Flies age faster once they’ve seen death

Scientists peered inside the Drosophila brain to see what happens when they see one of their own corpses.
single fruit fly
When tiny insects see or smell something tragic, it can have a life-changing impact. DepositPhotos

What we experience each day makes an impact on us, good or bad. After all, starting your morning with a smile from a loved one will brighten your mood quite a bit more than driving past roadkill on your a.m. commute. But, depending on how drastic the circumstances, witnessing something can have effects that last much longer than  the afternoon. 

For years, scientists have wondered and studied how exposure to certain things, such as childhood trauma or stress, impacts the way that we age. In an ongoing set of experiments, they’ve had fruit flies sub in for humans. As it turns out, when tiny insects see or smell something tragic, that has an impact on how quickly the invertebrates age.

[Related: Horny male fruit flies plunge into chaos when exposed to air pollution.]

For a study published in 2019, a group of scientists from the University of Michigan discovered that when a fruit fly or Drosophila melanogaster was exposed to a dead fruit fly, this exposure induced cues that were unattractive to other flies, changed their brain chemistry, decreased their fat stores, reduced starvation resistance, and accelerated aging. These kinds of reactions aren’t exactly rare—the authors cite how certain social insects like ants will move dead bodies out of their living spaces. Elephants, too, vocalize and inspect corpses when in the presence of dead elephants, and when female baboons mourn their dead they experience increased stress hormones.

The team now understands a bit more about what’s going on in a fly’s tiny brain upon seeing their deceased brethren, and published these findings in PLOS Biology on June 13.  For this latest experiment, the team investigated neural circuits and central signaling processes in the brains of traumatized Drosophila. 

[Related: Flies evolved before dinosaurs—and survived an apocalyptic world after the Permian extinction.]

The researchers used fluorescent tagging to see what occurred in the brain, and when exposed to other dead flies, there was an increased activity in the ellipsoid body. This part of the brain harbors cells called laminated ring neuron axons, which supply the ellipsoid body with nerves, and mediates sensory integration and motor coordination. To see which ring neurons were associated with this response, the researchers silenced them one by one. This revealed two ring neuron axons that hold a specific receptor, which binds with the messenger molecule serotonin, were necessary for the response. Later, the researchers artificially activated these same neurons and found that fruit fly life spans shrunk when they were turned on, even if the insect hadn’t come in contact with a dead fly previously. 

In a world obsessed with aging—how to prevent it, how to slow it, or even stop it altogether—this kind of research can, according to the researchers, help develop drugs that pause the clock for humans. But until then, the takeaway here is that even for creatures that are only a tenth to a fifth of an inch in size, coming to terms with death is complicated.