Belly Button Safari: Who's Living in There?

"We are covered in an ecological wonderland," declares Rob Dunn, a man with a strange idea of a wonderland. In the wild bacterial jungle that is our skin, Dunn has been studying an especially dark cave: the belly button. He's found out which microorganisms are the big game, which are the rare birds, and which ones may take up residence in your navel if you stop bathing.

Dunn is a biologist at North Carolina State University who studies the tiny life forms that share our personal space, from insects in our yards and houses to microbes on our bodies. The organisms we live with can affect our health, for better or worse. Yet researchers are only beginning to explore the various ecosystems we carry on ourselves. From our intestines to our faces to the bottoms of our feet, each of us holds a planet's worth of different habitats.

Out of all these bacterial habitats, the belly button is especially convenient to study. Everyone has one. Its shape and size don't change dramatically from person to person (among innies, anyway). And it's hard to scrub clean, making it a relatively undisturbed environment on the body. A national park, if you will.

The belly button also has crowd appeal, which is important for Dunn's citizen-science approach. He's interested in projects that involve unsqueamish members of the public. For his belly button research, Dunn recruited crowds at two 2011 events: One was Darwin Day at the Museum of Natural Sciences in Raleigh, North Carolina, and the other was the annual Science Online conference in Raleigh.

In total, 60 volunteers had their navels swabbed. Researchers extracted the bacterial DNA from each sample, sequenced it, and searched it for matches to particular species.

What the team found was diversity, and lots of it, as they report today in PLOS ONE. With a median of 67 different bacterial species per person, Dunn says the navel is at least as diverse as the other skin areas studied so far. That diversity varied widely; some people housed more than 3 times as many species in their belly buttons as others.

Out of the thousands of bacterial species the researchers found overall, the vast majority showed up in only a few people, or in just one person. Even though this habitat looks similar from one person to the next, we all have very different collections of rare critters running around in the undergrowth. No one bacterial species appeared in every belly button.

But there are common belly button denizens, too. On our umbilical safari, these are big obvious trees and loud monkeys that show up in most people's jungles. Eight species of bacteria were present in more than 70% of subjects. And wherever these species show up, they do so in large numbers. If you could dump the bacteria from everyone's belly buttons into one pile, members of the 8 king-of-the-jungle species would make up nearly half the heap.

Extending this group to the 23 most common bacterial species, the researchers looked at how the group's DNA compared to rarer bacteria. They found that the ruling bacteria were more closely related to each other than randomly selected groups of bacteria were—sort of a royal family. This suggests that the most common belly button bacteria share evolved traits that help them thrive in this environment.

The most surprising thing about the belly button bacteria, Dunn says, is their ultimate predictability. Even though thousands of species turned up in his study, he now knows which ones are most likely to live in someone's navel. "I expected that the common species would be far more random," he says. "But the truth was otherwise." There are only a few bacteria ruling the belly button jungle, and a diverse throng of others that make up their subjects. Dunn thinks we might be able to study what goes on in our skin's ecosystem by focusing on these few common bacteria.

Dunn hopes that eventually he'll be able to predict the specific bacteria living in someone's belly button based on their age, gender, habits, and history. "But I'll admit we are having an interesting struggle," he says. The research shows that people can be sorted into two or three "bacteriotypes," like blood types, based on the clusters of bacteria that inhabit their navels. But as to why a person is one type or another? "So far we can't explain what causes those differences," Dunn says. "It is a real mystery."

He's getting a little help in this area from one Science Online participant who claimed not to have showered or bathed in "several years." This subject's belly button swab turned up two species of archaea—single-celled organisms, entirely separate from bacteria, that often live in extreme environments. Until now, no one had found archaea on human skin.

This social non-conformer might represent the kinds of bacteria that our ancestors carried around. "Historically, no one washed very often," Dunn says. "This colleague of yours may be far more representative of how our bodies were for thousands, or even millions, of years than are most folks."

He adds, "That isn't saying I'm encouraging everyone to abandon washing."

Dunn suspects that belly button depth, too, might influence what species live there. But he's had a hard time studying this. "No one really wants to answer a question about the depth of their innie, no matter how anonymous we make the process," he says. However, his group's next study will look at a larger group of people, including outies.

Future safaris into our bodies' ecosystems might help scientists understand skin allergies and other health issues. Although belly button sampling is over for now, Dunn encourages people who want to get involved to join the mailing list at yourwildlife.org. He's currently looking at camel crickets in basements, ants in yards, and bacteria in bedrooms and kitchens.

"Armpits," Dunn adds—or perhaps threatens—"are also on the horizon."


Jiri Hulcr, Andrew M. Latimer, Jessica B. Henley, Nina R. Rountree, Noah Fierer, Andrea Lucky, Margaret D. Lowman, & Robert R. Dunn (2012). A Jungle in There: Bacteria in Belly Buttons are Highly Diverse, but Predictable PLOS ONE : 10.1371/journal.pone.0047712

Images: Copyright Belly Button Biodiversity.