Monotremata (Monotremes)

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Monotremata

Family: Echidnas
Family: Duck-Billed Platypus

(Monotremes)

Class Mammalia

Order Monotremata

Number of genera, species
3 genera; 3 species

Introduction

The three living species of monotreme have always been regarded as zoological enigmas, so much so that when the first specimen of a duck-billed platypus (a single skin) was shipped back to Europe from Australia at the close of the eighteenth century, it was widely believed to be a hoax, and an unconvincing one at that. It would not have been the first time a ship arrived from the East Indies with a cleverly faked "marvel." Previous hoaxes had included supposed mermaids (half fish, half monkey) and wildly exotic birds of paradise, so the scientific community at the time can be forgiven for being skeptical. However, the duck-billed platypus is one of those cases where fact is at least as strange as fiction. It is an unlikely looking amalgam of parts taken from other animals, including a stout, cylindrical body covered in fur, huge webbed feet, a flat paddle-shaped tail, and a unique rubbery bill. The English naturalist George Shaw examined the skin, and was initially dubious as to the validity of the specimen. After examining it carefully and failing to find any evidence of forgery, he formally described it in 1799. He named it Platypus anantinus, which literally translated means "flatfooted duck-like." The name was changed to Ornithorhynchus anatinus when it later transpired that the genus name, Platypus, had already been used to describe a species of flat-footed beetle. The platypus was not the first known monotreme. The short-beaked echidna (Tachyglossus aculeatus) had been described seven years earlier, also by Shaw, but had caused much less of a stir, presumably because its superficial similarity to the European hedgehog made it easier to accept. It was not until 1802 that an entire preserved specimen of the platypus arrived in Europe, providing unequivocal proof that it was a genuine animal. But this was just the first in a long series of zoological conundrums presented by this highly distinctive group. Closer examination of platypuses and echidnas has showed that these are far from regular mammals. The suggestion that they reproduced by laying eggs instead of giving birth to live young left most Victorian zoologists incredulous, and it was almost 100 years before this could be proved. These and many other disconcerting new discoveries have earned the monotremes an enigmatic reputation among mammals, and they are still far from being fully understood.

Evolution and systematics

The first reaction of many taxonomists on examining the monotremes was to classify them as an unusual group of furbearing reptiles. It took almost 200 years for monotremes to be declared unequivocally as mammals—indeed, some people remain unconvinced. The characteristics that qualify them as mammalian include a single bone in the lower jaw, three small bones in the middle ear, a high metabolic rate and warm blood, a body covering or hair, and the ability of females to produce milk to feed their young. This last feature is the most important. Milk is secreted by mammary glands; hence, the class name, Mammalia.

In order to emphasize the differences between monotremes and other mammals, the order is often placed in its own subclass, the Prototheria. The name, meaning "first beasts," is unfortunate, since it implies that monotremes are in some way

ancestral to other mammals. This is a common misconception. Many people still insist on describing monotremes as primitive, or inferior. This is demonstrably not the case. However, the Monotremata are certainly an ancient group—they split from the main branch of the mammalian phylogenetic tree sometime during the Cretaceous period, probably about 125–130 million years ago. This was before the divergence of marsupials and placental mammals, but at least 80 million years after the split between reptiles and mammals. Monotremes are no less advanced than any other living mammal group. Far from being ancestors of other mammals, they are cousins that have simply evolved in a different direction. They have retained many characteristics attributed to mammal ancestors, but they have also developed sophisticated adaptations lacking in other mammals. For example, they possess a remarkable "sixth sense" that enables them to sense the minute electric fields generated by other animals.

