A belly button, or navel, is a small scar on the abdomen. It serves as a lasting reminder of the umbilical cord, which connected a developing mammal to its mother during gestation. This article explores whether all mammals share this characteristic.
The Mammalian Navel: A Universal Mark
Nearly all mammals possess a belly button. This universal feature stems from the development of placental mammals within a mother’s womb. During this prenatal period, an umbilical cord forms, acting as a vital lifeline between the developing fetus and the placenta. The placenta, an organ unique to pregnancy, facilitates the transfer of oxygen and nutrients from the mother’s bloodstream to the growing young, while also removing waste products. This connection is essential for proper fetal development. Any mammal relying on this placental and umbilical cord system will inherently have a navel as a remnant.
From Cord to Scar: How a Belly Button Forms
The formation of a belly button begins right after birth when the umbilical cord is no longer necessary. In many mammalian species, including dogs and cats, the mother instinctively chews through the cord, severing the connection. For human newborns, the cord is typically clamped and cut by a medical professional. A small stump of the umbilical cord remains attached to the newborn.
This stump then undergoes a natural healing process. It dries out, shrivels, and darkens over several days or weeks. Eventually, the dried stump falls off, leaving behind the small scar that becomes the belly button. This process of detachment and healing is consistent across most placental mammals.
Variations in Mammalian Belly Buttons
While almost all mammals have a navel, its appearance can vary significantly across species. In many animals, such as dogs, cats, and cows, the belly button is often a small, flat, and inconspicuous scar. It can be difficult to spot, especially when hidden by fur. This contrasts with humans, whose navels tend to be more prominent, often appearing as an “innie” or “outie.”
The exceptions to this universal presence are marsupials and monotremes, two unique groups of mammals. Marsupials, like kangaroos and koalas, give birth to highly underdeveloped young that complete their growth in a pouch. While a temporary connection to a yolk sac or rudimentary placenta exists during their brief uterine development, it does not leave a lasting external scar like the navel of placental mammals. Monotremes, which include the platypus and echidna, are the only mammals that lay eggs. Since their young develop outside the mother’s body within an egg, they do not form an umbilical cord or placenta, and thus do not have a belly button.