Why don’t other animals have belly buttons like ours?

Jennifer Ireland is the curator of mammals at the North Carolina Zoo and is responsible for overseeing the management of the primate, marine mammal, wild cat, and small mammal collections as well as zebra, giraffe, and red river hogs. Here, she does a little navel gazing, explaining why our innies and outies seem to be uniquely human. Questions and answers have been edited.

Q: Belly buttons are a curious feature of the human physique. They stick out, they stick in, they collect lint, and are even home to an entire ecosystems of microbes. Why don’t other animals have belly buttons like ours?

A: You will not find belly buttons on animals like birds and reptiles, but you will find them on most, but not all, mammals. However, the belly buttons on squirrels, tigers, and even whales aren’t as noticeable as the belly buttons you would find on people. To know why, you have to first understand a few basics. Mammals can be divided into three groups – placental mammals, marsupials, and monotremes. Only placental mammals will have belly buttons. Marsupials, such as kangaroos, koalas and opossums, give birth to relatively underdeveloped young. Before they are born, they get their nutrients from a yolk sac in their mother’s womb. Newborn marsupials travel from the birth canal to their mother’s pouch, where they attach to a nipple and suckle until they have outgrown the pouch. Monotremes are very interesting mammals that lay eggs instead of giving birth to live young. There are only five monotreme species left in the world, the duck-billed platypus and four species of the spiny-backed echidna.

The placental mammals make up the vast majority of mammals, and these are the ones that will have belly buttons. Placental mammal females carry the unborn in their womb and nourish it through the placenta, a pancake-shaped organ that temporarily acts like the baby’s lungs, digestive system, and kidneys until those systems can function on their own. The placenta attaches to the fetus’s belly by the umbilical cord. When the fully developed offspring is born, the mother typically cuts the umbilical cord using her teeth. What is left behind is a scar, often flatter and smaller than the scar left on us humans. When you look at non-human mammals like dogs, cats, horses, and chimpanzees, you probably won’t notice these scars because they are usually covered with fur, making them hard to see.