Envision a dinner party where you're about to toast the arrival of summer. But as you raise your glass of wine you notice a fruit fly hovering right around the rim. You move in for the kill with your other hand, casting shadow upon the glass, and the fly is gone.
When fruit flies respond to the shadow—is it out of fear?
That's a difficult question to answer. It's impossible, of course, for flies to show human-like emotions of fear or sadness because their brains are vastly different than our own. Yet, researchers from the California Institute of Technology (CalTech) have found that when flies respond to visual threats, the way in which they react mirrors many of the elements that humans may associate with fear. Their findings were published online on May 14 in the journal, Current Biology.
To answer the question of fear from an objective angle, the team asked: Does a fly's response to shadows resemble our response to the sound of a gun?
Let's pretend for a second that James is walking down the street and a bystander shoots a gun.
Fear is persistent. This means that the fear James is feeling will last longer than the shot itself. If the bystander continues to shoot, the level of James' fear will increase, or scale, with the number of shots fired. But emotions also generalize. William T. Gibson, first author of the study and postdoctoral fellow, explained over the phone that fear will have the same effect regardless of whether he is "eating a hamburger" or "fighting with [his] brother" when the shots go off. And lastly, fear is trans-situational meaning that the loud clang of a pan or bang at the door would arouse the same kind of feeling.
To test these building blocks of emotion in flies, the researchers enclosed the flies in a walking arena, 30 millimeters in diameter, where they were exposed repeatedly to an overhead shadow. The study was done in the laboratory of David J. Anderson, a Seymour Benzer Professor of Biology and an investigator with the Howard Hughes Medical Institute.
One arena had food; the other didn't. Gibson explained that, in both cases, the test would last for about five mins with roughly 10 shadow stimulus spaced out in between. Their reactions increased depending on the number of times the fly was exposed to the shadows.
The real breakthrough here was persistent and scaled exposure. So by holding the flies in a tight space, they could test just how long, and how intense, some of their reactions were the more the shadow passed overhead.
Occasionally, the insects froze in place: a defensive behaviour, Gibson explained. But in other cases, the shadows caused hungry flies to leave a food source, suggesting the experience was negative. "They are starving and they want to eat, yet they are still leaving the food in response to the stimulus," Gisbon said.
"These experiments provide objective evidence that visual stimuli designed to mimic an overhead predator can induce a persistent and scalable internal state of defensive arousal in flies, which can influence their subsequent behavior for minutes after the threat has passed," Anderson said in a statement. "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. It suggests that the flies' response to the threat is richer and more complicated than a robotic-like avoidance reflex."
In other words, the study doesn't necessarily prove that flies feel fear, but they may be reacting to the shadow based on some primitive emotional state.
For past research on fear, mice were the common test subject. For this study, flies were chosen because their brains and nervous systems are easy to access. Neurogenic screens can be done on flies to identify which neurons are responsible for what behaviours, which means that they may actually be a faster path for fear research.
Gibson said that the most valuable aspect of this work is that it opens the door to using insects to discover molecular players in the brain that might be involved in defensive responses. "It's well known that whether you look into a worm, a mouse, a fly or a human, many of the brain chemicals that are involved in coordinating things like feeding or mating responses are conserved," Gibson said.
Regardless of the fact that there is no connection between the two brains; it does suggest that a fly could be a prime model for dissecting the elements of behavioural or emotional states.