Mammals are separated from other animal classes by characteristics such as milk-producing mammary glands, for feeding their young, and live birth. Animals in one small group of mammals, the monotremes, do not give birth to live young. The spiny anteater and the duckbill platypus, the only members of this order, are the only egg-laying mammals.
The duckbill platypus has a body and tail that are relatively flat, like a beaver’s. Its mouth is formed into a soft beak, giving the animal its duckbill name. Duckbills are amphibious creatures and capable swimmers. Platypus’ sense of smell enables them to forage for food underwater. They are found only in Australia.
There are two living species of echidna, the spiny anteater; both of them live in Australia. They have prickly and stiff fur along the back, giving them an appearance similar to hedgehogs or porcupines. The spiny anteater eats ants and termites, like the more familiar giant anteater, to which it is not related. For defense, a spiny anteater rolls itself into a ball and displays sharp prickly spines. Although the spiny anteater is a terrestrial animal, fossil records indicate that this species originally developed from the amphibious platypus.
Survival of Egg-Laying Mammals
Egg-laying mammals are living fossils, having survived evolutionary pressures for millions of years. The emergence of marsupials in Australia led to the disappearance of many egg-laying species that are known only through fossil records. However, the duckbill platypus and spiny anteater survived. A theory about the survival of the platypus and the spiny anteater stems from the fact that present-day monotremes are descended from amphibious animals; for defense against more highly developed animals, egg-laying mammals simply headed to the water, out of reach of predatory creatures.
The platypus and the spiny anteater both lay eggs. Other common characteristics are the lack of teats for hatched young to suckle milk. Instead, infants lick the region of the mammary glands, where milk is secreted through the mother’s skin. Lack of development of front and rear legs inhibits rapid movement to flee from predators. However, well-developed muscles in the forelegs allow platypuses to be extremely efficient swimmers, and spiny anteaters can burrow in world-record fashion.
Relationship to Other Animals
The evolutionary record considers monotremes to be the first mammals, exhibiting characteristics similar to both reptiles and mammals. The shape of the appendages and egg-laying are distinctly reptilian traits. Compared to other mammals, monotremes have half the number of genes devoted to olfactory development. Like other mammals, monotremes have fur, mammary glands, sweat-producing glands and the developed region of the brain known as the neocortex -- a particularly mammalian trait. Egg-laying mammals are considered the link between reptiles and the emergence of marsupials.