Coming to terms with death isn’t easy — even if you’re an insect that’s little more than an eighth of an inch in size.
For fruit flies, or Drosophila melanogaster, the mere sight of their companion’s corpses can trigger certain cues known to alter their brain chemistry, deplete fat stores and even cause other flies to avoid them, as if the traumatized insects still carried the stench of death.
What’s more, scientists from the University of Michigan have found that fruit flies who witness their dead comrades age faster and die sooner. They’ve now peered inside the insect’s pint-sized brain to better understand what happens when fruit flies are exposed to death, according to a study recently published in PLOS Biology. The research may have implications for pausing the clock on human aging, as well.
How Do Other Animals Respond to Death?
Across the animal kingdom, a number of species are aware of death’s looming specter, each reacting in their own unique way.
Social insects like ants and honeybees, for example, act as impromptu undertakers, carting dead colony members away from the nest. The behavior — known as necrophoresis — is, in part, triggered when the deceased insects no longer emit life-affirming chemical compounds, which their living companions can sense.
Other examples abound, too. When female baboons grieve a dead relative, they’re flooded with increased stress hormones, much like humans. Elephants, meanwhile, will mourn openly, including standing guard over the bodies of the deceased, touching the corpse with their trunk and even making noises.
“Apes are close to us, so you’d expect [that type of behavior],” says Christi Gendron, a neurobiologist at the University of Michigan and co-author on the new study. “But, specifically, the way that elephants mourn their dead and show altered behaviors has always been the most striking [to me].”
Since these responses share similarities across species, researchers like Gendron suggest that the physiological processes behind them might be shared, too, including among humans.
How Does Death Impact Fruit Flies?
Gendron and study co-author Tuhin S. Chakraborty found out how living fruit flies respond to the sight of dead ones almost by accident.
“[Chakraborty] stumbled upon it,” says Gendron. “The initial question that he was asking was whether flies can sense other sick flies. You hear stories of dogs that can sense sickness in humans, and we were trying to understand the neurobiology there.”
To study that, Chakraborty infected fruit flies with a bacterial pathogen. When those flies died, healthy flies housed with the cadavers began losing stored fat, were shunned by other flies and died quicker than their counterparts The scientists published those results in PLOS Biology in 2019.
For the latest experiment, the team wanted to better understand how the neural circuitry in living flies was translating the sensory perception of death into a diminished life span. They were able to track the fly’s brain activity by tagging them with a fluorescent green die. Dissecting those corpse-exposed flies showed heightened activity in the ellipsoid body, a part of the brain responsible for processing sensory information.
The scientists then identified the specific population of neurons necessary for that response. Later, by stimulating those same neuron clusters, the flies shed their mortal coil sooner, even if they had never been exposed to a dead fly before.
What Does This Mean for Humans?
Unfortunately, humans must confront death, too. And for those who are routinely exposed to stressful situations that surround death, like soldiers and first responders, the consequences can threaten their longevity, as well.
“[Those individuals] often suffer cardiovascular issues,” says Gendron. “They also suffer psychological issues, potentially, like PTSD and depressive disorders.”
By understanding the pathways responsible for processing death, even in a basic organism, the researchers hope to pave the way towards therapies, including drugs, for treating those individuals — and, in turn, potentially extending their lifespan.