Placental mammals, scientifically known as Placentalia, are the most widespread and numerous group within the class Mammalia, encompassing nearly 5,000 species. Their defining characteristic is the specialized organ that develops during pregnancy, from which they derive their name. This organ supports the developing offspring inside the mother’s body for an extended period.
The Function of the Placenta
The placenta is a temporary organ that forms within the uterus during pregnancy, connecting the mother and the developing fetus. It primarily facilitates the exchange of substances between the physically separate maternal and fetal circulations. This organ allows the fetus to receive materials essential for growth and development.
The placenta’s primary function involves nutrient and gas exchange. Maternal blood perfuses spaces within the placenta, allowing oxygen and nutrients like glucose to transfer to the fetus. Conversely, waste products such as carbon dioxide, urea, and creatinine diffuse from the fetal blood into the maternal circulation for removal by the mother’s body.
The placenta also functions as an endocrine organ, producing a range of hormones that help maintain the pregnancy. Human chorionic gonadotropin (hCG) is produced early in pregnancy and helps sustain the corpus luteum, which in turn produces progesterone. Progesterone relaxes uterine muscles, preventing premature contractions and maintaining the uterine lining, while estrogen helps prepare the uterus and breasts for childbirth.
Beyond nutrient and hormone exchange, the placenta provides immune protection. It acts as a selective barrier, regulating which substances can pass between the maternal and fetal bloodstreams. This barrier generally prevents maternal blood cells, larger proteins, and most microbes from reaching the fetus. Maternal antibodies can also be transferred across the placenta, providing the fetus with passive immunity for several months after birth.
How Placentals Differ From Other Mammals
Placental mammals employ a distinct reproductive strategy compared to the two other living groups of mammals: marsupials and monotremes. The key difference lies in the duration and method of internal fetal development. Placental mammals carry their young within the uterus to a relatively advanced stage before birth.
Marsupials, such as kangaroos and opossums, have a significantly shorter gestation period, often lasting between 12 and 33 days. Their offspring, known as joeys, are born in a highly undeveloped state, typically blind, hairless, and very small. After birth, the joey crawls into an external pouch, or marsupium, on the mother’s abdomen where it attaches to a nipple to complete its development.
While marsupials do form a choriovitelline placenta, it is generally less complex and supports the fetus for a much briefer period compared to the chorioallantoic placenta found in most placentals. This difference means marsupial young receive limited nutrition and immune protection within the uterus. The extended development and nursing within the pouch distinguish their reproductive approach from the prolonged internal gestation of placental mammals.
Monotremes, represented by the platypus and echidnas, are the only egg-laying mammals. Unlike placental and marsupial mammals, monotremes lay soft-shelled eggs which are then incubated by the mother. The young hatch in a very underdeveloped state and are nourished by milk secreted onto milk patches on the mother’s abdomen, as monotremes lack nipples.
The reproductive tract of monotremes also features a single opening, called a cloaca, for urinary, defecatory, and reproductive functions, similar to birds and reptiles. In contrast, placental females have separate openings for these systems. These differences in reproductive biology highlight the strategy of extensive internal fetal growth before birth in placental mammals.
The Diversity of Placental Mammals
The adaptive radiation of placental mammals followed the extinction of non-avian dinosaurs approximately 66 million years ago. This event opened up numerous ecological niches previously occupied by dinosaurs, leading to a rapid increase in mammalian diversity shortly after this mass extinction.
This group showcases a wide range of adaptations, enabling them to thrive in nearly every major habitat on Earth. Many placental mammals dominate terrestrial environments, including large herbivores like elephants and various ungulates such as cattle and horses. Primates, including humans, and rodents, such as mice and rats, also represent successful and numerous terrestrial groups.
Placentals have also adapted to the skies, with bats being the only mammals capable of sustained flight. These nocturnal creatures exhibit agility and navigation abilities. In aquatic environments, placental mammals like whales, dolphins, and manatees have evolved streamlined bodies and specialized respiratory systems for life in water.
Some placental mammals have adapted to subterranean lives, such as moles, which possess strong forelimbs and specialized sensory organs for burrowing. Carnivorans, including cats, dogs, bears, and seals, demonstrate diverse predatory strategies across various ecosystems. This vast array of forms and lifestyles underscores the evolutionary success and adaptability of placental mammals.