took.

This shows that when flies escape in response to a shadow, it is more than a momentary escape. Much like fear, it is a lasting physiological state. If this is the case, it means that flies could help scientists understand in a very fundamental way what emotions, including fear, are made of.

Dr. Gibson said:

“The argument that this paper makes is that the Drosophila system may be an excellent model for emotion states due to the relative simplicity of its nervous system, combined simultaneously with the behavioral complexity it exhibits.”

“Such a simple system, leveraged with the power of neurogenetic screens, may make it possible to identify new molecular players involved in the control of emotion states.”

The scientists now plan to dissect the neural circuitry involved in the fruit flies’ shadow response.

Citation: “Behavioral Responses to a Repetitive Visual Threat Stimulus Express a Persistent State of Defensive Arousal in Drosophila,” William T. Gibson, Carlos R. Gonzalez, Conchi Fernandez, Lakshminarayanan Ramasamy, Tanya Tabachnik, Rebecca R. Du, Panna D. Felsen, Michael M. Maire, Pietro Perona and David J. Anderson. Current Biology. Doi: 10.1016/j.cub.2015.03.058.

Video – Fly’s response to overhead shadow