“There’s a very special place for me in the flying hell,” says Christi Gendron, a neurobiologist at the University of Michigan in Ann Arbor.
Dr. Gendron earned that spot by studying how live fruit flies react to seeing dead fruit flies. To study this so-called death perception, you need corpses; Dr. Gendron and colleagues use hunger to get theirs.
This morbid work, published Tuesday in the journal PLOS Biology, revealed groups of neurons in the brains of insects that make them age faster after seeing dead flies. The results will help scientists understand how an animal’s brain converts what it perceives into physical responses in the body.
Animals throughout the kingdom of life are acutely aware of death. Elephants mourn their dead; crows hold “funerals”; and for bees, ants and termites, enterprise is a specialized task performed by only a few colony members.
Dr. Gendron and Scott Pletcher, a biologist at the University of Michigan, discovered how flies deal with mortality inadvertently. They tried to see if flies would exhibit a behavioral or physiological response, such as a boosted immune system, after being around other flies that had been made sick by a pathogen. “The only types of reactions we saw occurred after the flies we infested died,” said Dr. Pletcher.
Dr. Pletcher and Dr. Gendron discovered that flies that had seen corpses were evaded by other flies, as if they were scarred by death (how this works is still a mystery). The carcass viewers also lost stored fat quickly and died earlier than their untraumatized counterparts.
“Our lab has long been interested in how the brain controls aging,” said Dr. Pletcher, so they decided to investigate how the sensory perception of dead flies translated into a shorter lifespan of living flies.
The two scientists housed live flies in vials of fly carcasses for two days and monitored their brain activity with a fluorescent green dye. Dissecting these death-exposed flies revealed activity in the ellipsoidal body, which integrates sensory information into the brain.
Dr. Gendron and Dr. Pletcher then identified the major neurons in the ellipsoid body. When these were turned off, seeing dead flies did nothing to shorten the life of the living. When the researchers activated those clusters of neurons, flies were more likely to encounter their maker, even though they had never been exposed to dead flies.
“They show that a specific set of serotonin receptor-possessing neurons is used” by live flies to sense dead flies, said Marc Tatar, a biologist at Brown University who was not involved in the study. “That’s the beauty of this paper.”
It’s not super clear why seeing dead flies would cause those who are still alive to join them. Dr. Tatar suspects that dead flies are a sign of danger to those who are still alive, so he expects seeing them will cause flies to expend more energy on reproduction at the expense of longevity.
A 2022 paper reported that female flies exposed to dead bodies laid more eggs, but found no effect on longevity; Dr. Pletcher said the authors used “significantly less severe” cadaveric exposures, which could lead to different effects. In their experiments, Dr. Pletcher and Dr. Gendron did not increase reproductive output of death-exposed flies.
The other hypothesis is that the shorter lifespan is due to stress caused by the perception of death. Chronic stress in animals leads to health problems and shortens lifespan, and flies also have a stress response. “If we suddenly found ourselves in a sea of dead people, it would be very stressful,” said Dr. Pletcher.
The researchers hope to take a broader look at their results and look at other ways that social interactions, or lack thereof, influence aging in flies. And to understand whether aging faster after seeing death is somehow beneficial to the flies, says Dr. Tatar that we should take the time to study the fruit fly in its natural habitat – rather than just in the lab.
The types has “been in the lab for 120 years,” he said, adding, “We think of them as genetic organisms, rather than free, natural insects.”