Examples of Monotreme Animals

Scientists now know that duck-billed platypuses and their relatives the echidnas, or spiny anteaters, form a strange animal lineage known as the monotremes, and exhibit a bizarre combination of traits. Monotremes are furred like mammals and produce milk for their young, yet they reproduce by laying eggs. The five extant monotremes -- the duck-billed platypus (Ornithorynchus anatinus) and four species of echidnas (Tachyglossidae) -- live in New Guinea and Australia.

Rather than giving birth to live young like most mammals do, monotremes deposit one to three relatively small eggs. Spiny anteaters place their eggs in pouches for the one- to two-week incubation process. Duck-billed platypuses lack pouches; they incubate their eggs by pressing them against their bellies and holding them in place with their tails. Baby monotremes hatch relatively early in the developmental process, breaking out of their eggs with "milk teeth" they lose soon afterward. Monotremes lack nipples, so hatchling monotremes lap milk that flows directly from their mother’s mammary glands.

Duck-billed platypuses have a peculiar combination of traits, including their namesake mouths, beaverlike tails and webbed feet. Living in ponds, lagoons and rivers, duck-billed platypuses consume aquatic insect larvae, crustaceans and some plant material. While young, platypuses have teeth. But teeth are absent from the adults, who instead use a pair of horny plates located in the jaws to crush and chew food. Adult male platypuses have grooved spurs, attached to venom glands in their legs, that are likely used in combat with other platypus males.

Four living echidnas represent two genera; the short-beaked echidna (Tachyglossus aculeata) is the sole living member of its genus, while western (Zaglossus bruijnii), eastern (Z. bartoni) and Sir David’s long-beaked echidnas (Z. attenboroughi) demonstrate close kinship and share a genus. Short-beaked echidnas are native to, and common in, Australia; meanwhile, the three long-beaked echidnas are endangered inhabitants of New Guinea. The world’s rarest echidna species, the critically endangered Sir David’s long-beaked echidna, was thought extinct until an expedition in 2007 found a small population in the Cyclops Mountains. While short-beaked echidnas are relatively small, perhaps attaining 15 pounds in weight, long-beaked echidnas grow much larger, exceeding 35 pounds in some cases. Echidnas have tubular mouths and long tongues they use to slurp up ants, termites and other invertebrates. Prey is found through their sense of smell and through electroreceptive organs in their snouts. The dorsal surfaces or echidnas are covered in stiff hairs that protect the animals from predators.