While most mammals give birth to live young, a small and unique group defies this common understanding. These unusual creatures lay eggs, a trait more commonly associated with reptiles and birds, offering a glimpse into an ancient lineage of mammalian evolution.
The Egg-Laying Mammals
Egg-laying mammals belong to the order Monotremata, or monotremes. Their name comes from Greek, meaning “single opening,” referring to their cloaca. This single opening serves as a common exit point for their digestive, urinary, and reproductive systems, a characteristic shared with birds and reptiles but rare among mammals. Only five living species exist, all native to Australia and New Guinea: the platypus and four echidna species. Despite laying eggs, monotremes possess other defining mammalian traits, including fur, a high metabolic rate, and producing milk to nourish their young.
What Makes Them Special?
Beyond their egg-laying ability, monotremes exhibit several unique biological features. One is their method of milk production. Unlike other mammals that nurse through nipples, female monotremes secrete milk directly from specialized mammary glands onto skin patches or grooves on their bellies. Their young, often called puggles, then lap up the milk from the mother’s fur or skin.
The platypus possesses an extraordinary sensory capability known as electroreception. Its soft, rubbery bill detects faint electrical impulses from prey underwater, allowing it to hunt effectively even with its eyes, ears, and nostrils closed while submerged. Male platypuses are among the few venomous mammals, possessing a sharp, hollow spur on each hind ankle connected to a venom gland. The venom, which intensifies during breeding season, is primarily used in competition between males and can cause severe, long-lasting pain in humans, though it is not lethal.
Meet the Monotremes
The platypus (Ornithorhynchus anatinus) is a semiaquatic mammal with a distinctive duck-like bill, a flat, beaver-like tail, and dense, waterproof fur. It inhabits freshwater systems across eastern Australia and Tasmania. Platypuses are largely nocturnal or crepuscular, foraging underwater for aquatic invertebrates. They store collected food in cheek pouches before consuming it at the water’s surface.
Female platypuses lay one to three leathery eggs in burrows dug into riverbanks. After hatching, their young, or puggles, feed by licking milk from the mother’s specialized mammary patches.
Echidnas, often referred to as spiny anteaters, comprise four species known for their protective fur and keratinous spines, and elongated snouts. The Short-beaked Echidna (Tachyglossus aculeatus) is the sole echidna species found throughout Australia, adapting to diverse habitats. These echidnas primarily feed on ants, termites, and worms, using a long, sticky tongue to capture prey. Their powerful claws and strong limbs are well-suited for digging for food or protection, often burying themselves until only their spiny backs are exposed when threatened.
In New Guinea, three species of Long-beaked Echidnas (Zaglossus spp.) exist, characterized by coarser hair, more visible spines, and a longer, downward-curving snout. These species inhabit highland forests and feed predominantly on earthworms and insects, using their specialized snouts and strong claws to forage at night. Similar to platypuses, echidnas possess electroreceptors in their snouts, which aid in locating prey. Female echidnas lay a single egg into a temporary pouch, where the puggle hatches and remains for several weeks, suckling milk before developing spines and leaving the pouch.