What Are Placental Mammals? Definition and Examples

Placental mammals are the mammals whose unborn young grow inside the mother’s womb, fed and supplied with oxygen through a temporary organ called the placenta until they are born fairly well developed. They are by far the largest group of mammals, and you are one of them, along with your dog, a whale and a bat. The other two mammal groups, the pouched marsupials and the egg-laying monotremes, solve the same problem of raising young very differently.1PubMed Central. Paleovirology of ‘syncytins’, retroviral env genes exapted for a role in placentation

How the Placenta Keeps a Baby Alive

The placenta forms during pregnancy, attached to the wall of the womb and linked to the baby by the umbilical cord. Everything the growing baby needs, including nutrients and oxygen, crosses from the mother’s blood to the baby’s blood through it.2PubMed. Off to a good start: The importance of the placental exchange surface – Lessons from the mouse Waste products make the opposite trip, passing back into the mother’s blood to be cleared.3PubMed. Off to a good start: The importance of the placental exchange surface – Lessons from the mouse The two blood supplies never mix. A thin barrier of tissue divides them, and everything has to pass through it.4PubMed. Off to a good start: The importance of the placental exchange surface – Lessons from the mouse How well that barrier forms early on affects how well the baby can grow.

The placenta does not simply sit against the womb wall. Its tissue grows into the womb lining, and that contact is where the exchange happens. The umbilical cord is the baby’s side of the link, carrying blood between the baby and the placenta.

It is also more than a pipe. The placenta produces hormones that help maintain the pregnancy and prepare the mother’s body for feeding the newborn. It acts as a partial filter too, though an imperfect one, since some drugs, alcohol and certain infections can still cross to the baby.

Placentas are not all built alike across placental mammals.5PubMed Central. Captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants In dolphins the placenta spreads over the whole inside of the womb and keeps a complete layer of tissue between mother and baby.6UCSD Placentation. Bottlenose Dolphin Placentation In some other species, including humans, the baby’s tissue sits much closer to the mother’s blood.

Once the baby is born, the placenta is pushed out too, which is why it is often called the afterbirth. Many mammal mothers eat it, recycling its nutrients.

Placentals are not the only animals with a placenta, though. Marsupials form a brief, simple placenta-like connection, and some lizards and snakes grow placentas that mainly handle gas exchange. What sets placental mammals apart is a long-lasting, highly efficient one.

Familiar Examples of Placental Mammals

The group spans everything from shrews weighing a few grams to blue whales, the largest animal ever.12PubMed Central. Phylogenomic Analysis Resolves the Interordinal Relationships and Rapid Diversification of the Laurasiatherian Mammals Some members are surprising. Elephants are closer relatives of manatees and hyraxes than of rhinos.

Other animals are often wrongly assumed to be placentals. Kangaroos, koalas and wombats are marsupials, and so are opossums in the Americas. The platypus and echidnas lay eggs.

How Placentals Differ From Marsupials and Egg-Laying Mammals

Marsupials give birth to tiny, underdeveloped young that grow attached to a teat, often in a pouch. Placental mammals generally give birth to more developed newborns than either of the other two groups.13PubMed Central. Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype Those newborns still vary widely, from helpless (like mice and kittens) to ready to run (like foals and calves).14PubMed Central. Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype Species with well-developed newborns tend to have longer pregnancies for the mother’s size.15Nature. Gestation period, neonatal size and maternal investment in placental mammals

A newborn kangaroo, about the size of a jellybean, crawls to the pouch, while a calf stands and walks within hours. In placentals, much of the growing happens inside the mother’s body, fed through the placenta. In pouched mammals, much of it happens after birth, as the tiny young feed on milk from a teat. Egg-laying mammals hatch their young from soft-shelled eggs, and the young then drink milk that is secreted onto the skin rather than from nipples.

The two strategies carry different costs. A placental mother spends more energy each day per baby than a similar-sized pouched mother.16PubMed. Energetics of offspring production: a comparison of a marsupial (Monodelphis domestica) and a eutherian (Mesocricetus auratus) But the pouched approach takes longer overall from conception to independence.17PubMed. Energetics of offspring production: a comparison of a marsupial (Monodelphis domestica) and a eutherian (Mesocricetus auratus) So placentals pay more up front to produce young faster.

The bodies differ as well. Pouched and egg-laying mammals have small bones projecting forward from the hip, which living placentals lack.18PubMed. Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia Losing them likely went with longer pregnancies, since young no longer needed a long external attachment to the mother.19PubMed. Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia

The brain differs too. Only placentals have the corpus callosum, a thick bridge of nerve fibres joining the two halves of the brain.20PubMed Central. A pan-mammalian map of interhemispheric brain connections predates the evolution of the corpus callosum The other two groups still link the halves through an older, smaller pathway with a similar wiring plan.21PubMed Central. A pan-mammalian map of interhemispheric brain connections predates the evolution of the corpus callosum

The Virus Genes Behind the Placenta

Some genes needed for the placenta, called syncytins, originally came from viruses that inserted their DNA into ancestral mammals.22PubMed Central. Paleovirology of ‘syncytins’, retroviral env genes exapted for a role in placentation These genes help placenta cells fuse into a single continuous layer where mother and baby meet.23PubMed. From ancestral infectious retroviruses to bona fide cellular genes: role of the captured syncytins in placentation In mice, removing them stops the placenta forming, and the embryo dies.24PubMed Central. Paleovirology of ‘syncytins’, retroviral env genes exapted for a role in placentation

A virus that inserts its DNA into an egg or sperm cell can pass that DNA to every descendant of the animal that results. Most such DNA is harmless leftover that does nothing or is slowly damaged by mutation. A few viral genes happened to be useful, and natural selection kept them and reused them for new jobs. Syncytins are examples: the virus’s original trick of fusing cells was reused to fuse placenta cells.

