A study offers clues to the link between what the brain of an organism perceives and physical reactions in its body.
“There’s a very special place in fly hell for me,” said Christi Gendron, a neurobiologist at the University of Michigan in Ann Arbor.
Dr. Gendron earned that spot by researching how living fruit flies respond to the sight of dead ones. To study this so-called death perception, you need corpses; Dr. Gendron and colleagues use starvation to get theirs.
This morbid work, published on Tuesday in the journal PLOS Biology, revealed groups of neurons in the insects’ brains that make them age faster after seeing dead flies. The results will help scientists understand how an animal’s brain turns what it perceives into physical reactions in the body.
Animals across the kingdom of life are keenly aware of death. Elephants grieve for their dead; crows hold “funerals”; and for bees, ants and termites, undertaking is a specialized job performed by only some colony members.
Dr. Gendron and Scott Pletcher, who is a biologist at the University of Michigan, discovered how flies deal with mortality unintentionally. They were trying to see whether flies would show a behavioral or physiological response, like a heightened immune system, after being around other flies that had been made sick with a pathogen. “The only types of responses we saw happened after the flies that we infected died,” Dr. Pletcher said.
Dr. Pletcher and Dr. Gendron found that flies that had seen corpses were avoided by other flies, as if they’d been marked by death (how this works is still a mystery). The carcass-viewers also quickly lost stored fat and died sooner than their nontraumatized counterparts.
“Our lab has long been interested in how the brain controls aging,” Dr. Pletcher said, so they decided to dig into how the sensory perception of dead flies was being translated into a shorter life span in living flies.
The two scientists housed living flies in vials with fly cadavers for two days, and tracked their brain activity with a fluorescent green dye. Dissecting these death-exposed flies revealed activity in the ellipsoid body, which integrates sensory information in the brain.
Dr. Gendron and Dr. Pletcher then identified the key neurons in the ellipsoid body. When these were shut off, seeing dead flies did nothing to the life span of the living. When the researchers activated those neuron clusters, flies met their maker sooner, even if they had never been exposed to dead flies.
“They show that a specific set of serotonin-receptor-possessing neurons are used” by living flies to perceive dead ones, said Marc Tatar, a biologist at Brown University who was not involved with the study. “That is the beauty of this paper.”
It’s not abundantly clear why seeing dead flies would cause those still alive to rush to join them. Dr. Tatar suspects that dead flies are a sign of danger for those still living, so he expects that seeing them leads flies to put more energy into reproduction at the expense of longevity.
A 2022 paper reported that female flies exposed to dead bodies laid more eggs, but it found no effect on life span; Dr. Pletcher said the authors used “significantly less severe” corpse exposures, which could lead to different effects. In their experiments, Dr. Pletcher and Dr. Gendron didn’t see increased reproductive output from death-exposed flies.
The other hypothesis is that the shorter life span results from stress caused by perceiving death. Chronic stress in animals leads to health problems and shortens life spans, and flies have a stress response, too. “If we suddenly found ourselves in a sea of dead humans, that would be very stressful,” Dr. Pletcher said.
The researchers hope to take a broader view of their results and look at other ways that social interactions, or the lack of them, influence aging in flies. And to understand if aging faster after seeing death is somehow beneficial for the flies, Dr. Tatar says we need to take time to study the fruit fly in its natural habitat — instead of just in the lab.
The species has “been in the lab for 120 years,” he said, adding, “We think of them as genetic organisms, instead of as free, natural insects.”
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