Why Flies Die When They See Dead Flies

The ability to respond to death isn’t just a human trait. Honeybees recognize dead members of their colony and remove them from the hive. Scrub jays will gather around a dead jay and scream. And elephants will touch a fellow elephant’s corpse, try to lift it up, and trumpet. But fruit flies? After they see or smell other dead flies, they’ll start rapidly aging, and sometimes even die themselves. All of these responses require the animal to perceive and recognize death in some way. In a lot of species, this may happen via olfactory signals, aka smell. This could be the absence of pheromones an animal gives off when it’s alive, or chemicals produced as the body decays. Other animals seem to take cues from visual signs of death, like “black and floppy”.

Legend has it this was accidentally demonstrated by jackdaws who swarmed a researcher while he was taking his black swim trunks out of his pocket, in the exact same way they swarm dead birds. For fruit flies, it seems like both sight and smell play a role. In 2019, researchers at the University of Michigan used a T maze to see how live flies react to dead flies. A T maze isn’t what you’d probably picture as a maze. It’s more like a chamber with two arms at the top. Once the animals are inside, they can go one way or another, which scientists use to get information about what the animals prefer.

The researchers found that when one of the arms had only live flies and the other had live flies plus dead ones, the living flies avoided the side with the dead flies as much as they could. But as long as the corpses were a related species of fly, and the living flies could see them, they wanted nothing to do with the dead guys over there. Being able to see the dead flies seems to be critical for this, because it didn’t happen with blind flies or flies that lived in the dark. And the flies didn’t just avoid their dead friends. Living flies exposed to dead flies underwent physiological changes, including less storage of fats that can help them survive without food and an altered metabolic rate and less carbon dioxide production, which can affect the rate of aging.

And when they exposed the flies to dead flies repeatedly throughout their adulthood, the exposed flies only lived about 77% as long as unexposed flies on average. In this case, the flies needed both the visual and olfactory cues. Blind flies and those without the ability to smell didn’t show the effect as strongly as flies who could both see and smell. Basically, simply perceiving dead flies was enough to speed up their aging and make them die sooner. This isn’t the first study that’s shown a relationship between sensory perception and aging, at least in fruit flies and nematodes. See, in these invertebrates, some of the same neurons that are used for smell and taste can also regulate aging and lifespan.

And while we know that giving fruit flies less food to eat can make a fruit fly live longer, when flies on a diet just smell odors derived from nutrients, it actually undoes any of those lifespan benefits. And when honeybees detect a pheromone emitted from sisters that are being raised to be the queen one day, they will only live for about three to six weeks. Once the hive is no longer raising future queens, bees can live for twenty weeks or more. So there does seem to be a connection between sensory perception, physiology, and aging, at least in invertebrates. In fruit flies, serotonin seems to be at the center. Yeah, serotonin, the neurotransmitter famous for the role it plays in your mood. In flies, it appears to be involved in the relationship between lifespan and food perception.

Flies that lack receptors for serotonin can actually live longer when food is in short supply. But what’s driving the connection between flies’ senses and their lifespans has been a mystery. The same group from the University of Michigan took on that question. They found a small group of neurons with serotonin receptors that appear to be required for the “drop dead when you see a dead fly” thing. These neurons were located in the ellipsoid body, an area of the fly brain that helps coordinate sensory information and control movement. These neurons were active after flies had been exposed to dead flies for two days.

When those neurons were shut off, fly lifespans were totally unaffected by the dead. In fact, the same thing happened if just the serotonin receptors were shut off, even if the rest of those neurons were working properly. The other critical part of the process was a link to signaling proteins that also seem to be involved in depression. So, the researchers speculate that seeing dead flies may send the living flies into what they call a “depression-like state”, which ends up shortening their lifespan. Importantly, this isn’t really depression in the way humans experience it, with feeling sad and disinterested in things. They’re just looking at the flies’ neurons when they talk about depression, not their behavior or their feelings. After all, they don’t make couches small enough for flies to go to therapy, although that would be very cute.

And obviously the whole “dying simply when you see dead people” thing doesn’t happen in humans either, or funeral directors would get hazard pay. So, these studies aren’t directly relevant to people. But they could potentially tell us something about how sensory experiences affect physiology, and it does raise some questions about the relationship between depressive symptoms, serotonin, and aging. It’s still early in the research process for that, but who knows when it could lead to a breakthrough. After all, everyone knows that time flies.