Physical characteristics

Monotremes have retained a number of skeletal characteristics possessed by reptilian ancestors, most importantly the structure of the shoulder girdle and some features of the skull. The skull has a fairly large, rounded braincase and an elongated muzzle. Adults of the living monotremes have no teeth. Vestigial teeth are present in the jaws of juvenile platypuses, but they never erupt from the gums. The fact that they are present at all is an example of what evolutionary and developmental biologists call ontogeny-recapitulating phylogeny, which means the embryonic development of a young platypus follows a similar pattern to evolutionary development of the species. The same phenomenon is seen in frogs develop-

A short-beaked echidna (Tachyglossus aculeatus) forages for food. (Photo by Ann & Steve Toon Wildlife Photography. Reproduced by permission.)

ing from fish-like tadpoles, or land-dwelling crabs starting life as aquatic shrimp-like larvae. This theory is supported by the discovery of several fossil monotremes with fully developed dentition.

Living monotremes lack sensory whiskers. They have small, beady eyes and no external ears. Internally, however, the ears are much like those of conventional mammals, with three tiny ear bones—the incus, malleus, and stapes. These three bones, which help transmit vibrations from the eardrum to the inner ear, evolved in mammal ancestors from part of the jaw after the split from other reptiles.

The lack of development in certain sense organs such as ears and whiskers is more than amply compensated by the presence of another sense, which is unique to this order. In all living monotremes, the snout is covered in soft, rubbery skin, and pitted with tiny pores. These are lined with thousands of highly sensitive receptors that detect and transmit sensory information directly to the animal's brain. The shape of the snout varies considerably and is clearly adaptive. The snouts of the two echidna species are narrow and cylindrical, ideal for probing among leaf litter or into anthills. The bill of the platypus is flat and shovel-shaped for sweeping through the top layer of sediment on lake and river beds. It resembles that of a duck in shape alone; in living specimens, it is soft and moist, more like a dog's nose than a bird's hard beak.

All monotremes are hairy. The platypus has a particularly well-developed pelt of fine, dense hairs. The coat is an adaptation to the animal's semi-aquatic lifestyle, and serves to keep it warm by trapping a layer of air close to the skin. In the echidnas, as in placental hedgehogs, porcupines, and some insectivores, the body hairs are interspersed with spines. In fact, the spines are themselves enlarged hairs. They are made of the protein keratin and grow from follicles in the skin.

All montremes have short, powerful legs. Those of the platypus are adapted for swimming. Each of their large feet has five long toes, connected by a leathery webbing. The legs and feet of echidnas are adapted for digging and breaking open anthills and rotten logs in search of food. Both echidna species possess very well-developed claws. Male monotremes also have characteristic horny spurs on their ankles. In adult male platypuses, these are large and sharp with longitudinal grooves connected to ducts from glands in the thigh that secrete a highly potent venom. The spurs of male echidnas are smaller and less well developed.

Unlike most of the world's mammals, the digestive, excretory, and reproductive tracts of monotremes, in both males

A short-beaked echidna (Tachyglossus aculeatus) in a defensive ball. (Photo by C. B. & D. W. Frith. Bruce Coleman, Inc. Reproduced by per-

and females, all exit the body via a single opening, called the cloaca. Females have mammary glands but no teats as such; the mammary ducts open in pores on the female's furry abdomen. Male platypuses and echidnas do have a penis—it is forked like that of some marsupials, but is used only for delivering sperm and not for urination. In male and female monotremes, urine from the bladder passes via the cloaca.

Monotremes are warm-blooded, but they maintain their body temperature at a slightly lower level than placental mammals—usually somewhere between 86°F and 91.4°F (30-33°C). The blood is pumped by a four-chambered heart, which differs from that of other mammals in having an incomplete separation between the right atrium and ventricle.

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