Thanks to Linode for supporting this SciShow
video! You can get a $100 60-day credit on a new
Linode account at linode.com/scishow. Grieving is a complex and difficult process,
and none of it is fun, and it might feel like it’s knocking years
off your life… even if it isn’t really. But leave it to nature because that is totally
a thing elsewhere in the animal world. When fruit flies see their dead relatives, they literally go “guess I’ll die” and
keel over. And while that’s very just... what? It could also help us understand depression
and aging in humans. So let’s jump in. [♪ INTRO] 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. 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 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. And if you’re always flying from place to
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video! [♪ OUTRO]
