A belly button, or navel, is simply the scar left on the abdomen where the umbilical cord was severed after birth. This scar is a biological record of a long-term connection to the mother’s internal support system. Exploring the kangaroo’s anatomy reveals why this common feature among many mammals is largely absent in this iconic Australian creature.
The Anatomical Truth About Kangaroos
Kangaroos, like most marsupials, do not possess a prominent, permanent belly button as humans and other placental mammals do. The reason for this difference lies in the extremely short duration of their internal gestation. The umbilical connection that forms is temporary and non-scarring. Any mark that exists is a fleeting remnant of the brief attachment to the uterine wall. This connection quickly dries up and disappears, leaving no lasting navel. Instead of a noticeable scar, a kangaroo’s abdomen is smooth.
Placental Mammals Versus Marsupials
The fundamental difference lies in two distinct reproductive strategies. Placental mammals, or Eutherians, rely on a complex, highly developed placenta and a long-lasting umbilical cord to sustain the fetus internally. This structure facilitates the extended exchange of nutrients, oxygen, and waste products throughout a gestation period that results in a relatively well-developed infant at birth. The eventual separation of this robust umbilical structure leaves the permanent, visible navel scar.
Marsupials, however, employ a different approach, utilizing a rudimentary yolk sac placenta for a very brief internal development period. For a large red kangaroo, this internal gestation may only last around 33 days. This short-lived connection does not require the extensive vascularization and tissue structure that would create a lasting scar. The undeveloped nature of the marsupial embryo at birth means that the intense, long-term support provided by the umbilical cord is instead deferred to an external system.
Early Development: The Journey to the Pouch
The kangaroo’s unique life cycle exchanges the long-term internal connection for an external developmental phase within the mother’s pouch. The infant, known as a joey, is born in a highly altricial, or underdeveloped, state, often weighing less than one gram and resembling a jelly bean. This tiny, hairless creature must then immediately embark on an unassisted, instinct-driven climb from the birth canal up the mother’s fur to the safety of the pouch.
Once inside the marsupium, the joey permanently attaches itself to one of the mother’s teats, which swells to hold the infant securely in place. This teat connection functions as the joey’s alternative to the complex umbilical cord and placenta, providing continuous nutrition and support for many months. The joey remains fixed to the teat for approximately 60 to 70 days before its mouth and limbs develop enough to release the attachment. This external, long-term nursing stage is the defining feature of marsupial development, replacing the need for the long internal gestation that would otherwise create a belly button.