Monotremes

The monotreme order of mammals is comprised of two families, Ornithorhynchidae and Tachyglossidae, though some scientists have suggested alternative taxonomical Classifications. The first family contains just one genus and species, that of the duck-billed platypus (Ornithorhychus anatinus), while the second contains two genuses (Tachyglossus andZaglossus) and four living species of echidnas (short-beaked, Sir David's long-beaked, Eastern long-beaked, and Western long-beaked). These animals have much of the conventional appearance of mammals, but they are also quite different in some respects from the rest of the mammals. Both the platypus and the echidnas lay eggs which are incubated and hatched outside the body of the mother—they are the only mammals to do so.

Monotreme Anatomy

The monotremes are more closely related in evolution to the reptiles than are any other recent mammals. They are not the ancestors of the marsupials or the placental mammals, but rather they represent a distinct line of mammalian evolution. They possess certain mammalian features, such as hair and mammary glands, and they are warm-blooded. On the abdomens of the females, milk oozes from paired areas of tubular glands (they possess no teats for suckling) and is lapped up by the young. They have a four-chambered heart and some skeletal features associated with mammals. Other features, such as the presence of a cloaca, along with features of the vertebrae and ribs, are very reptilian.

In male monotremes, the penis is attached to the ventral wall of the cloaca, and is divided at the tip into paired canals used only for the passage of sperm. In female monotremes, the oviducts, which carry the eggs from the ovary, open separately into the cloaca. In the oviducts, the eggs are fertilized and then covered with albumen (a protein like that found in chicken egg whites) and a flexible, sticky, leatherlike shell. This reptilian feature of a shell-covered egg is found in no other order of mammals.

For living animals, the range of the monotremes includes only Australia, Tasmania, and New Guinea. Most fossil records of monotremes, which tend to be larger than the extant species, are also found in this region. However, at least one monotreme fossil (a tooth from a platypus relative; the modern adult platypus has no teeth) has been found in Argentina, indicating that ancestral species were once more widely distributed.

Duck-Billed Platypus

When platypus specimens were first sent from Australia to Europe, they were considered by many to be an elaborate hoax. The unusual combination of features such as a seemingly ducklike bill, a beaverlike tail, webbed feet, and a venomous barb on the hind feet of male specimens was unlike anything documented before. W. H. Caldwell, in 1884, first demonstrated to Western science that platypuses lay eggs, unlike other mammals, further adding to the curious nature of the animal.

There are still many unknowns regarding platypus reproduction and development, especially in terms of biochemistry and endocrinology. Copulation generally takes place during the spring months of August and September, and usually occurs in water. The female then withdraws to a rather complicated nesting burrow dug into the side of a streambed. Inside the burrow is a nesting chamber consisting of a bed of grass, reeds, and leaves. Generally, two sticky eggs are laid here. There is no pouch for the eggs to incubate in, and the exact mechanism of incubation remains unclear. It is likely that the female curls around the eggs and protects them with her body heat. Incubation is seven to ten days, after which the hatchlings use an egg tooth (much like reptiles) to emerge from the egg. At birth the hatchlings are about 17 millimeters (0.65 inches) long. They feed on ill-defined milk patches hidden under dense fur on the ventral side of the mother. Hatchlings have very large, strong forelimbs that are used for holding on to the fur over the milk patches. Because they are so small and well protected in the mother's dense fur, hatchlings are nearly impossible to detect by looking at the mother.

As the young grow, the mother’s mammary glands also grow. These mammary glands are very similar to those seen in other mammals, though there is no nipple, and the chemical composition of the milk is also similar. Until they emerge from the burrow, hatchlings feed on only the milk from their mother. Young platypuses emerge from the burrow for the first time in December or January, generally being thirty to thirty-five centimeters (twelve to fourteen inches) long. They will eventually grow to forty to forty-five centimeters as adult females, or fifty to fifty-five centimeters as adult males. Adults feed on worms, crustaceans, and insects on the muddy bottoms of their freshwater habitats, which they locate using sensitive electroreceptors on their bills. Adult males are thought to use their venomous barbs in competition with each other as well as for defence; the platypus is one of only a few venomous mammals, and its venom is strong enough to kill a dog and cause excruciating pain to a human.

Echidnas

Previously known as spiny anteaters, echidnas have compact, rounded bodies covered with short, thick spines. They have elongated, slender snouts and strong limbs, and are powerful diggers. They have no teeth and, like other ant- and termite-eating mammals, have a long sticky tongue that reaches well beyond the snout. Termites, ants, and other small arthropods are swept into the mouth by its action. The short-beaked echidna is widespread throughout Australia and is also found on Tasmania and southwestern New Guinea. The three species of long-beaked echidna are found only in new Guinea and are all listed by the International Union for Conservation of Nature and Natural Resources (IUCN) as critically endangered on its Red List of Threatened Species.

Adult echidnas are solitary animals, except when it is time to mate. During the breeding season (late June to September), males will follow females for several weeks at a time before being allowed to mate. Often several males will be seen in single file following a female, this being referred to as an echidna train. It is not known exactly how males find breeding females, although it is likely that females secrete a scent that can be detected several miles away.

Females develop a brood pouch during breeding season. Following mating, a single fertilized egg is transferred from the cloaca to the pouch, although the actual mechanism for this transfer is not clear. The leathery-shelled eggs average 14-17 millimeters (0.55-0.65 inches) in length. The egg is incubated for ten days in the pouch before the hatchling appears. The young echidna (who is about 1.2-1.3 centimeters, or 0.5 inches, in length when it is born) remains in the brood pouch for an additional eight weeks until its spines develop. During this time it feeds on milk flowing from the mammary ducts onto tufts of hair within the pouch.

When the youngster emerges from the pouch, it is hidden by its mother in a protected spot. It appears that the young echidna then begins a short period of hibernation, during which continued development occurs. Echidnas are reproductively mature at one year of age.

Principal Terms

brood pouch: a temporary external pouch created by folding the skin of the abdomen together; used to carry young as they continue to develop

cloaca: a bodily opening at the end of the gut into which both the waste disposal and reproductive systems open

echidna: a long-snouted, insect eating, egg-laying mammal; also known as the spiny anteater

monotreme: reptilelike mammals, distinguished from other mammals by the fact that they lay eggs and have a cloaca

platypus: web-footed, duck-billed, semiaquatic, egg-laying mammal

Bibliography

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"Monotremes: Egg-Laying Mammals." University of California Museum of Paleontology, Regents of the University of California, 2006, www.ucmp.berkeley.edu/mammal/monotreme.html.

Rismiller, Peggy, and Roger Seymour. "The Echidna." Scientific American, February, 1991, 96-103.

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