The existence of mammals that lay eggs presents a fascinating biological puzzle, seemingly defying the fundamental definition of the class Mammalia. These animals belong to the order Monotremata, meaning “single opening” in Greek, which refers to the cloaca—a single duct for their urinary, defecatory, and reproductive systems, a trait shared with reptiles and birds. Monotremes represent the most ancient split from the lineage that led to all other living mammals, diverging an estimated 220 million years ago. This archaic group provides a living link to the earliest forms of mammalian life.
Identifying the Egg-Laying Mammals
Only five species of monotremes exist today: the duck-billed platypus and four species of echidnas, sometimes called spiny anteaters. Their geographic range is restricted to Australia, the island of Tasmania, and New Guinea.
The semi-aquatic platypus has a distinctive, rubbery bill, webbed feet, and a flattened tail that aids its lifestyle in freshwater streams and rivers. In contrast, echidnas are terrestrial, covered in coarse hair and sharp spines for protection. The echidna group includes the short-beaked echidna, widespread across Australia and New Guinea, and three species of long-beaked echidnas found exclusively in New Guinea.
How They Maintain Their Mammal Status
Despite the reptilian trait of laying eggs, monotremes possess all the characteristics that classify them as true mammals. Like all other mammals, they have hair or fur covering their bodies, and they are endothermic, meaning they generate internal body heat to maintain a stable temperature. However, their metabolic rate and body temperature are generally lower than that of placental mammals, an adaptation that helps them conserve energy.
The most defining mammalian trait is the production of milk to nourish their young. Monotremes possess functional mammary glands but lack nipples or teats. Instead, the milk is secreted through specialized pores onto a patch of skin or fur called the mammary patch.
The hatchling must lick the milk directly from the fur, which is a primitive form of lactation not seen in other living mammals. This unique method of nourishing their young confirms their place within the class Mammalia. They also share other traits with mammals, such as a single bone in the lower jaw and three tiny bones in the middle ear.
The Unique Monotreme Reproductive Cycle
The reproductive cycle begins with the fertilization of an egg that is retained inside the mother for several weeks as it develops. The egg is small, typically measuring 13 to 15 millimeters in diameter, and is encased in a soft, pliable, leathery shell, similar to that of a reptile. A large yolk sustains the embryo during the short incubation period.
A female monotreme typically lays only one or two eggs at a time, which pass out of the cloaca. The mother then incubates the egg, though the method differs between the two groups. A female platypus lays her eggs in a specially constructed, sealed-off nesting chamber within a burrow on a riverbank.
Incubation Methods
The echidna, however, develops a temporary, external fold of skin on her abdomen that functions as a rudimentary pouch. She transfers the single egg into this pouch after laying it. The egg is incubated there for a brief period, usually around ten days, before hatching.
The young, referred to as puggles, hatch in an extremely altricial, or undeveloped, state, resembling a fetus more than a fully formed animal. At the time of hatching, a puggle is tiny and blind, but has well-developed forelimbs used to navigate the mother’s fur. The puggle crawls to the milk patch, where it feeds on secreted milk for an extended period, often several months, until it is developed enough to leave the burrow or pouch.