The platypus, a semi-aquatic mammal native to eastern Australia and Tasmania, has a duck-like bill, beaver-like tail, and lays eggs, yet it is classified as a mammal because the female nourishes her young with milk. The simple answer to the question of lactation anatomy is that the platypus, alongside its relative the echidna, does not possess nipples. This anatomical difference is a defining characteristic of its ancient mammalian lineage.
How Monotremes Feed Their Young
The female platypus still feeds its offspring milk, but the delivery mechanism is fundamentally different from placental and marsupial mammals. Instead of having specialized teats, the mother secretes milk directly through numerous specialized pores in the skin on her abdomen. These mammary glands are modified sweat glands, a trait that points to an ancient evolutionary origin for lactation.
This secretion flows out onto the mother’s fur, where it collects in shallow abdominal grooves or forms “milk patches.” The young, which are born blind and helpless, cannot suckle in the traditional sense. They instead lap or slurp the rich milk directly from the mother’s wet fur.
The platypus milk is notably dense, containing about 39% solids, which is much higher than the milk produced by most other mammals. The mother often lies on her side or back, allowing her young, called puggles, to gather beneath her to feed. This simple, external system is believed to be the primitive form of mammalian milk feeding, predating the evolution of the nipple.
The Unique Mammalian Family
The platypus belongs to the Order Monotremata, a classification that represents the most ancient living lineage within the Class Mammalia. This group includes only the platypus and the four species of echidnas. All mammals are broadly divided into three main groups based on reproduction: Monotremes, Marsupials, and Placental mammals.
Marsupials, such as kangaroos, give birth to highly underdeveloped young that complete development in a pouch. Placental mammals, like humans and dogs, nourish their young through a placenta for a long gestation period. Monotremes are unique in that they lay eggs, an ancient characteristic retained from their evolutionary past.
The name “monotreme” itself means “single opening,” referring to the cloaca, a single orifice used for urinary, defecatory, and reproductive functions, a feature common in reptiles and birds. The absence of nipples is considered an ancestral characteristic of this group, highlighting their early divergence from the rest of the mammalian family tree.
Laying Eggs and Caring for Pups
The reproductive cycle of the platypus begins when the female constructs a specialized nesting burrow, which is often sealed with soil plugs for protection and insulation. Following mating, the female typically carries a clutch of one to three soft-shelled, leathery eggs, with two being the most common number. The gestation period lasts approximately 27 to 28 days before the eggs are laid.
The female then incubates the eggs for a short period of around ten days by curling her body around them and holding them against her abdomen with her tail. The newly hatched puggles are tiny, hairless, and blind, possessing a temporary structure called an egg tooth to help them break out of the shell.
The mother will leave the burrow only for brief foraging trips, using the soil plugs to squeeze water from her fur and keep the nest dry. The puggles remain inside the burrow, feeding on her secreted milk for about three to four months. Once the young are fully developed and ready to emerge, they are considered independent.