Why do armpits smell so much more than the rest of the body? We were told it was because they were warm, moist, and hard to wash. But it turns out the real reason is because they squirt out bacteria food. And they may do for the exact same reason that skunks squirt out stink.
Top image: Chris Korhonen/Flickr.
Hello and welcome to another episode of Your Disgusting Body, where we teach you to feel shame, using science. I'd grown up under the delusion that armpits were just grosser than the rest of the body because they are little nursery areas for bacteria, the way ponds are nursery areas for mosquitoes. Armpits just happened to have the right environment. That isn't the case. Actually, armpits are grosser than the rest of the body because they are little nursery areas for bacteria they way ponds are nursery areas for ducks. Not only are they the right environment — but if the animals settle there, humans regularly give them food.
Most of the body is covered with eccrine sweat glands that secrete water and a little salt. This doesn't feed anything, and so the sweat evaporates without stinking. So far, so good. It turns out, though, that around the rest of the body there are things called apocrine sweat glands. They're just about everywhere people have thick body hair, and both the hair and the glands really get going only after puberty. These glands don't just give off water and salt. They mix in sialomucin. Mucins, as the name suggests, result in a lot of the body secretions that could be described as gelid. Sialomucin is a component in breast milk. It feeds children, when secreted by the right body part, but when secreted by the apocrine sweat glands, it feeds a lot of bacteria. The armpits are the best place on the body to get it, since they are always warm and moist, have no cleaning systems, and almost always have a free buffet.
That's what the bacteria get out of living in an armpit, but what do humans get out of this arrangement? The fact that sialomucin secretion starts at puberty indicates it might be attractive to mates, but then again pimples start at puberty, and no one has ever pretended that they're hot. Some researchers think that the fatty secretion might help maintain the hair.
One theory, in particular really strikes me as interesting. Puberty isn't just a time of life that's all about attracting a mate, it's often also a break away from parents. No one is looking out for the kid anymore. So the powerful smell, and the fact that humans sweat when they get an adrenaline rush, indicates to some that humans might produce stink as a warning signal to cleaner animals. Considering carnivores often smell, and carrion-feeders are used to feeding on things that are actively rotting, it has to be some stench. While there are plenty of theories that are competing in this area, I like the idea that our ancestors shared a defense strategy with the noble skunk. "Seriously, I'm slow and weak and naked and I have no fangs or teeth, but take a whiff of this, Mister Lion. Don't you think you'd rather eat a nice, clean gazelle?"
Skunk Image: Dan & Lin Dzurisin
Via Mayo Clinic, and How Stuff Works.