Flies Have Similar Cardiac Responses to Danger as Humans

According to a study published today in Current Biology, the hearts of fruit flies respond to danger in very much the same way human hearts do.  

“We were quite surprised by this result,” recalls Marta Moita, the neuroscientist that led the project. “We know that when vertebrates face a threat, their autonomic nervous system kicks into action, generating the changes in cardiac activity that we are all familiar with. However, this system doesn’t exist in insects, and so it was unclear whether they would exhibit similar cardiac alterations.”  

The research team from the Champalimaud Centre for the Unknown followed the fruit fly’s cardiac activity through the fly's transparent exoskeleton while it was walking about by lighting up the heart's cells with fluorescent molecules. Occasionally, a dark expanding circle appeared on a large screen in front of the fly, mimicking an approaching threat.

"Amazingly, just like in humans, the fly's heart changed its activity depending on which defense response is assumed. If the fly decided to escape, the heart accelerated, but if the fly froze in place for a sustained period of time, its heart slowed down," the authors explain. 

The team also found that the direction of pumping (since the fruit fly body is essentially divided into two sealed compartments, the heart alternates pumping in two directions) also changed depending on the fly's defensive response, and discovered that in both cases—escape and freezing—the heart was pumping more actively toward the front section of the fly. 

"Pumping more nutrients into the front section while escaping makes sense. This is where the brain, legs and wings are located, so that's where the action is. But we didn't expect to see this while the fly was freezing," the team adds.  According to the researchers, freezing is considered an energy-saving behavior. Indeed, the slowing down of the heart indicates just that. But then, what was the reason the heart was pumping more actively towards the front?

To test this hypothesis, the team compared the sugar levels of flies that froze with sugar levels of flies that were exposed to neutral images, and therefore didn’t exhibit any defensive behaviors. The results were striking: flies that froze had significantly lower sugar levels. 

"This finding refutes the generally held belief that freezing is a passive, energy-saving behavioral state," Moita argues. "Instead, it suggests that freezing is a state of active preparedness. Now, the question is—what is the fly preparing for? What is the range of actions that may follow freezing, and how is the choice between actions made by the brain?" 

These questions join a string of new research avenues raised by this study. One of the most pressing is identifying the neural structure that controls cardiac responses to danger in flies, and deciphering how it works.  "Since flies and humans share many genes, the hearts of flies are commonly used to study various aspects of cardiology, particularly relating to disease," the authors explain. "However, little attention has been directed towards how the fly's heart responds to danger."