In some people, wool from clothing does not collect in the navel, while others have to clean the fibers every day.
Jason J. Goldman reveals why this force is so strong in some.
There are two key things you need to know about the fiber that forms inside the belly button.
The first is that it is scientifically called "belly button wool," although those who write about it in the scientific literature sometimes call it "belly button cotton" (BBL).
The second is that belly button hair forms more often in middle-aged, hairy men, especially those who have recently gained weight.
These are the findings of researcher Karl Krushelnicki from the University of Sydney.
Dr. Karl, as he is known to his fans, hosts an Australian science radio show, and one of his listeners wrote in to ask where belly button wool comes from and how it is created.
This inspired Krushelnicki to conduct an online survey, which concluded that BBL is mostly a feature of middle-aged men with sufficient body hair.
For his research, Krušelnicki won Ig Nobel Prize for 2002, which is an award given for research that "first makes you laugh, then makes you think."
Wool may help keep the belly button cleaner by collecting bacteria as it forms.
Along with an online questionnaire, Krushelnicki and his colleagues collected samples from willing volunteers and also asked them to shave their pubic hair.
It turns out that shaving the hair off the stomach actually prevented the accumulation of fiber.
While he may not be a leading expert on the subject, Dr. Karl and his colleagues have come up with an explanation for the formation of belly button wool that, at least, makes intuitive sense.
Navel hair, they think, functions like a “one-way ratchet mechanism,” stealing tiny fibers from the inside of your clothing and storing them inside your belly button.
Older clothes, less fiber
Krushelnicki is not the only person who has tried to unravel the mystery of what forms the wool that fills the navels of people around the world.
A researcher at the Vienna University of Technology named Georg Steinhauser published the hypothesis in an unusual journal in 2009. Medical hypotheses.
For reasons known only to himself, Steinhauser collected wool from his own navel every evening for three years.
Although he insists on maintaining good personal hygiene, which includes showering every morning, his belly button would be completely filled with wool by the end of each day.
Overall, Steinhauser collected 503 samples from his own navel.
Their combined weight did not even reach one gram.
On average, one piece weighed 1,82 milligrams, although seven pieces weighed more than 7,2 milligrams, and the winner was a real belly button monster, weighing an incredible 9,17 milligrams.
The hairs themselves peel tiny fibers off the shirt and then direct them towards the navel where they accumulate.
“It was obvious that the wool had been collected from cotton fibers,” Steinhauser wrote, “because the fibers were the same color as the T-shirt in question.”
He collected fewer fibers when he wore older shirts, probably because they were already stripped of stray fibers, and when he wore classic button-down shirts.
Steinhauser ultimately came to the same conclusion as Krušelnicki: those hairs surrounding the navel are the culprits for collecting fibers.
He reasoned that the hairs themselves peel tiny fibers from the shirt and then direct them towards the navel, where they accumulate.
"The scales of the hair act like a kind of 'serrated hooks,'" he said.
He also shaved the hair around his own navel on one occasion.
As with Dr. Karl's volunteer participants, he found that this was enough to stop the accumulation in his navel.
Wool usually takes on the color of the cotton fibers of which it is mostly composed.
But Steinhauser took his own research one step further.
He analyzed the chemical composition of a BBL sample he collected after wearing a plain white 100 percent cotton T-shirt.
If his belly button hair is composed exclusively of fibers from his T-shirt, then analysis will reveal that the fibers are composed exclusively of cellulose.
What he discovered, however, was that other waste had become entangled in the umbilical cord fibers as well.
Based on the chemical reading, Steinhauser suspects that the remaining matter is made up of house dust, skin flakes, fat, protein, and sweat.
Stomach hair, it seems, does not discriminate.
Based on this, he reasoned that those whose belly buttons accumulate wool may have cleaner and more hygienic belly buttons overall, because removing the fibers brings with it the ride and all.
Although only a handful of other researchers have devoted time and energy to investigating the ontogeny of umbilical cord wool - perhaps even Krushelnicki and Steinhauser are the only ones.
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North Carolina State University conducted research into what else lives in our belly buttons.
Rob Dunn, a researcher in the Department of Biology and the Keck Center for Behavioral Biology at NCSU, founded a citizen science project called the "Navel Diversity Project."
In 2011, Dunn and his colleagues collected samples from more than 500 volunteers at the 2011 Science Online Conference in Raleigh, North Carolina, and Darwin Day at the Raleigh Museum of Natural History.
But the researchers weren't too interested in fiber.
Instead, they wanted to understand umbilical microbiome.
"The navel is one of the habitats closest to us, yet it remains relatively unexplored," they wrote.
And that's why they decided to discover what bacteria live in our belly buttons.
Since that initial study (a second round of collection has since taken place), Dan and his team have discovered an enormous microbial diversity in navels, a veritable treasure trove of microscopic life forms.
Adjusting the navel
In the 60 samples they initially examined, they counted at least 2.368 species and suspect that this figure is likely much lower than the real number.
To put that into context, that's more than twice as much biodiversity as North American birds or ants.
But most of these species were rare: 2.128 of them were present in the navels of fewer than six people.
Moreover, most could be found on just one individual.
Despite the incredible diversity, the vast majority of bacteria found in the human navel come from just a handful of species.
Although there was no single type common to all individuals, eight types were present in at least 70 percent of the participants.
Together, these eight species accounted for almost half of all bacteria found.
The researchers also found three species of archaea, a type usually found only in extreme environments.
To make things even more interesting, two out of three came from just one individual who said he hadn't showered or bathed in several years.
Why so much diversity in the navel?
Dan and his team suspect that a cluster of common species have adapted to life on human skin—or perhaps even the navel itself—while others are only occasional visitors to the far shores of the navel.
They draw a parallel with fish in a pond.
Permanent residents have adapted to the pond habitat, while other species that appear only for a short time may not be equipped for a long-term stay.
Likewise, a disproportionate number of trees in any rainforest are uniquely adapted to life in the tropics.
Other trees may be able to grow in the rainforest soil, but they cannot form a strong community.
Although sheer diversity makes it impossible to predict which types of bacteria might be found inside the navel of any given human individual, what researchers can do is predict which species are most common and which are less common.
So, if your belly button isn't routinely collecting fibers to form wool balls, don't worry: your belly button is still a very exciting place.
Truly, he is teeming with life.
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