Every placental mammal on Earth develops an umbilical cord during pregnancy, from a blue whale calf to a newborn mouse. But the cord is not exclusive to the furry animals you might first picture. Certain reptiles, a family of freshwater fish, and even an ancient armored fish that swam Devonian seas all developed or develop structures that function as umbilical cords. The story of which animals have them, and which surprisingly do not, reveals how evolution has independently invented the same solution to the same problem: keeping a developing embryo alive inside its mother’s body.
Placental Mammals and the Classic Umbilical Cord
The group most strongly associated with umbilical cords is the placental mammals, also called eutherians. This group includes humans, dogs, cats, horses, elephants, whales, bats, and rodents. During pregnancy, the placenta anchors to the uterine wall, and the umbilical cord runs from the placenta to the developing fetus. Specialized transient structures, including parts of the allantois, chorion, and yolk sac, are incorporated to form the placenta and cord together, supporting the fetus’s nutritional, respiratory, and waste-removal needs throughout gestation.1PubMed Central. Embryonic and extraembryonic tissues during mammalian development: shifting boundaries in time and space
The cord itself is more than a simple tube. It typically contains blood vessels surrounded by a soft, spongy tissue called Wharton’s jelly. In humans and most other placental mammals, you will find two arteries carrying waste-laden blood away from the fetus and one vein returning oxygen- and nutrient-rich blood from the placenta.2PubMed. The biomechanics of the umbilical cord Wharton Jelly: Roles in hemodynamic proficiency and resistance to compression Wharton’s jelly cushions these vessels against compression and kinking, which is critical since the fetus is constantly shifting around in the womb.3PubMed Central. The Pathophysiology of Wharton’s Jelly and Its Impact on Fetal and Neonatal Outcomes: A Comprehensive Literature Review
That three-vessel arrangement is the standard pattern, but it is not universal across all mammals. A comparative study of cord structure across species found that some mammals have four-vessel cords (which tend to occur in species with larger neonates), while muroid rodents like mice and rats have just two vessels. Cord length also varies enormously: it scales roughly with birth weight, so a whale’s cord is vastly longer than a hamster’s. Species with coiled cords tend to have longer cords for their body size than those with uncoiled cords.4bioRxiv. Umbilical cord structure shapes feto-maternal heat exchange across mammals
What About Marsupials?
Marsupials like kangaroos, koalas, opossums, and wombats occupy an interesting middle ground. They do develop a placenta and a rudimentary umbilical cord, but the arrangement is far simpler and shorter-lived than in placental mammals. A marsupial embryo spends only a brief period attached to the uterine wall before being born at a very early stage of development, often looking more like a pink, jelly-bean-sized larva than a recognizable baby animal. After birth, the real work of nourishment transfers from the cord to the teat. As one review put it, marsupials have “exchanged the umbilical cord for the teat,” relying on a long and physiologically sophisticated lactation period to do the heavy lifting that the placenta handles in eutherian mammals.5Placenta. Review: Marsupials: placental mammals with a difference
So marsupials do technically have umbilical cords, but the cord’s role is dramatically compressed. The joey’s survival depends far more on crawling to the pouch and latching onto a nipple than on any prenatal cord connection. This is why marsupials are sometimes left out of casual discussions about umbilical cords, even though they belong in the conversation.
Animals That Lay Eggs Do Not Have Them
The egg-laying mammals, the monotremes, are the clearest exception within the mammal family tree. Platypuses and echidnas lay leathery eggs and never form a placental connection between mother and embryo. The developing young is nourished entirely by yolk inside the egg, so there is no umbilical cord and no navel scar.
Birds and most reptiles fall into the same category. A chicken embryo inside its egg has a yolk sac and blood vessels running along the inner membrane, but those structures are contained within the egg and are not connected to the mother. There is no maternal-fetal interface and therefore no cord. The same is true for turtles, crocodilians, and the vast majority of snakes and lizards that reproduce by laying eggs.
Viviparous Lizards With True Placentas
Here is where the story gets genuinely surprising. A handful of reptile species give birth to live young and have evolved structures that closely parallel the mammalian placenta and umbilical connection. The best-studied example is the three-toed skink, Chalcides chalcides, a small lizard native to the Mediterranean region. This species was described in detail as far back as 1855 by Italian anatomist Cesare Studiati, who recognized that the arrangement of fetal and maternal tissues met the criteria for a true placenta, complete with a functional role in delivering both oxygen and nutrients to the embryo.6PubMed. Classics revisited. History of reptile placentology: Studiati’s early account of placentation in a viviparous lizard
Modern microscopy has confirmed that this lizard has evolved what researchers describe as a true epitheliochorial placenta, a type that shares many features with the placentas of mammals but also has unique characteristics of its own. The fetal membranes develop specialized layers that change in structure as pregnancy progresses, and both fetal and maternal blood vessels are well developed at the interface.7PubMed. Glycosylation of the materno-foetal interface in the pregnant viviparous placentotrophic lizard Chalcides chalcides: a lectin histochemical study The connection between the embryo and this placenta functions as an umbilical link, even if it does not look exactly like a mammalian cord wrapped in Wharton’s jelly.
