The sight of their dead comrades is enough to drive fruit flies to an early grave, according to researchers, who suspect the creatures keel over after developing the fly equivalent of depression.
For a species that spends much of its life feasting on decayed matter, the insects appear to be particularly sensitive to their own dead. Witnessing an abundance of fruit fly carcasses speeds up the insects’ ageing process, scientists found, cutting their lives short by nearly 30%.
While the researchers are cautious about extrapolating from 3mm-long flies to rather larger humans that can live 400 times longer, they speculate that the insights might prove useful for people who are routinely surrounded by death, such as combat troops and healthcare workers.
“Could motivational therapy or pharmacologic intervention in reward systems, much like what is done for addiction, slow ageing?” the authors ask in Plos Biology. The possibility could be tested in humans today, they added, using drugs that are already approved.
Researchers led by Christi Gendron and Scott Pletcher at the University of Michigan raised fruit flies in small containers filled with food. While some of the containers held only living flies and tasty nutrients, others were dotted with freshly dead fruit flies as well, to see what impact they had on the feeding insects.
When fruit flies were raised among dead ones, they tended to die several weeks earlier than those raised without being surrounded by carcasses. Those exposed to death appeared to age faster, losing stored fat and becoming less resilient to starvation.
To investigate further, the scientists turned to a biological tool that helped them pinpoint which brain circuits switched on when fruit flies hung out with dead comrades. The more active neurons became in the flies’ brains, the more of a fluorescent protein those neurons produced.
When the scientists dissected the fruit fly brains after the experiments, they found that two specific types of neurons, known as R2 and R4, were activated when the insects were raised in an environment full of dead flies. When they stimulated these same neurons in healthy fruit flies, it had the same life-shortening effect: the flies died younger, despite not witnessing the dead bodies of others.
Understanding more about how flies’ brains transform their physiology to accelerate the ageing process might pave the way for new treatments to slow ageing in humans, the scientists suggest. But one possibility is that all that death simply gets the flies down, and eventually becomes too much. “Given our findings,” the authors write, “it seems plausible that the sight of dead conspecifics elucidates a “depressive-like” state that results in decreased longevity.”
