The order Monotremata (one-holed creatures) is comprised of two families, the Ornithorhynchidae, including the platypus, and the Tachyglossidae, including the long- and short-beaked spiny anteaters or echidnas. Monotremes are found only in Australia, Tasmania, and New Guinea. Monotremes are a derivative of an ancient mammal stock but there is no direct evidence of what it might have been.
Monotremes are not closely related to marsupials or placental mammals, but rather they evolved from a distinct group of reptilian ancestors. Despite sharing some reptilian features, monotremes possess all the major mammalian characteristics: air breathing, endothermy (i.e., they are warm-blooded), mammary glands, a furred body, a single bone in the lower jaw, and three bones in the middle ear.
Monotremes have a reptilian-like shoulder girdle with distinct coracoid bones and a T-shaped interclavicle. Other reptilian-like skeletal features are present, including certain ribs and vertebral processes, as well as epipubic or "marsupium" bones. These bones are rudimentary and analogous to those that support a pouch in present-day marsupials. However, it seems more likely that these bones are a vestige from reptilian ancestors, associated with the attachment of strong abdominal muscles to support large hindquarters.
Unlike higher mammals with separate reproductive and excretory systems, monotremes have a cloaca, with only one external opening for excretion and reproduction, as in birds and reptiles. In male monotremes, the penis is used only for the passage of sperm and not for urination as in other mammals. The overall pattern of reproduction is mammalian with a brief, vestigial period of development of the young in an external, soft-shelled egg. Once fertilized in the oviduct, the egg is covered with albumen and a tough, leathery shell forms. The egg is rounded, large-yolked, and compressible, rather than brittle like the eggs of birds. Echidnas develop a temporary pouch to incubate the egg and care for the young. The platypus does not develop a pouch and typically lays a single egg in a leaf nest. The mammae lack nipples, so the young lick milk from two lobules in the echidna's pouch or from the abdominal fur of the platypus. A three to six month period of maternal care is typical for monotremes.
Certain shrews and monotremes are the only venomous mammals. In echidnas, the poison gland is present, but non-functional. Only the male platypus is capable of producing the venom and conveying it to a horny spur on the back of the ankle. Delivered by a forceful jab of the hindlimbs, the venom is powerful enough to cause agonizing pain in humans and can kill a dog. Although the exact nature of the venom system is unknown, it may have originated as a defense against some long extinct predator. Today, dingoes occasionally prey on echidnas, but in historical terms, dingoes are relatively recent arrivals in Australia. Because echidnas are widely hunted as food and the platypus is quite sensitive to changes in its habitat, monotremes are considered vulnerable in status.
Betsy A. Leonard