EGG LAYING MAMMALS | TechGape

EGG LAYING MAMMALS

-Rahul Rohitashwa

Animals are generally divided into two main groups, the first are the vertebrates which have a backbone or vertebral column and the other is the invertebrates which lack backbones. The vertebrates include fishes, amphibians, reptiles, birds and mammals. Animals such as insects, worms, snails, octopus, starfish, etc. fall into invertebrates which are devoid of backbones. 

Vertebrates which appeared on this planet earth approximately 480 million years ago are the most advanced animals. Today, among them the most highly evolved are the mammals which came into existence almost 200 million years ago. All mammals are warm-blooded and have a body covering of either fur or hair. With their highly developed nervous system and other copious adaptations, mammals occupy almost every environment on earth that supports life.

Most mammal babies grow in their mother’s womb and are born quite well developed. But in marsupial mammals such as Kangaroos, Koalas etc. the babies are born tiny and are underdeveloped. They continue their next phase of growth in the safety of their mother's pouch called a marsupium. In this context, the most exciting and exceptional case is that of the monotremes. 

The order monotremes fall into subclass Prototheria which are considered to be the most primitive mammals which are restricted only in Australia and its adjacent neighbouring islands i.e. Tasmania and New Guinea. Monotremes do not give birth to babies at all. They lay eggs like birds and reptiles and eventually the eggs hatch into babies. They have no placenta and their digestive and urogenital tract opens into a cloaca. They are oviparous and the corpus callosum of their brain is either feebly developed or absent.

Perhaps the oddest of all mammals are the monotremes or egg-laying mammals. The three important representatives of monotremes left today in the world are the Platypus and four species of Echidnas. They are the main inhabitants of Australia, New Guinea and its adjoining islands.

The Echidnas:

Echidnas sometimes known as spiny anteaters belong to the family Tachyglossidae. There are four extant species, of Echidnas which, together with the duck-billed platypus, are the only surviving members and are the only extant mammals that lay eggs. Their diet consists largely of ants and termites. They live in Australia and its adjoining islands. Echidnas and the platypus are the only egg-laying mammals, known as monotremes. 

The female lays a single soft-shelled, leathery egg 22 days after mating, and deposits it directly into her pouch. Hatching takes place after ten days. The young echidna called a puggle, then sucks milk from the pores of the two milk patches (monotremes have no nipples) and remains in the pouch for 45 to 55 days. At this time it starts to develop spines. The mother digs a nursery burrow and deposits the puggle, returning every five days to suckle it until it is weaned at seven months. The average wild echidna can grow as old as 16 years.

The Echidnas look almost similar to porcupines being covered with long, sharp and protective spines. They use their powerful front claws to dig soil or termite rests and then they lick up their prey using their long sticky tongue. The female Echidna keeps usually her single egg or sometimes two to three eggs in her temporary brood pouch which develops on her belly. The incubation time of the eggs varies from ten to twelve days. Then she feeds the tiny helpless baby on her milk like other mammals.

There are mainly two species of Echidnas which are confined in Australia and its nearby islands. These two species are the Short-beaked Echidnas and the Long-billed Echidnas. The short-beaked Echidnas grow to about 35 cm in length while the long-beaked Echidnas grow to about 70 cm in length. But both species of Echidnas use their strong claws for digging up their prey and in excavating burrows.

The short-beaked Echidna is quite common in Australia and New Guinea. It is metropolitan in nature and is found in almost all types of habitats ranging from dry deserts to cold uplands, scrubland and forests whereas the long-billed Echidna is found only in the highlands of Papua New Guinea. Despite its name, as spiny anteater the long-beaked Echidna also feeds occasionally on earthworms which it catches with its tough spines on its tongue.

The Duck Billed Platypus:

Platypuses are the main inhabitants of Australia and Tasmania. They are a semi-aquatic egg-laying mammal. It is the sole living representative of its family Ornithorhynchidae. It lays its eggs in the nest made up of grass and leaves consisting of a very long breeding burrow in the riverbank. The platypus is one of the strongest of all mammals known till date. This fact stems its origin late back in 1798 when scientists belonging to Royal Imperial College of Medicine, London, studied a preserved platypus body and acclaimed that it was a fake compendium made up of several animals sewn together. 

As for instance the beak or bill is broad and flat just like a duck, hence they are named as the duck-billed platypus, their flattened tail resembles like that of a beaver and their webbed feet resemble like that of an otter. Its eyes are small with nictitating membrane while external pinnae are totally absent. Both the webbed feet and flattened tail and its streamlined body feature make it ideally suited to swim and therefore it spends up to half its time nosing and grubbing in the water for food. It uses its tail for dual purpose i.e. as a rudder and also to store a reserve of fat against a rainy day.

Usually, the male and female look alike but males can be easily distinguished from their female counterparts. The male has a spur on each hind limb connected with a poison gland thus showing an example of sexual dimorphism. Platypuses are nocturnal animals and are usually active at night. Nature has provided these animals with an extraordinary mode of feeding. They hunt at night and detect the prey hidden in the mud by receiving the electrical impulses given continuously out by the muscles of the prey body. The platypus senses these types of impulses by its sensory bill which is very sensitive to touch and movements. During hunting, it keeps its eyes and nostrils tightly closed.

Electro Location:

Monotremes are the only mammals known to have a sense of electroreception. They locate their prey in part by detecting electric fields generated by its muscular contractions. The platypus' electroreception is the most sensitive of any monotremes. The electroreceptors are located in rostrocaudal rows in the skin of their bill.

The platypus can determine the direction of an electric source, perhaps by comparing differences in signal strength across the sheet of electroreceptors. The platypus feeds neither by sight nor smell. Rather, when it digs in the bottom of streams with its bill, its electroreceptors detect tiny electrical currents generated by muscular contractions of its prey, so enabling it to distinguish between animate and inanimate objects.

Connecting Link:

Both the animal i.e. Echidnas and Platypus are considered today as connecting link because they share some characters which are common to both reptiles and mammals. As for example their egg-laying habit, reptilian type of excretory system, hairs on their skin like mammals etc. Once very rare, these animals are now protected by law and have become common in some regions. But there are reports that of and on these animals are killed illegally by poachers for its skin and other body parts which is certainly a matter of great concern. 

Though the IUCN (International Union for Conservation of Nature) lists the platypus on its Red List as least concern, the underlying threats to its survival could not be ignored. But due to the rigorous efforts of some NGOs and wildlife activists once the rampant killing of these marvellous and strange animals have been banned in many areas of Australia, Tasmania and other nearby islands. Consequently, now these animals witness better survival opportunities and are in a fair way of success.

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Rahul Rohitashwa
Freelance Science Writer: Environment and Wild Life Naturalist, Keen Bird Watcher & Involved in various Nature Conservation Campaign and as well as in Educational Activities. Winner of Prestigious “Earth Eco-Warrior Award- 2008” by the Honorable Vice President of India in the field of Nature and Eco-Conservation activities in Bihar Twice Winner of “Prithvi Mitr (Friends of the Earth)” Award & “Yuva Rashtriya Pratibha Samman” by Earth Matters Foundation and Govt. of Bihar respectively.
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