Originally published on Student Engagers on March 19, 2014. This post became hugely popular after it was quoted in a Buzzfeed article (and translated into a number of European languages) in which I sound much more of a navel expert than I actually am.
This question was thrown at me at the end of a conversation about juvenile bone growth, and I was completely blindsided. I know my cat definitely has a bump in the place his navel should be, and I assumed all placental mammals have them.
Further research shows that indeed, all placental mammals startwith a belly button (or navel, or umbilicus if you’re scientific). The navel is the remnant of the umbilical cord, which attaches a fetus to the mother’s placenta to deliver nutrients in utero. Thus animals that hatch from eggs don’t have them – this includes marsupials like kangaroos and wombats, which have not evolved a placental structure and instead incubate their young in a pouch. However, in most other mammals (and certain humans) they’re obscured by fur, and in some species they are a thin scar rather than a small bump, and fade over the course of the animal’s lifetime.