视听｜为什么苍蝇看到死蝇会死去

苍蝇看到死蝇会死去？这一现象背后有什么科学原理？今天推介的视频解释了苍蝇对死蝇的特殊反应，以及可能导致其死亡的原因。通过清晰的解说和有趣的视觉展示，观众将深入了解这个奇怪的现象，并从中获得更多包括昆虫行为在内的生物学知识。

Why Flies Die

When They See Dead Flies

When fruit flies see their dead relatives, they literally go guess I'll die and keel① over and while that's very just what it could also help us understand depression and aging in humans, so let's jump in. The ability to respond to death isn't just a human trait.

① keel /kiːl/ n. the principal structural member of a ship or boat, running lengthwise along the centerline from bow to stern, to which the frames are attached 龙骨

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, trying to lift it up and trump it.

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 olfactory② signals, aka smell. This could be the absence of pheromones③ an animal gives off when it's alive or chemicals produced as the body decays.

② olfactory /ɒlˈfæktəri/ adj. relating to the sense of smell; connected to the sense of smell or the organs involved in smelling 嗅觉的，与嗅觉有关的

③ pheromones /ˈfɛrəmoʊnz/ n. chemical substances produced by animals, especially insects, that are released into the environment and elicit specific behavioral responses or attract individuals of the same species for purposes such as mating or marking territory 信息素，动物特别是昆虫产生的化学物质

Other animals seem to take cues from visual signs of death like black and floppy④ legend has it that 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.

④ floppy /ˈflɑːpi/ adj. lacking stiffness or firmness; limp or flexible 软弱无力的，松弛的

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, which is also what I would do.

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, 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 percent as long as unexposed flies on average.

In this case, the flies needed both the visual and olfactory cues lined 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 nematodes⑤. In these invertebrates⑥, 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.

⑤ nematodes /ˈnɛmətoʊdz/ n. worms of the phylum Nematoda, which are typically small, slender, cylindrical in shape 线虫

⑥ invertebrates /ɪnˈvɜːrtɪbreɪts/ n. animals that do not have a backbone or vertebral column 无脊椎动物

It actually undoes any of those lifespan benefits and when honeybees detect a pheromoneemitted 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. They can live for 20 weeks or more, so there does seem to be a connection between sensory perception, physiology and aging at least in invertebrates.

In fruit flies serotonin⑦ 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 actually live longer when food is in short supply, but what's driving the connection between fly's senses and their lifespans has been a mystery.

⑦ serotonin /ˌsɛrəˈtoʊnɪn/ n. a neurotransmitter and hormone that is found in the brain, blood platelets 血清素

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 ellipsoid⑧ body, an area of the fly brain that helped coordinate sensory information and control movement.

⑧ ellipsoid /ɪˈlɪpsɔɪd/ n. a three-dimensional geometric shape that is stretched in one direction and compressed in the other two directions 椭球体

These neurons were active after the flies had been exposed to dead flies for two days. When those neurons were shut off, fly life spans 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 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 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 and if you're always flying from place to place, this video's sponsor Lenode could help you out.

来源：SciShow