A belly button, or navel (clinically called the umbilicus), is the first scar on a placental mammal’s body. It marks the point where the umbilical cord connected the developing fetus to the mother. Virtually all placental mammals, which constitute the vast majority of the class Mammalia, possess a navel. This scar is a direct consequence of the reproductive strategy that supports extended development within the mother’s womb.
The Biological Necessity of the Navel
The navel is fundamentally tied to the necessity of the placenta, an organ unique to placental mammals. The placenta forms during pregnancy, acting as a temporary life support system for the developing young. It functions as a biological exchange site, transferring oxygen and nutrients from the mother to the fetus while simultaneously removing metabolic waste products.
This exchange is facilitated by the umbilical cord, a structure containing blood vessels that runs from the placenta to the fetus’s abdomen. When the offspring is born, this connection is no longer needed, and the cord must be separated. In the wild, the mother typically severs the cord by chewing through it, or the cord naturally breaks during birth.
The remaining stump of the umbilical cord, attached to the newborn’s body, then dries out and falls off within a few days or weeks. This natural process of healing leaves behind a permanent, fibrous scar on the abdominal wall. This scar tissue serves as a lasting anatomical record of the placental connection.
The Difference in Appearance
If all placental mammals have a navel, they are rarely seen on animals like cats, dogs, or livestock due to visual characteristics and concealment. Unlike the pronounced depression or protrusion seen on humans, the navel in most non-human mammals heals into a small, flat, or linear scar. This subtle formation makes it far less noticeable.
The primary factor concealing this scar is the dense fur or hair that covers the abdomen of most mammals. Even in large animals, such as horses or cattle, the healed umbilical scar is often just a small, smooth patch of skin or a slight tuft of hair. The visual prominence of the human navel is partly due to our relative lack of body hair and the way the underlying abdominal fascia heals.
In marine mammals like whales and dolphins, the navel is present but appears as a smooth, faint line or groove on their uniform skin. Among primates, the navel is often more distinct and resembles the human form due to a similar healing process and less dense body hair. The variation in appearance across species is a matter of scar tissue formation and external coverage, not a difference in its biological origin.
What About Marsupials and Monotremes?
The exceptions to the rule of a mammalian navel are found in the two other groups of mammals: marsupials and monotremes. Monotremes, which include the platypus and echidnas, are egg-laying mammals and bypass the need for a placenta and umbilical cord entirely. Since their young develop within a shell outside the mother’s body, they do not form the abdominal scar that becomes a navel.
Marsupials, such as kangaroos and opossums, have a brief gestation period and give birth to highly underdeveloped young. While they form a temporary connection to the mother’s uterus, this structure is a primitive yolk sac, not the complex placenta found in placental mammals. The connection is extremely short-lived, and the young crawl to the pouch almost immediately after birth.
The yolk sac connection in marsupials does not typically leave a permanent, discernible scar. The tiny point of attachment heals quickly and completely, leaving no obvious mark on the adult animal. A notable exception is the bandicoot, which is a marsupial that has evolved a more developed, though still temporary, placental structure.