These lizards are not a one-off oddity. Viviparity, giving birth to live young rather than laying eggs, has evolved independently in squamate reptiles (lizards and snakes) over a hundred times by some estimates. Not all of those viviparous species develop a complex placenta; many simply retain the egg internally and the embryo lives off yolk. But the ones that do develop placental connections represent a remarkable case of convergent evolution, arriving at a solution strikingly similar to the mammalian umbilical cord through a completely independent evolutionary path.
A Family of Fish With Ribbon-Like “Cords”
Perhaps the most unexpected animals with an umbilical-cord equivalent are the goodeid fish, a family of small freshwater fish found mostly in Mexico. Goodeid embryos develop structures called trophotaeniae: branching, ribbon-like extensions that sprout from the embryo’s perianal region and trail into the fluid of the mother’s ovarian cavity. These ribbons absorb dissolved maternal nutrients from the surrounding ovarian fluid, functioning as a pseudoplacenta.8PubMed Central. Prenatal regression of the trophotaenial placenta in a viviparous fish, Xenotoca eiseni
The trophotaeniae are derived from the embryo’s hindgut and consist of multiple ribbon-like processes extending outward from a tube-like mass of tissue.9PubMed. The trophotaenial placenta of a viviparous goodeid fish. II. Ultrastructure of trophotaeniae, the embryonic component At least 21 species within the Goodeidae family have been documented with these structures.10Journal of Morphology. The trophotaeniae of the goodeidae, a family of viviparous cyprinodont fishes Unlike a mammalian umbilical cord, which connects embryo to placenta via enclosed blood vessels, trophotaeniae work more like external gills turned into feeding organs: they dangle in the mother’s ovarian fluid and absorb nutrients across their surfaces. Before birth, these structures regress and are reabsorbed, leaving the fry looking like an ordinary fish by the time it emerges.8PubMed Central. Prenatal regression of the trophotaenial placenta in a viviparous fish, Xenotoca eiseni
Calling trophotaeniae an “umbilical cord” stretches the term a bit, since they are not physically attached to a maternal organ the way a mammalian cord connects to a placenta. But the function is the same: channeling maternal resources to a developing embryo retained inside the mother’s body. Biologists refer to the system as a trophotaenial placenta, acknowledging both the analogy and the difference.
Caecilians and the Road Not Taken
Caecilians, the legless amphibians that look like oversized earthworms, offer a fascinating contrast. Several species are viviparous, carrying their young inside oviducts rather than laying eggs. Once the embryo exhausts its yolk supply, it does not develop a placenta or cord. Instead, the fetuses use specialized temporary teeth to scrape and feed on the thickened lining of the mother’s oviduct.11Elsevier (Zoologischer Anzeiger). Maternal investment in the viviparous caecilian amphibian Typhlonectes natans (Gymnophiona: Typhlonectidae) This is a completely different solution to the same problem: how does a mother feed an embryo that has already used up its yolk? Mammals evolved the cord, goodeids evolved the trophotaeniae, and caecilians evolved tiny teeth for eating their mother’s uterine tissue from the inside. Evolution is resourceful when it needs to be.
The Oldest Umbilical Cord in the Fossil Record
The question of when umbilical cords first appeared in evolutionary history got a dramatic answer in 2008, when Australian researchers announced the discovery of a 380-million-year-old fossil placoderm fish preserved with an intact embryo and a mineralized umbilical cord still connecting the two. Placoderms were armored jawed fish that went extinct long before dinosaurs appeared. This specimen was not only the first fossil embryo ever found with an umbilical cord but also the oldest known evidence of any creature giving birth to live young.12Museum Victoria. Fish 380 Million Years Old Found With Unborn Embryo
The discovery pushed the known history of internal fertilization and live birth back by roughly 200 million years compared to what scientists had previously confirmed in the fossil record. It also showed that the umbilical cord as a concept, a physical tether between mother and offspring, is far older than mammals. The basic engineering of “connect the baby to the mother with a nutrient pipeline” predates the dinosaurs, the colonization of land by vertebrates, and the evolution of every mammal that has ever lived.
