In echidnas, the poison gland is present, but nonfunctional. A three to six month period of maternal care is typical for monotremes.Ĭertain shrews and monotremes are the only venomous mammals. 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. The platypus does not develop a pouch and typically lays a single egg in a leaf nest. Echidnas develop a temporary pouch to incubate the egg and care for the young. The egg is rounded, large-yolked, and compressible, rather than brittle like the eggs of birds. Once fertilized in the oviduct, the egg is covered with albumen and a tough, leathery shell forms. The overall pattern of reproduction is mammalian with a brief vestigial period of development of the young in an external, soft-shelled egg. In male monotremes, the penis is used only for the passage of sperm and not for urination as in other mammals. 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. 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. These bones are rudimentary and analogous to those that support a pouch in present-day marsupials. Other reptilian like skeletal features are present, including certain ribs and vertebral processes, as well as epipubic or “marsupium ” bones. Monotremes have a reptilian like shoulder girdle with distinct coracoid bones and a T-shaped interclavicle. 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 are not closely related to marsupials or placental mammals, but rather they evolved from a distinct group of reptilian ancestors. Monotremes are a derivative of an ancient mammal stock but there is no direct evidence of what it might have been. Monotremes are found only in Australia, Tasmania, and New Guinea. 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.
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