Seemingly, the monotremes have never been a particularly large or predominant group. The fossils identified to date include just eight extinct species, most of which have been found in Australia. However, these are quite diverse, including two species representing extinct families, the Kollokodontidae and Steropodontidae. There are also three extinct species of echidna and three long-dead species of platypus. The earliest known monotreme is also the earliest known mammal in Australia, a fossil known as Stenopodon galmani. It was discovered in rocks about 110 million years old during the excavations of an opal mine at Lightning Ridge in New South Wales. Only a fragment of jaw and a few teeth have been recovered, but this is enough to show that monotremes were as much a feature of Cretaceous Australian landscape as dinosaurs. Another important fossil find includes a 62-million-year-old platypus tooth from Patagonia. This fossil, known as Monotrematum sudamericanum, the South American monotreme, is significant because it suggests that the ancestors of modern monotremes may once have been widespread on the prehistoric southern landmass of Gondwana.

The most informative monotreme fossil discovered to date comes from the extraordinarily rich fossil beds of Riversleigh in Queensland, Australia. Estimated at 13 million years old, the specimen is an almost perfectly preserved skull that closely resembles that of a modern platypus, except that the jaw is full of developed teeth. The modern platypus only has baby teeth (milk teeth), which are, in the adult, replaced by flat horny pads that are used like millstones to crush and grind food before swallowing. The long-extinct relatives of the duck-billed platypus probably had a broad insectivorous diet much like that of modern-day hedgehogs and shrews. The Riversleigh platypus was given the generic name Obdurodon, meaning "enduring tooth." It is considered one of the mammalian fossil finds of the century, and is one reason River-sleigh has been designated a World Heritage Site.

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 developing

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

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.

Distribution

Modern monotremes, and all but one of the fossil species so far described, are confined to the continent of Australia and the island of New Guinea. The short-beaked echidna is the most common and widespread of the three living species. It occurs throughout Australia and Tasmania and in central and southern New Guinea. The platypus is more restricted— it occurs only in eastern Australia in the states of Queensland, New South Wales, Victoria, and Tasmania. There is an introduced population on Kangaroo Island off the coast of South Australia. The long-beaked echidna (Zaglossus spp.) is endemic to New Guinea and is increasingly restricted to remote areas that remain inaccessible to humans.

Habitat

For a small group of specialized animals, monotremes occupy a surprisingly wide range of habitats. The duck-billed platypus is semi-aquatic and is dependent on permanent rivers or freshwater pools. Thus, it is restricted to parts of Australia with a relatively high rainfall. Both males and females construct simple burrows on the banks of rivers or pools and catch most of their food underwater. Polluted waterways and those that have undergone severe bankside development or canalization are generally not suitable. However, platypuses are increasingly common in suburban settings, due to legal protection and environmental restoration projects.

Short-beaked echidnas are among the most ubiquitous Australian mammals. They have no specialist habitat requirements other than an adequate supply of ants for food, and live everywhere from tropical rainforest to suburban gardens and city parks. There is enough moisture in their diet to sustain them even in the arid central desert of Australia, although there they are much more sparsely distributed. The long-beaked echidna occupies a more restricted range of habitats, mainly montane forests and damp alpine meadows in the higher parts of New Guinea. It is much less tolerant of dry conditions than its short-beaked cousin.

Behavior

As general rule, monotremes are nocturnal or crepuscular, and the best time to watch them is around dusk and dawn. They are active by night in order to avoid the heat of the day. However, daily activity is dictated to some extent by climate and the degree of disturbance. The two species whose range extends into temperate parts of Australia, the duck-billed platypus and the short-beaked echidna, are often active by day, especially in winter when the nights can be quite cold.

Cold weather and food shortages can induce the short-beaked echidna to enter short periods of torpor—a deep sleep, during which metabolic processes slow down and energy is conserved. The species is also the only monotreme capable of full hibernation. In most parts of the species' range, this is never necessary, but in the Snowy Mountains of New South Wales, winters can be sufficiently long and harsh so that echidnas spend up to four months asleep. They may wake periodically to investigate their surroundings, and even move to another den before going back to sleep. During hibernation, the echidna's body temperature may drop as low as 39.2°F (4°C), much lower than during shallow summer torpor. A hibernating animal uses very little energy, but even so, a long winter can take a serious toll on an individual's reserves of fat, and it may emerge in spring some 10–20% lighter than when it first began hibernating.