Different lineages, including primates, rodents, rabbits and carnivores, captured different viral genes independently.25PubMed Central. Captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants This may help explain why placentas vary so much between species.26PubMed Central. Captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants

Viral DNA may also protect the baby. Part of the viral gene that once helped viruses dodge immune defences may help stop the mother’s immune system attacking the baby.27PubMed. From ancestral infectious retroviruses to bona fide cellular genes: role of the captured syncytins in placentation

Placental mammals also switch some genes on or off depending on whether they came from the mother or the father, a pattern concentrated in the placenta.28PubMed Central. Evolution of parent-of-origin effects on placental gene expression in house mice Errors in this system can cause growth problems or loss of the baby.29PubMed Central. The Evolutionary Advantage in Mammals of the Complementary Monoallelic Expression Mechanism of Genomic Imprinting and Its Emergence From a Defense Against the Insertion Into the Host Genome

The Four Big Branches of Placental Mammals

Laurasiatheria is named after the ancient northern landmass Laurasia, where it and Euarchontoglires likely began. The names reflect where each group is thought to have arisen, but members have since spread, so they are not a map of where animals live today. You sit in Euarchontoglires, with rodents and rabbits as relatives.

For a long time scientists disagreed on how placental groups were related, because body shapes gave conflicting answers.34PubMed. Molecular phylogenetics and the origins of placental mammals Large DNA studies then settled on these four groups, confirmed by several independent kinds of genetic evidence.35Systematic Biology. The Adequacy of Morphology for Reconstructing the Early History of Placental Mammals The African and northern groups each evolved swimming, hoofed and insect-eating forms separately.36PubMed. Parallel adaptive radiations in two major clades of placental mammals So looks alone can mislead about which mammals are related.

Golden moles are a good case. They look and dig like European moles, yet they belong to the elephant group, not the shrew group.

Placental Look-Alikes Among Marsupials

Convergent evolution is when distantly related animals evolve similar traits because they live similar lives.37Nature. Kangaroo Mob ~ Marsupials: The Evolution Backstory The extinct thylacine, or Tasmanian wolf, was a marsupial that strongly resembled a wolf.38Wikipedia. Marsupial Sugar gliders (marsupials) and flying squirrels (placentals) both glide on skin flaps that evolved separately.39Wikipedia. Marsupial Australian marsupial moles burrow in desert sand much like placental moles.40Nature. Kangaroo Mob ~ Marsupials: The Evolution Backstory These look-alikes that evolved separately exist because marsupials spread across Australia filling roles that placentals filled elsewhere.41Science World. Placental Mammals and Their Marsupial Counterparts

To tell a look-alike’s true group, check whether females have a pouch or teats where young attach. Where the animal lives is one clue among several, since Australia’s native mammals are mostly marsupials. Brain and hip bones differ even when the outer shape matches. Kangaroo rats hop on their hind legs like kangaroos, but they are North American rodents, so placental.

When Placental Mammals Appeared

The wider lineage leading to placentals existed by about 170 million years ago, alongside dinosaurs.42Wikipedia. Placentalia Genetic evidence suggests true placentals began around 90 million years ago.43Wikipedia. Placentalia The oldest undisputed placental fossils, though, date to the extinction that ended the dinosaurs.44Wikipedia. Placentalia Some scientists argue placentals arose only after that extinction.45PubMed. The placental mammal ancestor and the post-K-Pg radiation of placentals Most agree the group’s big burst into many forms came just after the dinosaurs vanished.46PubMed Central. Rapid morphological evolution in placental mammals post-dates the origin of the crown group

The earliest placentals were small, probably insect-eating animals. Larger brains and more complex forms came later, as rapid diversification filled the spaces left by dinosaurs.

Are Placental Mammals at Risk Today

About one in four mammal species is in danger of extinction, and more than half of mammal populations are shrinking.47PubMed Central. Multiple ecological pathways to extinction in mammals Mammals of every body size can be at risk, depending on how they live.48PubMed Central. Multiple ecological pathways to extinction in mammals Large plant-eaters and meat-eaters have lost much range and numbers in recent times.49PubMed Central. The contribution of historical processes to contemporary extinction risk in placental mammals Species whose populations were small long ago are more likely to be threatened today.50PubMed Central. The contribution of historical processes to contemporary extinction risk in placental mammals Some placentals moved by people, like invasive mammals in Europe, harm native wildlife and can spread diseases to humans.51PubMed Central. Introduction, spread, and impacts of invasive alien mammal species in Europe The biggest threats include habitat loss from farming and building, and hunting and trade.

Risk varies because a species with a narrow range or a very specialized diet has fewer options when its habitat changes. Slow-breeding animals such as elephants and whales recover slowly from losses, so even modest hunting can hurt them. Small mammals that breed fast can recover more easily, but they are still at risk if their habitat disappears.