While most mammals give birth to live young, a small, distinctive group defies this by laying eggs. This unique reproductive strategy sets them apart, highlighting the remarkable diversity within the mammalian class.
The Rare Egg-Laying Mammals
The mammals that lay eggs belong to the order Monotremata. This ancient lineage, comprising the platypus and four species of echidnas, represents a unique branch of mammals. Monotremes are primarily found in Australia and New Guinea. Despite their egg-laying habit, they possess fundamental mammalian traits, distinguishing them from other vertebrates.
The Platypus: An Aquatic Enigma
The platypus, Ornithorhynchus anatinus, is a semi-aquatic monotreme with a duck-like bill, a beaver-like tail, and otter-like feet. After mating, the female lays one to three leathery-shelled eggs. These eggs are incubated in an underground nesting burrow, where the mother holds them against her belly with her curled tail. Male platypuses possess a venomous spur on each hind leg, which can deliver a painful venom.
Platypus eggs incubate for about 10 days. Young platypuses, called puggles, hatch with features that aid their exit from the egg. After hatching, the puggles remain in the burrow for three to four months, feeding on milk secreted by the mother.
Echidnas: Terrestrial Spiny Wonders
Echidnas are terrestrial monotremes distinguished by their protective spines and long, slender snouts. There are four species of echidnas. Unlike the platypus, a female echidna lays a single soft-shelled, leathery egg.
This egg is deposited directly into a temporary pouch on the mother’s abdomen, where it is incubated. The egg hatches after 10 days, and the young puggle remains in the pouch, suckling milk from specialized pores, as echidnas lack nipples. Puggles stay in the pouch for 45 to 55 days, developing spines before they are moved to a nursery burrow.
Why They Are Still Classified as Mammals
Despite their egg-laying reproduction, monotremes are classified as mammals due to several shared characteristics. A primary defining trait is the presence of mammary glands, which produce milk to nourish their young. While most mammals secrete milk through nipples, female monotremes release milk through pores onto skin patches, which their young then lap up.
Monotremes also possess hair or fur covering their bodies. Like all mammals, they are warm-blooded, maintaining a constant internal body temperature. Other shared features include a single bone in their lower jaw and three middle ear bones.