Detailed studies of the duck-billed platypus and the long-beaked echidna are hampered by the fact that these are rather shy and elusive animals. The same cannot be said of the short-beaked echidna, which is bold by comparison and especially tolerant of humans. In fact, Australian echidnas have few enemies and little to fear from any predator. They are too big to be threatened by cats or even foxes, and their sharp spines are usually sufficient to deter large dogs such as the dingo or birds of prey. If a large animal approaches too close for comfort, the echidna curls its body into a tight ball and raises its spines for protection. Should evasive maneuvers be necessary, the echidna can burrow extremely quickly, literally sinking into the ground until all that remains are a few protruding spines. Despite their lack of spines, adult platypuses are large enough to have few natural predators.

The platypus spends much of its time hidden away in waterside burrows, and emerges to feed only in the quiet hours after dusk and just before dawn. It tends not to travel far from home and usually slips straight into the water. When moving on land, the platypus uses a brisk but relatively inefficient waddle, but it is a superb swimmer and spends much of its time beneath the surface. Its perfectly streamlined body is propelled swiftly and silently through the water with the large webbed front feet. The back feet act as rudders and brakes, and the animal is able to twist and turn with a speed and agility comparable to that of a bird in flight. The platypus returns to the surface every minute or so to take a breath, but it does so silently and without a splash. At the start of each dive, it rolls forward in the water, its sleek form barely breaking the surface. Echidnas can swim, too—their spines help make them surprisingly buoyant and they can make rapid progress in water using a steady doggy paddle.

The monotremes are for the most part solitary animals, except for mothers with young. Single animals occupy a home range that may overlap with those of several others, but they are not territorial and show very little interest in each other except during the breeding season.

Less is known of the New Guinean long-beaked echidna than its two Australian cousins. It too is nocturnal and generally lives alone.

Feeding ecology and diet

Monotremes are highly specialized feeders on invertebrate prey, but the diets and foraging behaviors of the living species are all very different. The short-beaked echidna specializes in feeding on ants, an abundant food resource exploited by relatively few other Australian animals, which is another reason for the species' great success. The echidna's long narrow snout or "beak" is thought to be equipped with an additional sense that enables the animal to detect electrical activity, but probably the most important sense when it comes to feeding is smell. The echidna shoves its snout into ant nests and rotten logs. If ants are detected, the animal uses its claws to rip open the nest and begins lapping up the insects with its long, sticky tongue. Mouthfuls of ants are mashed between the tongue and the hard palette of the echidna's mouth before being swallowed.

Like the short-beaked echidna, the duck-billed platypus faces very little competition for food. The platypus hunts underwater in the dark, but the gloomy conditions are no handicap. The platypus has reasonable eyesight and hearing, but it closes both its eyes and ears when underwater and relies wholly on the information transmitted to its brain by nerves serving the snout. Not only is the snout sensitive to touch, it also contains about 850,000 tiny receptors, able to detect the minute electrical fields generated by the bodies of other living animals, even very small ones such as those of insect larvae. It is difficult to imagine how this extra sense works—it is similar to the lateral line sense of fish, but more finely tuned. Larger prey animals are snapped up and crushed against the

hard palate, while smaller ones are strained out of the water or sediment. The food is then pushed into large cheek pouches and stored while the platypus searches for more. The platypus returns to the surface periodically to process and eat its catch. Food is crushed and ground between horny plates that line each jaw, and swallowed. It may take several minutes to finish such a meal, during which time the platypus drifts at the surface with its feet spread wide.

The third species, the long-beaked echidna of New Guinea, is though to feed mainly on earthworms, which it unearths in humid forests. Its extra long nose is used to probe deep into the humus layer or into the topsoil, and once detected, prey is quickly excavated with the front feet, collected with the aid of a long, mobile tongue that is armed with hook-like spines in a central groove. The worms are lightly mashed in the mouth, then swallowed.

