The question of whether animals possess a belly button, or navel, is an inquiry into the biological history of a species. The belly button is simply a scar on the abdomen that signifies a past connection to the mother. It is formed by the detachment of the umbilical cord, which served as a lifeline during development. The general answer is yes, but this characteristic is not universal. Only specific groups of animals share this anatomical feature, making the presence of a navel a direct result of reproductive strategy.
The Biological Necessity of the Umbilicus
The formation of a true navel depends on the development of the placenta. This temporary organ develops within the mother’s uterus and acts as a biological interface between the mother and the developing fetus. The placenta filters and transfers oxygen, glucose, and other necessary nutrients from the maternal bloodstream to the fetus.
The physical link carrying these resources is the umbilical cord, a structure containing blood vessels that run from the fetus’s abdomen to the placenta. The cord also removes metabolic waste products from the fetal circulation, sending them back to the mother for disposal. This system ensures the fetus can grow and develop until birth.
Once the offspring is born, the placenta and umbilical cord are no longer needed. The mother animal often instinctively chews through or breaks the cord, or it may simply dry out and detach naturally. When the remaining stump of the cord dries up and falls off, the resulting fibrous tissue forms the umbilicus, leaving behind the small, permanent scar known as the belly button.
Where True Navels Are Found
True, permanent belly buttons are found exclusively on placental mammals, the largest and most diverse group of mammals. This includes common domestic animals like dogs, cats, cows, pigs, and horses, as well as marine mammals such as whales and dolphins, and large terrestrial species like elephants. All these animals undergo a long gestation period supported by a fully developed placenta and umbilical cord.
Despite having this feature, the navel on most placental mammals is far less noticeable than it is on humans. This difference lies in the healing process and external anatomy. The umbilical cord scar in animals like dogs and cats tends to heal very smoothly and flatly against the skin, forming a small, linear scar rather than the concave or convex “innie” or “outie” seen in primates.
This subtle scar is typically covered by hair, fur, or thick skin, making it difficult to locate. For example, a horse’s navel appears as a thin, slit-like line of hairless tissue on its lower abdomen, while a whale’s navel is a smooth, flat depression on its underside. The lack of a noticeable “button” is simply a difference in scar tissue formation and the absence of fat tissue accumulating around the site, which is common in human abdominal anatomy.
How Other Species Separate From the Mother
Mammals that are not placental, specifically marsupials and monotremes, do not develop a true navel in the same way. Marsupials, such as kangaroos and opossums, have a very short gestation period and utilize a primitive, yolk sac-based placenta. The young are born in an extremely underdeveloped state, sometimes weighing less than a gram.
The rudimentary connection, which is more of a yolk stalk than a true umbilical cord, detaches quickly. The attachment point often heals while the offspring is still making its journey to the mother’s pouch or shortly thereafter. This process typically leaves no permanent, recognizable scar on the adult animal.
Monotremes, which include the platypus and echidna, do not give live birth. Because these animals lay eggs, they never develop any form of placental or umbilical cord connection to the mother. They obtain all necessary nutrients from the yolk sac inside the egg, meaning they emerge without any abdominal scar.
Even non-mammalian vertebrates, like birds, reptiles, and fish, which also develop inside an egg, do not possess a true navel. The developing embryo is tethered to the nutrient-rich yolk sac via a yolk stalk. When the animal hatches, the yolk sac is absorbed into the body, and the connection site closes, leaving a temporary umbilical scar that fades quickly.