Mammals are warm-blooded animals characterized by hair or fur and nourishing their young with milk produced by mammary glands. While most mammals give birth to live young, two distinct species defy this norm by laying eggs. These fascinating creatures are unequivocally classified as mammals despite their egg-laying habit.
The Enigmatic Platypus
The platypus is a semi-aquatic mammal native to eastern Australia, including Tasmania. Its distinctive appearance includes a duck-like bill, webbed feet, and a flattened, beaver-like tail. Covered in dense, waterproof fur, its streamlined body is well-adapted for an aquatic lifestyle, with eyes, ears, and nostrils that close tightly underwater. Adult platypuses lack teeth, instead using horny grinding plates to process their food.
This solitary animal primarily inhabits freshwater systems like rivers and lakes, digging burrows in their banks. As a carnivore, the platypus forages underwater, consuming aquatic invertebrates. It detects prey using electroreception through its highly sensitive bill. Male platypuses also feature a venomous spur on each hind limb, primarily used in territorial disputes.
The Spiny Echidna
Echidnas, often referred to as spiny anteaters, are terrestrial mammals found across Australia, Tasmania, and New Guinea. Their most recognizable feature is a coat of coarse hair interspersed with sharp spines, which are modified hairs. They possess a long, slender snout, a small, toothless mouth, and an exceptionally long, sticky tongue. Echidnas use their powerful, clawed limbs and backward-pointing hind feet for efficient digging.
These animals occupy diverse habitats, thriving wherever their insect prey is abundant. Echidnas are insectivores, mainly eating ants, termites, and earthworms. They locate prey using an acute sense of smell and specialized electroreceptors in their snouts. When threatened, an echidna can rapidly burrow into the ground or curl into a protective, spiky ball.
Beyond the Egg: Why They’re Mammals
Despite their egg-laying, both the platypus and echidna are classified as mammals due to several defining biological characteristics. They possess mammary glands, which produce milk to nourish their young. Unlike most other mammals, they do not have nipples; instead, milk is secreted through pores onto patches of skin or into grooves, from which the young lap it up.
Another key mammalian trait is hair or fur covering their bodies. The echidna’s spines are, in fact, modified hairs. Both species are also warm-blooded. These shared features firmly place them within the Mammalia class, distinguishing them from reptiles or birds. They belong to the unique mammalian order Monotremata, which diverged early in mammalian evolution.
How Monotremes Reproduce
Monotremes, including the platypus and echidna, share a distinct egg-laying reproductive process. After mating, the female undergoes a gestation period of approximately 12 to 28 days before laying her eggs. These eggs are soft-shelled and leathery, similar in texture to those of reptiles.
A female platypus typically lays one to three eggs in a specially constructed nesting burrow. She then incubates these eggs by curling her body around them for about 10 days until they hatch. In contrast, the female echidna usually lays a single egg directly into a temporary pouch on her abdomen, where it incubates for approximately 10 to 11 days. Upon hatching, the tiny, underdeveloped young, sometimes called puggles, emerge using a specialized egg tooth. These hatchlings are then nourished by lapping milk secreted onto the mother’s fur, remaining within the protective burrow or pouch for several months as they grow and develop.