All placental mammals, including rats, possess an umbilicus, commonly known as a belly button. This feature is the resulting scar tissue left after the umbilical cord detaches following birth. The scar is a biological certainty because the developing fetus requires a physical connection to the mother during gestation. In rats, the umbilicus exists but looks vastly different from the prominent feature seen in humans.
The Umbilical Cord Connection
The presence of an umbilicus confirms that the rat is a placental mammal. During the approximately 21-day gestation period, the umbilical cord acts as the sole lifeline between the developing rat fetus and the mother’s placenta. This structure contains blood vessels—specifically, two arteries and one vein—that perform a continuous exchange of materials.
The umbilical vein carries oxygen and nutrient-rich blood from the placenta into the fetal circulation, sustaining the pup’s growth. Simultaneously, the umbilical arteries transport deoxygenated blood and metabolic waste products back to the mother’s bloodstream for disposal. Once the rat pup is born, this connection is no longer required.
The mother rat typically severs the umbilical cord shortly after birth by gnawing through the tissue. The remaining stump of tissue dries up and eventually falls off, leaving behind the rat’s small, permanent umbilicus on the abdomen.
Identifying the Rat Umbilicus
The common question of whether rats have a belly button stems from its subtle appearance. Unlike the deep indentation or raised knot found in humans, the rat umbilicus is a small, pale, and remarkably flat scar. It does not form a distinct “innie” or “outie” structure.
The scar is generally a thin, linear mark, sometimes slightly round, located on the midline of the lower abdomen. Finding this mark requires careful examination of the rat’s ventral side. The dense fur covering the body effectively conceals this small scar from casual view.
Because the scar is flat and lacks the three-dimensional structure of the human navel, it blends seamlessly with the surrounding skin tissue. This minimal scarring results from the natural way the cord is severed and the small size of the umbilical opening at birth.
Umbilicus Across Mammalian Species
The appearance of the umbilicus varies significantly across the mammalian class, yet its fundamental existence is a shared trait for all placental species. Domestic animals, including dogs and cats, possess a flat, inconspicuous umbilicus similar to that of the rat. In these animals, the scar is typically hidden by fur, making it challenging to locate without parting the hair.
Primates, such as humans and monkeys, are among the few mammals whose navel forms a more noticeable depression or protrusion. In livestock, like cattle and horses, the umbilicus often appears as a simple, flat patch of hairless skin or a thin line. These differences are largely due to variations in the umbilical cord’s width, the method of detachment, and the subsequent healing process.