The human belly button is a familiar scar, prompting curiosity about whether other creatures in the animal kingdom share this characteristic. Exploring the presence of belly buttons in different species reveals much about their unique reproductive strategies and developmental processes.
The Umbilical Cord and Its Scar
A belly button, or navel, is a scar left where the umbilical cord once attached. During gestation, the umbilical cord serves as a link between a developing fetus and its mother. This structure transfers oxygen and essential nutrients from the maternal circulation to the fetal circulation, and also removes waste products, such as carbon dioxide and urea, from the fetus, returning them to the mother for elimination. The umbilical cord typically contains two umbilical arteries, which carry deoxygenated blood and waste away from the fetus, and one umbilical vein, which delivers oxygen-rich blood and nutrients to the fetus.
After birth, the cord is either severed naturally or by the mother, or it is clamped and cut. The remaining stump dries and falls off within weeks, leaving behind the characteristic scar known as the belly button.
Animals with Belly Buttons
Animals with belly buttons are primarily placental mammals. Like humans, these animals develop inside the mother’s womb, receiving nourishment and oxygen through a placenta and an umbilical cord. The presence of this direct maternal-fetal connection during gestation is the defining factor for having a navel.
Common examples of placental mammals include dogs, cats, cows, horses, and various primates like monkeys and chimpanzees. Even marine mammals like whales and dolphins, and terrestrial giants such as elephants, have belly buttons. While present, their belly buttons are often less noticeable than those of humans, often appearing as a flat scar, a slight fold, or a small patch of differently textured skin. This subtle appearance is often due to being covered by fur, or in aquatic species, being streamlined for hydrodynamics.
Animals Without Belly Buttons
Many animals do not have belly buttons because their reproductive methods do not involve an umbilical cord and placenta. This group includes egg-laying animals such as birds, reptiles, amphibians, and most fish. These creatures develop inside an egg, relying on the yolk sac within the egg for their nutritional needs, rather than a direct connection to a mother.
Marsupials, like kangaroos, koalas, and opossums, typically lack a visible belly button. Although they have a rudimentary placental connection, their young are born at an early, underdeveloped stage and migrate to a pouch to complete their growth. The umbilical cord, if present, is often very small and leaves little to no lasting scar.
Monotremes, which are egg-laying mammals like the platypus and echidna, do not develop an umbilical cord and therefore do not have a navel. Invertebrates, such as insects, worms, and jellyfish, also do not possess belly buttons as their developmental processes do not involve internal gestation or an umbilical cord.