Reproductive biology

One of the most remarkable montreme features, and the one that initially seemed to be the biggest obstacle to their inclusion in the class Mammalia, is the fact that females lay eggs instead of giving birth to live young. The eggs are subsequently brooded and hatched outside the mother's body, as in reptiles and birds.

Monotremes typically breed slowly. It takes a mother platypus about six months to raise a small litter of one or two young to independence, seven months in the case of the short-beaked echidna, which typically has only one baby at a time. Long-beaked echidnas have larger litters of four to six young, but still only breed once a year. By investing a large amount of parental care in a few young, the young have a high rate

of survival. They are also surprisingly long lived. Wild platypuses usually survive into their early teens, whereas long-beaked and short-beaked echidnas may live well into the 20s, while captives have lived 30 and 50 years, respectively.

By early spring, courtship and rivalry among platypuses is well underway and males become very aggressive. They will fight for dominance and the right to mate with the females living within their range. They have no teeth and their claws are blunt, but the sharp spurs on the ankles are deadly. Normally, they are kept folded away to avoid snagging, but during battle they are raised. Fights occur in the water, where the animals are most agile, and combatants swim in tight circles, each attempting to spike the other and inject a debilitating dose of venom. The venom is toxic enough to kill a dog and cause agonizing pain and prolonged paralysis in humans. The male duck-billed platypus is the world's only venomous mammal. The spurs in male echidnas are small and sharp, but lack the deadly venom. Having seen off his rivals, the victorious male woos the female with a courtship involving a slow circular dance, during which he holds her tail in his bill. Both courtship and mating take place in the water.

Rivalry among male echidnas is equally intense, though not quite as violent. At the start of the breeding season, male echidnas begin following females around. After two or three weeks, some females have attracted a following of six or seven suitors, that follow her every move in a line known somewhat whimsically as a "love train." As the female comes into breeding condition, the males begin circling her, creating a circular trench from which each male attempts to evict his rivals. The last male left in attendance claims the right to mate.

Beyond courtship and mating, male monotremes have nothing more to do with the rearing of their young. The females, on the other hand, are diligent parents. After mating, females are busy preparing their nests, which are built in deep burrows. The echidnas excavate burrows for nesting or take advantage of natural dens such as rock crevices or hollow logs. Platypus burrows are simple oval tunnels with a sleeping chamber at the end. Breeding females also build more extensive nesting burrows. These may extend as far as 65 ft (20 m) into the bank, with branching tunnels that twist and turn, some leading to living chambers, others to dead ends. Unlike most other mammals, which do their best to keep nesting areas snug and dry, the atmosphere inside a platypus nest is very humid. The nest is made of damp leaves and other vegetation collected from the water or the banks and carried to the burrow clasped under the body by the tail. The breeding tunnel is blocked every few feet with loose earth, which the female shifts and replaces every time she comes and goes.

As in marsupials, most development takes place outside the mother's body and pregnancy itself is very short—just two weeks in both the platypus and the short-beaked echidna. The eggshells are rubbery, not brittle, and surround each embryo while it develops in the uterus. Each egg contains a very large yolk to sustain the embryo until it has developed sufficiently to hatch out and sustain itself on milk. The eggs are small and almost spherical. Those of the short-beaked echidna are laid directly into a temporary fold of skin, like a marsupial pouch, and can be carried with the mother. The platypus has no pouch and, once she has laid her eggs, she stays with them in the nest, her body curled around them, never leaving them for more than a minute or two, for fear they become chilled. The eggs are flexible and slightly sticky, so once laid they tend not to roll around.

The young of both species hatch after an incubation period lasting about 10 days. They cut their way out of the egg using a special milk tooth to pierce and tear the leathery shell. For the echidnas, this single tooth is the only one they will ever possess. In the platypus, baby teeth do develop, but they never become functional.

