There might be a weird benefit to leaving dead flies where they fall on your windowsill.
Research has shown that when fruit flies of the species Drosophila melanogaster are exposed to the carcasses of their dead friends, their lifespan shrinks in a significant and measurable way.
They start acting withdrawn, lose body fat, and their aging accelerates to the point that they die sooner than fruit flies that don't see their dead buddies just lying where they fall like some macabre fruit fly graveyard.
And now scientists have a better idea about why this happens. Two neuron types receptive to the neurotransmitter serotonin become activated when fruit flies perceive dead comrades, and this increased activity accelerates the flies' aging process.
"Understanding the neural circuits through which death perception impacts these phenotypes may inform future work directed toward understanding the consequences associated with this, and perhaps other sensory experiences in individuals, including humans, and may provide insight into how specific neural states impact behavior and physiology," write a team of researchers led by physiologists Christi Gendron and Tuhin Chakraborty of the University of Michigan.
Sensory processes can influence aging, but we're not particularly knowledgeable about how this happens. The team's previous research exhaustively demonstrated that perceiving dead flies of the same species – known as conspecifics – does have a demonstrable effect on fruit flies, showing that they waste away and die early, but the reasons for this were unknown.
We've seen similar effects in other animals: necrophoresis, or the removal of dead conspecifics, in eusocial insects; vocalization and corpse inspection in elephants; or an increase in levels of regulatory hormones called glucocorticoids in nonhuman primates.
In fruit flies, the changes seemed to involve serotonin, an important neurotransmitter that carries signals between nerve cells, and one of serotonin's receptors, 5-HT2A. The researchers started here, investigating which 5-HT2A-expressing neurons might be involved in the physical effects of what the researchers call "death perception".
They injected live flies with a fluorescent protein and showed them dead conspecifics, which does not exactly sound like a rollicking good time for any of the flies involved. Then, the researchers observed and noted which parts of the living flies' brains lit up when they clocked the carcasses.
Finally, they started artificially activating neurons in these regions in flies that had not seen dead friends until they found two that had the same effect as the death perception, named R2 and R4. So, now we know.
There isn't a lot we can do with that information yet. A fly brain is very different from a human's, but the team hopes that one day it will help better understand how our brains and the aging process work, particularly in people who play challenging roles in society.
"By contributing to an understanding of the physiological effects of death exposure and the biological mechanism(s) that drive them," the researchers write, "our results may provide insight for treating individuals who are routinely exposed to stressful situations surrounding death, including active combat soldiers and first responders."
The research has been published in PLOS Biology.