The biological definition of a mammal includes possessing hair or fur and nourishing young with milk produced by mammary glands. Since the vast majority of mammals—specifically, the placental and marsupial groups—give birth to live young, this is often considered a universal mammalian trait. The existence of egg-laying mammals is a fascinating biological anomaly, challenging the simple classification of the animal kingdom.
Monotremes: The Only Egg-Laying Mammals
The Order Monotremata represents the sole exception to the rule of live birth among mammals. These animals are the only living members of the subclass Prototheria, distinguishing them from placental and marsupial mammals. This small, ancient order contains only five extant species, indigenous exclusively to Australia and New Guinea.
The five species of monotremes include the duck-billed platypus and four species of echidna, also known as spiny anteaters. The four echidna species are comprised of the short-beaked echidna and three different kinds of long-beaked echidnas.
Defining Characteristics of Monotremes
Monotremes are classified as mammals because they share the defining traits of the class, even while retaining several primitive features typically associated with reptiles. Like all other mammals, they are endothermic, possess hair or fur, and feed their young with milk produced by specialized glands. They also possess the characteristic three middle ear bones and a single bone in the lower jaw, hallmarks of mammalian anatomy.
In contrast to placental and marsupial mammals, monotremes possess a cloaca, a single opening used for the digestive, urinary, and reproductive tracts. The name “monotreme” itself derives from the Greek for “single hole,” referring to this unique shared passage. Furthermore, they lack nipples; instead, milk is secreted through specialized pores or patches on the mother’s abdomen, which the young then lap up. These animals also exhibit a more sprawling, reptile-like gait due to extra bones in their shoulder girdle, such as the interclavicle and coracoid, which are absent in their live-bearing relatives.
Unique Reproductive Cycle
The reproductive cycle of monotremes begins with the development of a soft-shelled, leathery egg, resembling the eggs of reptiles. The fertilized egg is retained within the mother, receiving nutrients directly before being laid. The echidna typically lays a single egg and deposits it into a temporary pouch on her abdomen. The platypus lays one or two eggs in a burrow nest and curls around them for warmth.
The incubation period is short, often lasting ten days or less before the young hatch. The newly hatched infant, called a puggle, is altricial, meaning it is blind, hairless, and larval-like. The puggle crawls to the mother’s abdominal milk patch to feed. This period of postnatal care is prolonged, with the young remaining dependent on the mother’s milk for several months until they are developed enough to leave the burrow or pouch.