Witnessing a death could cause of death for flies?

“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 got that slot by examining how living fruit flies respond to the sight of dead ones. To examine this so-called death perception, you need corpses; Dr. Gendron and colleagues utilize famine to collect theirs.

This gruesome discovery, published on Tuesday in the journal PLOS Biology, showed regions of neurons in the insects’ brains that made them age quicker after witnessing dead flies. The data will help scientists understand how an animal’s brain transforms what it observes into physical reactions in the body.

Animals across the realm of life are profoundly aware of death. Elephants cry for their dead; ravens conduct “funerals”; and for bees, ants and termites, undertaking is a specific duty done by just select colony members.

Dr. Gendron and Scott Pletcher, who is a scientist at the University of Michigan, found how flies deal with death accidentally. They were aiming to observe whether flies would display a behavioral or physiological reaction, like a heightened immune system, after being around other flies that had been rendered sick with a disease. “The only types of responses we saw happened after the flies that we infected died,” Dr. Pletcher added.

Dr. Pletcher and Dr. Gendron observed that flies that had seen corpses were shunned by other flies, as if they’d been marked by death (how this works is still a mystery). The carcass-viewers also swiftly lost stored fat and perished sooner than their nontraumatized counterparts.

“Our lab has long been interested in how the brain controls aging,” Dr. Pletcher said, so they chose to delve into how the sensory experience of dead flies was being translated into a shortened life span in living flies.

The two scientists kept living flies in vials with fly cadavers for two days, then recorded their brain activity with a luminous green dye. Dissecting these death-exposed flies revealed activity in the ellipsoid body, which integrates sensory input in the brain.

Dr. Gendron and Dr. Pletcher then identified the main neurons in the ellipsoid body. When these were cut off, viewing dead flies did nothing to the life span of the alive. When the researchers stimulated certain neuron clusters, flies met their maker sooner, even though they had never been exposed to dead flies.

“They show that a specific set of serotonin-receptor-possessing neurons are used” by surviving flies to recognize deceased ones, said Marc Tatar, a biologist at Brown University who was not involved with the work. “That is the beauty of this paper.”

It’s not abundantly obvious why viewing dead flies would drive those still living to rush to join them. Dr. Tatar thinks that dead flies are a sign of danger for those still surviving, thus he expects that seeing them drives flies to put more energy into reproduction at the price of lifespan.

A 2022 research stated that female flies exposed to dead corpses lay more eggs, but it found no influence on life duration; Dr. Pletcher noted the authors employed “significantly less severe” corpse exposures, which might lead to different results. In their trials, Dr. Pletcher and Dr. Gendron didn’t find higher reproductive production from death-exposed flies.

The alternative idea is that the shortened life span arises from stress produced by seeing death. Chronic stress in animals leads to health issues 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 added.

The researchers aim to take a broader view of their data and look at additional ways that social contacts, or the absence of them, impact aging in flies. And to understand if aging quickly after viewing death is anyway helpful for the flies, Dr. Tatar believes we need to take time to study the fruit fly in its natural habitat — instead of simply 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.”