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

The world's first platypus (Ornithorhynchus anatinus) twin puggles born in captivity are shown together for the first time during a full health check at Taronga Zoo's veterinary clinic in Sydney, Australia, March 28, 2003. (Photo by AFP PHOTO/Torsten BLACKWOOD. Reproduced by permission.)

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 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 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 pla-cental 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.

Pregnancy And Childbirth

Pregnancy And Childbirth

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