Do All Animals With Umbilical Cords Have Belly Buttons?
In humans, the navel is the scar left after the umbilical cord detaches. Other placental mammals form the same kind of scar, but it is much less conspicuous. In large mammals like dolphins, orangutans, and whales, the navel is visible as a mark on the belly. In smaller mammals like dogs, cats, and mice, the scar is small and usually hidden under fur.13The Conversation. Curious Kids: Do all animals have belly buttons? If you have ever tried to find your cat’s belly button, you have probably noticed it is not easy: it sits roughly midway along the abdomen, typically as a thin, flat line or a tiny bare patch rather than the deep or protruding button familiar in humans.
Animals that never had an umbilical cord, such as birds, egg-laying reptiles, platypuses, and echidnas, do not form a navel. The chick embryo’s yolk-sac attachment leaves a small scar on the abdomen, but it is not the same thing as a mammalian belly button and heals over completely in most species. The viviparous lizards and goodeid fish discussed earlier do not leave a permanent navel scar either, since their connection structures are reabsorbed or detach without creating the kind of thick tissue remnant that scars in mammals.
When the Cord Goes Wrong in Other Species
Umbilical cord complications are well known in human obstetrics, but they are a real concern in veterinary medicine too, particularly in horses. A mare’s umbilical cord can become excessively twisted during gestation, cutting off blood flow to the foal. Cord torsion is a recognized cause of fetal death and abortion in horses, most commonly seen in mid to late pregnancy. Cord length plays a role: in one study of Thoroughbred mares, cords shorter than about 84 centimeters were considered normal, while abnormally long cords were associated with excessive twisting and tissue death in the surrounding membranes.14Journal of Equine Veterinary Science. Late-term abortion associated with umbilical cord torsion in the mare: case report
In cattle, umbilical hernias are a frequent issue in calves, where a weakness at the cord attachment site allows abdominal contents to push through the body wall. Dogs occasionally experience neonatal problems when the umbilical cord is not severed cleanly by the mother, leading to infection. These veterinary parallels underscore that the cord, while essential for prenatal survival, introduces a structural vulnerability that persists across mammalian species. The cord is an elegant engineering solution, but it is also a single point of failure: if blood flow is blocked or the attachment site is compromised, the consequences are severe regardless of whether the animal is a foal or a human infant.
Why the Cord Keeps Evolving Independently
The fact that umbilical-cord-like structures have appeared independently in placoderms, mammals, lizards, and fish tells us something about the constraints of viviparity. Any species that retains developing young inside the mother’s body for a significant period faces the same engineering challenge: the embryo needs a continuous supply of oxygen and nutrients and a way to get rid of waste. If the embryo is small and the gestation is short, diffusion through the surrounding fluid or tissue may be enough. But as embryos get larger or gestation gets longer, passive diffusion stops being sufficient. At that point, natural selection tends to converge on the same handful of solutions: a direct vascular connection (the mammalian cord), an absorptive surface bathed in maternal fluids (the goodeid trophotaeniae), or a thinned-out interface where maternal and fetal blood run close together (the lizard placenta).
The diversity of cord-like structures across the animal kingdom is a textbook example of convergent evolution. These organisms are not closely related and did not inherit their cords from a common ancestor that already had one. They each arrived at a similar structure because the physics and biology of keeping an internal embryo alive push evolution toward a narrow set of workable designs. The mammalian umbilical cord is the version we know best, but it is just one chapter in a much older story that stretches back nearly 400 million years.
Cord Blood and Stem Cell Interest Across Species
One reason scientists have paid such close attention to umbilical cord anatomy in recent decades is the discovery that cord tissue, especially Wharton’s jelly, is a rich source of mesenchymal stem cells. In human medicine, these cells are being studied for use in regenerative therapies, from cartilage repair to immune-system modulation. But the research interest extends to veterinary species as well. Equine Wharton’s jelly and cord blood are being investigated for treating joint injuries in horses, and similar work is underway in dogs. The protective tissue that keeps blood vessels safe during pregnancy turns out to have properties that make it medically useful long after birth, a second life for a structure that most people think of as biological waste.
Wharton’s jelly varies in composition across mammalian species, which is part of what makes comparative studies valuable. Understanding how the jelly’s collagen and hydrated molecules differ between, say, a horse and a human can help researchers figure out which animal models are most useful for developing stem cell therapies.2PubMed. The biomechanics of the umbilical cord Wharton Jelly: Roles in hemodynamic proficiency and resistance to compression The cord, it turns out, is not just an evolutionary curiosity or an obstetric concern. It is also a piece of tissue whose basic biology continues to yield surprises well beyond the moment it is cut.