Many mammals have virtually no noticeable belly button. We humans, however, are left with an umbilical scar that is not only obvious but which varies dramatically. Some are seen as beautiful. Others are deemed ugly.
Aki Sinkkonen at the University of Helsinki in Finland thinks the belly button, aka the umbilicus, serves a greater purpose than mere cosmetics: It may be an indicator of mating potential in fertile women.
"I propose that umbilicus, together with the surrounding skin area, is an honest signal of individual vigor," Sinkkonen wrote in the latest issue of The FASEB Journal. "More precisely, I suggest that the symmetry, shape, and position of umbilicus can be used to estimate the reproductive potential of fertile females, including risks of certain genetically and maternally inherited fetal anomalies."
Innie vs. outie
During pregnancy, the umbilical cord supplies a baby with nutrients and oxygen from the mother. After birth, it's clamped and snipped, leaving a short stump that eventually falls off.
It's not known exactly why belly buttons end up in so many different configurations. What is clear: People have preferences.
In reviewing other studies, Sinkkonen found we prefer belly buttons that are t-shaped or oval and vertical, with a little hooding, and which don't protrude. In short, we dislike outies and don't favor the cavernous variety either.
Sinkkonen reviewed other studies of the umbilicus and essentially connected some dots. He notes that the umbilical cord is critical during pregnancy, and suggests that many problems can be reflected in scar tissue, which is essentially what's left behind as a belly button.
"I suggest that umbilicus is a fitness signal," Sinkkonen told LiveScience in an email. "I may be wrong. However, many mammals do not have a visible umbilicus. We have, but it does not have an obvious function, except signaling."
Not just navel gazing
This novel navel notion has a serious aspect.
"If further research confirms the signaling hypothesis, female umbilici may be routinely measured to detect risk pregnancies of several fetal abnormalities," Sinkkonen wrote in the journal.
And for anyone who worries their love life might be doomed by an imperfect belly button?
"Don't worry," Sinkkonen said. "Nobody's perfect except Angelina Jolie. Seriously, I know wonderful women who believe that their umbilicus/belly button is ugly. Nobody should pay too much attention on the details of his/her body."
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Robert is an independent health and science journalist and writer based in Phoenix, Arizona. He is a former editor-in-chief of Live Science with over 20 years of experience as a reporter and editor. He has worked on websites such as Space.com and Tom's Guide, and is a contributor on Medium, covering how we age and how to optimize the mind and body through time. He has a journalism degree from Humboldt State University in California.