Monotremes are mammals that lay eggs. Only five species exist today: the duck-billed platypus and four species of echidna (also called spiny anteaters). They are the sole surviving members of an ancient mammalian lineage that split from all other mammals roughly 220 million years ago, making them the oldest branch on the mammalian family tree.
What Makes Monotremes Different
The defining feature is egg-laying. Female monotremes produce leathery-shelled eggs after a short gestation of just 15 to 21 days. The eggs are then incubated outside the body for 10 to 12 days before hatching. The hatchlings emerge at an extremely early stage of development, comparable to an early fetus in other mammals. They’re tiny, blind, and almost entirely helpless.
Despite laying eggs, monotremes are true mammals. They have fur, they’re warm-blooded (though with some caveats), and they nurse their young with milk. But they don’t have nipples. Instead, milk seeps through patches of skin on the mother’s abdomen, and the hatchlings lap it up directly from the fur. This milk contains specialized proteins with strong antimicrobial properties, which is critical since the young are so underdeveloped and vulnerable to infection.
The name “monotreme” comes from Greek, meaning “single opening.” All monotremes have a cloaca, a single body opening used for reproduction, urination, and waste elimination, a feature shared with reptiles and birds but not with other mammals.
Lower Body Temperature Than Other Mammals
Monotremes run cooler than virtually every other mammal. While most mammals maintain a body temperature around 37°C (98.6°F), active platypuses and echidnas hold steady at roughly 31 to 32°C (about 88 to 90°F), rarely exceeding 34°C. Their body temperature also fluctuates more than in other mammals, dipping lower in cold conditions and rising in heat. Early researchers found echidnas as low as 22°C on a cold morning and as high as 36.6°C in fierce midday sun, a range one scientist called “reptilian.”
This isn’t a deficiency. The average body temperature of placental mammals would actually be lethal to monotremes. Their physiology simply operates at a different set point, one that likely reflects the ancestral condition of early mammals before higher, more tightly regulated body temperatures evolved in other lineages.
The Five Living Species
The platypus is the most recognizable monotreme. It lives in freshwater river systems across eastern Australia, where it digs burrows into riverbanks. It’s a semi-aquatic hunter, finding most of its food underwater. One of its most remarkable adaptations is electroreception: the platypus bill contains specialized receptors that detect the tiny electrical fields generated by muscle contractions in prey. This allows it to locate shrimp, insect larvae, and other small animals in murky water with its eyes closed.
Male platypuses also carry a venomous weapon. A hollow, keratinous spur on each hind leg connects to a venom gland. All platypuses are born with this system, but the spur regresses in females by age one, while males develop functional venom production by age two. Venom output ramps up during the spring breeding season alongside rising testosterone levels. Males use their spurs against rival males during mating competition, delivering venom that causes immobilization. In humans, a platypus sting is not fatal but produces intense pain, swelling, and heightened sensitivity that can persist for weeks.
The short-beaked echidna is the most widespread monotreme, found across eastern Australia, Tasmania, and New Guinea. It looks somewhat like a hedgehog, covered in coarse spines with a long snout for probing into ant and termite mounds. Unlike the platypus, it’s entirely terrestrial.
The remaining three species are all long-beaked echidnas in the genus Zaglossus, restricted to the island of New Guinea. They are larger and rarer than their short-beaked cousin, with longer snouts curved slightly downward. The eastern long-beaked echidna is classified as Vulnerable. The western long-beaked echidna and Attenborough’s long-beaked echidna are both Critically Endangered. The western species has been recorded only on the Vogelkop Peninsula in northwest New Guinea and the nearby island of Salawati, where it is probably now extinct. These animals are so elusive that basic information about their behavior and population sizes remains limited.
Where Monotremes Live
Every living monotreme species is found in either Australia or New Guinea, and nowhere else on Earth. The platypus occupies freshwater streams, rivers, and ponds in eastern Australia, though its range has shrunk since the 19th century. The short-beaked echidna has the broadest distribution of any monotreme, inhabiting forests, grasslands, and scrublands across eastern Australia, Tasmania, and parts of New Guinea. All three long-beaked echidna species are confined to the mountains and forests of New Guinea.
An Ancient Lineage
Monotremes diverged from the lineage leading to marsupials and placental mammals approximately 220 million years ago, during the late Triassic period when dinosaurs were just beginning to diversify. This makes them far older as a group than the split between marsupials (like kangaroos) and placental mammals (like humans), which occurred tens of millions of years later. Many of the traits that seem unusual in monotremes, such as egg-laying, a cloaca, lower body temperature, and milk delivery through skin patches rather than nipples, are likely inherited from the earliest mammals. Monotremes aren’t primitive versions of “real” mammals. They represent a parallel path that mammals took, one that has survived for over 200 million years in its own right.