Monotremes

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 nonfunctional. Only the male platypus is capable of producing the venom and conveying it to a

KEY TERMS

Albumen— The white of an egg.

Cloaca— The cavity into which the intestinal, genital, and urinary tracts open in vertebrates such as fish, reptiles, birds, and some primitive mammals.

Coracoid— A bone or cartilage projecting from the scapula toward the sternum.

Endotherm— An animal that uses its metabolism as a primary source of body heat and uses physiological mechanisms to hold its body temperature nearly constant.

Placenta— A vascular, membranous organ that develops in female mammals during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord. Following birth the placenta is expelled.

Vestige— A small degenerate or rudimentary organ or part existing in an organism as a usually nonfunctioning remnant of an organ or part fully developed and functional in a preceding generation or earlier developmental stage.

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

Monotremata (class Mammalia, subclass Prototheria) An order comprising the duck-billed platypus (Ornithorhynchus anatinus, see ORNITHORHYNCHIDAE) and the echidnas or spiny ant-eaters (Tachyglossus and Zaglossus, see TACHYGLOSSIDAE). There were some extinct forms, of which very few are known in detail, though of those some attained large sizes. The echidnas have no fossil record older than the Pleistocene, but a fossil platypus, Obdurodon, is known from the Miocene and in the early 1990s teeth of an undoubted platypus-like form, Monotrematum sudamericanum, were discovered in Palaeocene deposits in Patagonia. Two Cretaceous genera, Steropodon and Kollikodon, are also known. In view of their reptilian affinities they are thought to represent a separate and direct line of descent from the earliest Mesozoic animals, possibly the Docodonta, independent of the line leading to other mammals, but the dental features of Steropodon are considered by some to point to affinities with the Eupantotheria. They retain many primitive features and are quite unlike marsupials or eutherians. The rectum and urinogenital system open to a common cloaca (the name ‘monotreme’ is derived from the Greek monos meaning ‘alone’ and trema meaning ‘hole’, although the feature is shared by marsupials and some insectivores). The male is heterogametic as in other mammals. The young are hatched from large, yolky eggs, incubated in a nest by the female platypus and in a pouch in the echidnas. The embryos develop a caruncle and egg-tooth. After hatching they are fed milk secreted by the female from specialized sweat glands which do not open through central nipples. The diaphragm is fully developed, and the heart possesses a single left aortic arch as in other mammals. The larynx is developed, and monotremes make sounds. The tarsus of the male has a grooved erectile spine to which (in the platypus) poison is fed from a gland in the thigh. The poison (said to be capable of killing a dog) may serve to immobilize the female during mating. Adults lack teeth but possess bills. The skull is specialized but retains primitive features. The jaw consists of a single bone. The cervical ribs are not fused, the shoulder girdle is reptilian in form, the pelvis is reminiscent of that of marsupials. The body is covered with hair, in the echidnas partly modified into spines on the back. The limbs are modified for digging (echidna) or swimming (platypus); it has recently been discovered that both platypus and echidna locate their prey by detecting weak electrical fields around the snout. Echidnas hibernate, and in hot weather all monotremes shelter in burrows or caves. Monotremes are known only from Australia and New Guinea, where they may have survived because of their high degree of specialization and the isolation of that continent.

Monotremes

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

KEY TERMS

Albumen

—The white of an egg.

Cloaca

—The cavity into which the intestinal, genital, and urinary tracts open in vertebrates such as fish, reptiles, birds, and some primitive mammals.

Coracoid

—A bone or cartilage projecting from the scapula toward the sternum.

Endotherm

—An animal that uses its metabolism as a primary source of body heat and uses physiological mechanisms to hold its body temperature nearly constant.

Placenta

—A vascular, membranous organ that develops in female mammals during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord. Following birth the placenta is expelled.

Vestige

—A small, degenerate, or rudimentary organ or part existing in an organism as a usually nonfunctioning remnant of an organ or part fully developed and functional in a preceding generation or earlier developmental stage.

Monotreme

Monotremes are an ancient group of mammals in the order Monotremata, which probably split from the lineage leading to marsupials (those with no placenta and having a pouch in the abdomen) and placental mammals early in mammalian evolution. The earliest fossil occurrence of monotremes is in the lower Cretaceous, approximately 110 million years ago.

Monotremes retain some of the primitive characteristics of mammalian ancestors, the therapsids. Monotremes lay eggs, have a somewhat reptilian posture, and retain a cloaca , a body cavity into which the reproductive, urinary, and excretory systems empty. Monotremes lack teeth as adults and have an unusual cranial shape. However, monotremes possess several critical mammalian features. They have fur, four-chambered hearts, single dentary (lower jaw) bones, and mammalian ear structure, and they lactate, or produce milk. Females lay one to three small, leathery eggs and incubate them outside of the body. Upon hatching, the young lap milk from the mother's mammary glands, which lack a nipple.

There are two families and three species of monotremes. The family Tachyglossidae includes two species: the spiny anteater, found in Australia, Tasmania, and southern New Guinea; and the long-nosed anteater, found only in New Guinea. The family Ornithorhynchidae includes a single species, the duck-billed platypus, an aquatic species that is found in eastern Australia and Tasmania. All three species eat primarily invertebrates and are prodigious burrowers. Populations of the long-nosed anteater are currently threatened by overhunting. Platypus is a protected species, and both the spiny anteater and platypus populations seem stable as of 2001.

see also Mammal; Marsupial

Tanya Dewey

Bibliography

Anderson, Sydney, and J. Knox Jones, Jr., eds. Orders and Families of Recent Mammals of the World. New York: John Wiley & Sons, Inc., 1984.

Nowak, Ronald M., ed. Walker's Mammals of the World. Baltimore, MD: Johns Hopkins University Press, 1991.

Vaughan, Terry A. Mammalogy. Philadelphia, PA: Saunders College Publishing, 1986.

Monotremata (class Mammalia, subclass Prototheria) An order comprising the duck-billed platypus (Ornithorhynchus anatinus) and the echidnas or spiny ant-eaters, Tachyglossus and Zaglossus. There were some extinct forms of which very few are known in detail, though of those some attained large sizes. The echidnas have no fossil record older than the Pleistocene, but a fossil platypus, Obdurodon, is known from the Miocene and in the early 1990s teeth of an undoubted platypus-like form, Monotrematum sudamericanum, were discovered in Palaeocene deposits in Patagonia. Two Cretaceous genera, Steropodon and Kollikodon, are also known. In view of their reptilian affinities they are thought to represent a separate and direct line of descent from the earliest Mesozoic animals, independent of the line leading to other mammals. They retain many primitive features and and are quite unlike marsupials or eutherians (placental mammals).