The class Mammalia includes a diverse range of animals, all sharing a common set of defining characteristics. One of the most recognized features of this class is viviparity, the process of bearing live young. The vast majority of species, including placental mammals and marsupials, carry their developing offspring internally until birth. However, a small, ancient group of mammals retains a reproductive method that is an exception to this general rule.
Monotremes: The Exception to the Rule
The biological order Monotremata represents the only group of mammals that does not give birth to live young. Monotremes instead reproduce through oviparity, meaning they lay eggs. This ancient lineage split from the rest of the mammalian family tree millions of years ago, long before the evolution of live birth in other mammals. Their name, Monotremata, is derived from the Greek words meaning “single opening,” which references their cloaca. Like reptiles and birds, monotremes possess a single posterior opening that serves as the exit point for the digestive, urinary, and reproductive tracts.
Meet the Egg-Laying Mammals
There are only five living species of monotremes today, all found in Australia, Tasmania, and New Guinea. These species belong to two distinct families: the duck-billed platypus and the echidnas, also known as spiny anteaters. The platypus (Ornithorhynchus anatinus) is a semi-aquatic mammal with a rubbery bill and webbed feet, endemic to eastern Australia and Tasmania. It is the sole living member of its family, Ornithorhynchidae.
The other family, Tachyglossidae, comprises four species of echidnas. This includes the widespread short-beaked echidna and three species of long-beaked echidna (Zaglossus species), primarily found in New Guinea. Echidnas are terrestrial, covered in coarse fur and sharp spines, and use a long, tubular snout to feed on insects.
Unique Reproductive Strategy
The reproductive process in monotremes is a mosaic of mammalian and reptilian features, beginning with the egg itself. Unlike bird eggs, the monotreme egg is small (13 to 15 millimeters in diameter) with a leathery, porous shell. It contains a large yolk, similar to that of a reptile, which provides the developing embryo with its nutrition. Following a short gestation of 12 to 20 days, the female typically lays only one or two eggs at a time.
Once laid, the eggs require external incubation for about ten days before they hatch. The method of incubation differs between the two families. The female platypus curls around her eggs in a moist nest within a riverbank burrow. Conversely, the female echidna develops a temporary, muscular fold of skin on her abdomen that functions as a brood pouch, placing the egg directly inside for incubation. Upon hatching, the tiny, underdeveloped young use a temporary structure called an “egg tooth” to break out of the shell.
Definitive Mammalian Traits
Despite their egg-laying method, monotremes are classified as mammals because they exhibit traits unique to the class Mammalia. The most telling trait is the ability to produce milk for their young via mammary glands. Unlike all other mammals, monotremes lack nipples; instead, milk is secreted through specialized pores onto the mother’s abdominal skin. The young then lap up the milk from tufts of fur or within the echidna’s temporary pouch. Monotremes are also endothermic (warm-blooded), although their average body temperature is slightly lower than that of placental mammals. They possess hair or fur, a single bone in their lower jaw, and three middle ear bones.