The vast majority of mammals give birth to live young, a process known as viviparity. However, a small group of mammals defies this general rule by reproducing through laying eggs. These species, known collectively as monotremes, represent an early evolutionary branch of the mammalian lineage that retained this reptilian trait. Their existence provides a fascinating window into the history of mammalian reproduction.
The Monotremes: Identifying the Two Species
The two mammals that lay eggs belong to the Order Monotremata. This name references their single opening, or cloaca, for their urinary, digestive, and reproductive systems. This group consists of the duck-billed platypus and the echidnas. The platypus is a single species, Ornithorhynchus anatinus, which is semi-aquatic and found exclusively in eastern Australia and Tasmania.
Echidnas comprise the other family, Tachyglossidae, which includes four distinct species. The short-beaked echidna is the most widespread, inhabiting Australia, Tasmania, and parts of New Guinea. The three species of long-beaked echidnas are found only in New Guinea. This small order of five species is endemic to Australia and New Guinea.
Defining Mammals: Features Beyond Egg Laying
Monotremes are classified as mammals because they share several traits with other groups. One primary characteristic is the presence of hair or fur covering their bodies, which helps them maintain a consistent internal temperature. This ability to regulate body heat means they are endothermic, or warm-blooded, a trait shared with all other mammals.
Female monotremes produce milk to nourish their young. Unlike other mammals, they lack nipples and instead secrete milk through specialized mammary gland ducts and pores on a patch of skin on their abdomen. The newly hatched young, often referred to as puggles, then lap the milk directly from the mother’s fur.
Unique Biology of the Platypus
The platypus, with its distinctive bill and webbed feet, is adapted for its semi-aquatic environment. Its bill is soft and pliable, covered in approximately 40,000 electoreceptors and 60,000 mechanoreceptors. This sensory equipment allows the platypus to employ electroreception, enabling it to detect the faint electrical signals generated by the muscle contractions of its invertebrate prey while underwater. When diving, the platypus closes its eyes, ears, and nostrils, relying entirely on these receptors to navigate and hunt on the riverbed.
The male platypus possesses a venom delivery system. It has a hollow, keratinous spur on each hind ankle connected to a venom gland in its thigh. The venom is potent enough to inflict excruciating pain in humans, often lasting for days or weeks. This venom is not used for hunting prey but is believed to be employed during the breeding season in competition with other males for dominance.
Unique Biology of the Echidna
Echidnas are terrestrial monotremes recognized by the dense covering of spines that cover their backs and sides. These spines are modified hairs that serve as a defense mechanism, allowing the echidna to curl into a prickly ball when threatened. They are insectivores, feeding on ants and termites, which they find using their long, slender snout and excellent sense of smell.
To capture prey, echidnas use a long, sticky tongue that can rapidly shoot out, sweeping up insects. After laying a single soft-shelled egg, the female deposits it directly into a temporary abdominal fold, or pseudo-pouch. The hatchling emerges after about ten days and develops inside this temporary pouch. It nurses from the mother’s milk patch until its protective spines begin to grow, at which point it is moved to a nursery burrow.