The umbilical cord is a temporary biological structure that serves as a necessary link for the developing offspring of many species during gestation. This specialized organ facilitates the continuous exchange of substances between the mother and the fetus inside the womb. The presence of this cord is directly tied to the reproductive strategy a species employs to nourish its young before birth. Understanding which animals possess this feature requires examining the major classifications of live-bearing species, particularly within the mammalian family.
The Essential Link: Defining the Umbilical Cord
The umbilical cord functions as the physical connection between the developing fetus and the placenta, an organ attached to the mother’s uterine wall. This connection allows the placenta to act as a substitute for the fetus’s developing respiratory, digestive, and excretory systems. The cord is typically a tube-like structure containing three main blood vessels: one vein and two arteries. The umbilical vein transports oxygenated, nutrient-rich blood from the placenta into the fetal circulatory system, while the two arteries carry deoxygenated blood and metabolic waste products away. These vessels are housed within a gelatinous connective tissue known as Wharton’s jelly, which provides a protective cushion against compression.
The Primary Classification: Placental Mammals
The vast majority of animals that possess a true umbilical cord are classified as placental mammals, or Eutheria. This group includes nearly all species commonly recognized as mammals, utilizing a complex, long-lasting placenta to support fetal development over an extended gestation period. The cord’s structure supports the nourishment required for offspring to be born in a relatively advanced state of development. This reproductive method is seen across a massive range of biological niches and environments.
Terrestrial examples include common farm animals such as cattle, horses, pigs, and sheep, as well as companion animals like cats and dogs. In these species, the cord permits the lengthy in-utero development necessary for newborns to stand and move shortly after birth. Even mammals that have adapted to specialized environments, such as bats, rely on this placental system during their pregnancies.
Marine mammals, known as cetaceans, also utilize an umbilical cord system, which is evident by the presence of a navel on species like whales and dolphins. The cord is fundamental to the successful development of their large, complex offspring that must be prepared to swim and surface for air immediately upon delivery. In some mammals, the structure of the cord even varies, as cattle and sheep have two distinct umbilical veins, unlike the single vein found in humans.
Related Animals Without a True Cord
While the umbilical cord is characteristic of placental mammals, two other mammalian groups employ different reproductive strategies that bypass the need for this structure. Marsupials, such as kangaroos, koalas, and opossums, do not develop a complex, long-term umbilical cord. Instead, their gestation period is exceptionally short, often lasting less than 30 days.
During this brief internal phase, the embryo is nourished primarily by a yolk sac, and a complex placenta does not fully form. The young are born in an extremely underdeveloped, almost embryonic state and must crawl from the birth canal into a protective pouch, or marsupium, where they attach to a teat for months to complete their development. This early birth strategy avoids the immune conflict that a long internal gestation would cause without a fully formed placenta.
The other exception is the Monotremes, which include the platypus and echidna. These animals are the only mammals that reproduce by laying eggs. Since the offspring develop externally within a leathery eggshell, the internal connection provided by an umbilical cord is not necessary. The developing young utilize the nutrients within the yolk sac of the egg until they hatch. Furthermore, species outside the mammalian class, such as fish, amphibians, reptiles, and birds, also do not develop umbilical cords.
What Happens to the Umbilical Cord After Birth
Once an offspring is born, the umbilical cord is no longer functional because the newborn begins to breathe air and feed independently. In the wild, the severance of the cord occurs naturally, often without the need for external intervention. Many mammalian mothers, particularly carnivores and omnivores like dogs and cats, instinctively gnaw through the cord to separate the young from the placenta. This action is often coupled with consuming the placenta, which helps to clean the area and eliminate scents that might attract predators.
In large herbivores, such as horses and cattle, the cord typically snaps on its own when the newborn stands up shortly after delivery. Similarly, the explosive expulsion of the placenta in marine mammals, combined with the movement of the water, causes the cord to break. Regardless of the method of severance, the remaining cord stump on the newborn’s abdomen dries, shrivels, and eventually falls off. This natural healing process leaves behind a permanent scar, the navel, found on nearly all placental mammals.