Not every nook and cranny is an adaptation. Some things just happen (as a byproduct), and don’t have enough selective pressure behind them to change*. This seems to be the case with the human belly button, although one wonders why it is so prominent a hole, prone to collecting odd things like lint and bacteria. Some people find it sexy, and it has been censored in the past for that reason, but by my reckoning at least, that doesn’t seem to be universal.**  Breasts and genitals sure, but navels?
I guess you could try to make it sound like an adaptation, like the amount navels differ from an ‘at rest’ round shape is a hard to fake indicator of fitness. You can suck in your gut but if you’re actually flabby your navel won’t be in that typical tight navel shape. But navel shapes and ab muscles vary too much to make that sound convincing. There are other hypotheses suggesting the navel as an indicator that could be selected for, but I remain of the mind that it just happens to heal like that and it doesn’t matter.

I recently acquired a little dog*** which came complete with a hilarious, cute and endearing belly button. I was shocked the first time it rolled on it’s back exposing it’s almost perfect donut-like (or bagel-like if you don’t have a sweet tooth) protrusion. Until that very point I had, apparently, gone my whole life without once thinking about navels on other animals. Funny, the things that can pass you by. I was more shocked at my own surprise though, rather than the navel itself. After all why wouldn’t a mammal have a navel (even if they were hidden or healed differently)?

The placental technicality can be refined and comparisons to yolk sacs can be made (and man, there’s a whole world of interesting topics there) but for this post that would just be a tangent. The main point I wanted to get across is this: my familiarity with human belly buttons, combined with my unfamiliarity with animal belly buttons has led to some seriously advanced and entertaining pet anthropomorphizing.

As a kid, I would often worry that the “seal” on my navel could unravel somehow. I knew my organs wouldn’t come out—I mean, c’mon, the hole would be way too tiny f’reals. But I was more worried that the stuff that I would find in my navel occasionally would now fall into my abdominal cavity. I have since allayed those fears (or perhaps rendered them moot) by filling my abdominal cavity with alcohol and cheetos. The outward pressure my insides now exert on my navel are sufficient to prevent anything from getting in. Mission accomplished.

Speaking of stuff you find in your navel, science writer and friend of the blog, Carl Zimmer once had his navel flora sampled. He wrote about his bizarrely unique bacterial wonderland on his blog, The Loom.

This post was brought to you by Dubnium (Db).
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*Man-nipples come to mind.

**I could be wrong. Maybe navels are mad sexy and are sexually selected for, but I found no reason to assume that as a general rule.

***Colloquially known as a ‘puppy’

**** and if you think they do, that sentence is the least of your worries on this site.

*****yea I made that up.