Newly hatched monotremes are barely 1 in (2.5 cm) long. The body is pink, naked, and almost transparent. Their skin is so delicate that they would shrivel and die in minutes if exposed to the sun. But in the humid environment of the mother's pouch or the nest, young echidnas and platypuses are safe from desiccation as long as they can find milk. As for all mammal babies, the first urgent task for a newly hatched monotreme is to reach the mammary ducts on the mother's abdomen, when they start leaking milk about 10 days after birth. The development of lactation in mammals is one evolutionary mystery on which the monotremes have been able to shed some light. Mammary glands are thought to have evolved from sweat glands. In the ancestors of mammals, the young of animals that laid eggs like monotremes must have benefited from the secretion of a sweat-like substance from cutaneous glands on their mother's brood pouch. To begin with, they may have simply absorbed extra salts or moisture, but once this small nutritional advantage was established, natural selection favored lineages in which the glands became more and more active. Lactation in mammals was obviously well established by the time the monotremes diverged from the placental and marsupial lineages, but the former apparently never developed specialized structures for the delivery of milk to the offspring, namely teats. In marsupials and placental mammals, the release of milk is triggered by giving birth and is sustained by the stimulus of young sucking on a teat. The situation in monotremes is different, since milk is not needed until 10 days after giving birth and there are no teats for the young to latch on to. Instead, the milk seeps into the mother's fur. Young platypuses lap up the milk as it accumulates in the fur, while baby short-beaked echidnas suck vigorously at the mammary pores.

A young echidna may ride in its mother's makeshift pouch for up to three months, but not surprisingly it is evicted as soon as its spines begin to grow. Then it will be left in the nest while its mother goes out to feed. Likewise, as young platypuses become able to maintain their own body heat, their mother can leave for longer periods. The young are well protected, even when left home alone—each time she leaves, the mother carefully replaces a plug of earth as a deterrent to predators and to prevent her offspring from getting out.

Young platypuses are weaned at three or four months. Young short-beaked echidnas first venture outside the pouch at about the same age, but are not capable of feeding themselves for a further three months.

Conservation

The long-beaked echidna is listed as Endangered by the IUCN. Never as widespread or abundant as its two Australian relatives, Zaglossus is now threatened by habitat loss and severe over-hunting for meat. The logging industry in New Guinea is not only responsible for the destruction of huge areas of forest, it also leaves the remaining habitat much more accessible to hunters. The latest estimates put the total population at about a quarter of a million.

In contrast, the short-beaked echidna is apparently thriving. It is one of few native Australian mammals for which the arrival of European settlers and introduced wildlife has not resulted in a serious decline. It is not hunted for meat, and its spines are ample protection from most animal predators. Its diet of ants means it can survive in a wide variety of habitats and is not adversely affected by many forms of development. Both echidnas and their prey can even survive bush fires, by burrowing underground and waiting for the flames to pass above.

The duck-billed platypus is something of a conservation success story. It was hunted extensively for its fur, which is thick and silky like that of an otter. The platypus also suffered indirectly from the actions of humans, as rivers were polluted by industrial and mining effluent and waterways were modified around human settlement. Concrete banks are not good for burrowing, and human-made structures such as weirs, drain guards, and dams are all potential platypus death traps. Many thousands have been drowned in fishing nets. However, the story has a happy ending. Unlike many other threatened mammals, the decline of the duck-billed platypus did not go unnoticed and, since the 1960s, it has been a well-protected species. Many waterways have been restored specifically to meet platypus needs and it is an increasingly common species, even in some towns.

Significance to humans

Echidnas and platypuses are charismatic animals, which do no harm to human interests. They are also of considerable novelty value. "Platypus-spotting" is one of the many wildlife encounters offered by the highly lucrative Australian ecotourism industry. Sightings are rarely guaranteed, but for many people, a glimpse of one of these enigmatic creatures slipping silently into view and disappearing once more will remain a treasured highlight of a visit down under.