We are riddled with bacteria. For every human cell in our bodies, there are another 10 microorganisms hanging around. That's a good thing, too, because without them our bodies wouldn't function properly at all. Yet exactly how our microbiomes develop, and how that affects our health, is far from clear. But according to new research, social interactions may play a role—at least in chimpanzees.
"Animal sociality facilitates the transmission of pathogenic microorganisms among hosts, but the extent to which sociality enables animals' beneficial microbial associations is poorly understood," biologist Andrew Moeller and his colleagues write today in Science Advances. In plain English, that statement translates to an obvious (yet curiously unanswered) question: If animals catch colds and flus and other bugs from each other, shouldn't they also be able to catch friendlier germs too?
The answer to that question, Moeller and his team argue, is yes. Getting to that answer required some hard, dirty work—eight years spent with the Kasekela chimpanzees (the ones Jane Goodall made famous), watching their interactions and ... well, collecting their poop. As it happens, Kasekela social interactions vary substantially with the seasons: In winter, they tend to spend more time foraging together in larger groups, while in the summer they split off into smaller groups. That transforms the original question into a simpler one: How do the seasons affect chimps' feces—more specifically, how do they affect the colonies of bacteria living inside the chimps gastrointestinal tracts, where that feces came from?
Moeller and his colleagues' first observation is that chimps' microbiomes were more similar to each other (in the sense of harboring a similar number of a wide variety of different microbes) in the winter than in the summer. What's more, the overall degree of sociability during a given season was a reliable, though not particularly strong, predictor of microbiome similarity: The more social the chimps were, the more their gut bacteria resembled each other's. At the same time, social interactions seemed to increase the diversity of each chimp's microbiome: The more social interactions, the more species there were to be found in each chimp's stomach and intestines.
Intriguingly, chimps' microbiomes were about as similar to their immediate family members as they were to those outside their families, indicating that while some of chimpanzees' microbiota are inherited from mothers, just as many come from interactions with a host of other chimpanzees in the broader community. Social interactions may therefore help keep the community's "pan-microbiome" stable over long periods of time, the authors suggest.
And what does this all mean for us humans? "Given the stark differences between human and chimpanzee social structures, our results indicate a need to thoroughly explore the relationship between social networks and microbiome composition in human societies," the researchers write. "Our results suggest that changes in the human pan-microbiome occurring today as a result of modern lifestyles could be inherited by future generations."
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