Intermezzo : Marsupials and Monotremes

When you ask around about Australian animals, everyone knows a koala and a kangaroo… But there are a couple more special ones over there.

These animals are part of some unique kind in the world, the MARSUPIALS (or pouch animals). This means that their young are birthed very early and fatten a couple of months afterwards in the pouch.

Another special group are the MONOTREMES or egg-laying mammals…

MARSUPIALS

Marsupials evolved in North America, found their way to South America, and then into Australia via Antarctica when the southern continents were joined as Gondwana. In Australia they diversified to fill many niches. Most of the 140 species of marsupials in Australia are found nowhere else in the world; some of them are also found in New Guinea which was connected to Australia in more recent geological times.

A marsupium or pouch is one of the features that characterize marsupials although not all have a permanent pouch and a few have none at all. They are similar to mammals in being covered in fur and bearing live young which are suckled by the mother. In marsupials the gestation period is very short resulting in the birth of undeveloped young. Although blind, without fur, and with hind limbs only partially formed these tiny newborns have well developed forelimbs with claws that enable them to make their way into the pouch and attach to a teat and continue their development.

The trade-off of the short pregnancy is the lengthy period of lactation during which the young remain in the pouch and the composition of the milk produced by the mother changes depending on the developmental stage of the young.

This kind of behavior is well suited for the extreme dry continent.
In periods of great drought, the mothers stop their birthing-cycle to save energy. If the drought keeps on, even the little one in the pouch dies, but once the rains have fallen, the mothers are able to start up their pregnancy cycle very fast and within a couple of weeks, the mothers have once again two babies… One in their uterus and one in their pouch.
From kangaroos it is known that they have up to three little joeys at the same time, one in their uterus, one in their pouch and one jumping around, but still drinking milk…

Hereby a list of marsupials (with the most famous ones) :
kangaroos, wallabies and euros
koalas
gliders and possums
wombats
bandicoots
Tasmanian devils

MONOTREMES

There are only five living monotreme species: the duck-billed platypus and four species of echidna (also known as spiny anteaters). All of them are found only in Australia and New Guinea. Monotremes are not a very diverse group today, and there has not been much fossil information known until rather recently.
In some ways, monotremes are very primitive for mammals because, like reptiles and birds, they lay eggs rather than having live birth. In a number of other respects, monotremes are rather derived, having highly modified snouts or beaks, and modern adult monotremes have no teeth. Like other mammals, however, monotremes have a single bone in their lower jaw, three middle ear bones, high metabolic rates, hair, and they produce milk to nourish the young.