Have you ever said, “I know it like the back of my hand”? Well, how well do you know the back of your hand? Or the back of your knee? Or behind your ears? Probably not as well as scientists at the National Institutes of Health who just completed a survey of the bacteria that live on the skin at various parts of the body. They found that different sites harbor different bugs, results published in the May 29th issue of the journal Science.

The skin, they point out, is a large ecosystem that includes a variety of ecological niches. Your moist armpit is like a tropical rainforest, whereas, not far away, your forearm is as smooth and dry as a desert. So the scientists took samples from 20 different body regions in 10 volunteers. And found that the microbes in your armpit are more like those in someone else’s armpit than the ones on your own forearm.

Dry skin showed the greatest diversity with an average of 44 different species found on the forearm. While the least diverse mix, with only 19 species, lives aft of your ears. So science-savvy moms: don’t worry about washing behind the ears. Instead, bug your brood to give those grubby forearms a good scrubbing.

—Karen Hopkin