Topography matters, a lot, the researchers reported. If a moist, hairy underarm is like a rain forest, the dry inside of the forearm is a desert. They harbor distinctly different bacteria suited to those distinctly different environments. In fact, the bacteria under two unrelated people's underarms are more similar than the bacteria that lives on one person's underarm and forearm.
Mom's advice to wash behind your ears notwithstanding, that spot contained the least diverse bacteria — 19 species on average. The most diverse spot: the forearm, which averaged 44 species.
How many are supposed to live there? That's not clear yet. Some certainly could be tourists, picked up as we go about our day. When researchers re-checked five of these volunteers a few months later, the bacteria in some spots — the moist nostril and groin, for example — proved pretty stable while other spots, including the forearm, had changed quite a bit.
