Vertebrates_Concept_1
Magnets
Magnets
Distribution
Marsupials are a distinct branch of the mammalian tree . After first evolving in North America some 85 million years ago, they migrated to Australia (the oldest marsupial fossils that have been discovered there are about 25 miliion years old), where they developed in isolation from placental mammals until the arrival of humans 40000 years ago. Now there is only one marsupial native to North America, a species of opossum, though there are still over 70 species in South America.
It is in Austratia, New Guinea and Tasmania that the greatest variety of marsupiais still exist. These include the kangaroo (which leaps on two legs), koala (a marsupial sloth), wombat (a burrowing marsupial similar to a large rodent) and opossum (a marsupial shrew).
Birth and development
Whereas placental mammals produce fully formed live young that have grown to a viable level of development within a womb, marsupials have a two-stage development.
The first stage is within the mother's womb. The fertilised embryo is neither attached to the mother via a placenta nor encased in a shell: it is simply supplied by a yoke sac. After less than one month of growth, when the yoke sac is empty, the partially forrned marsupial will have to venture out of the womb. At this stage a kangaroo, for example, will not yet have developed eyes or back legs; but it will possess very strong forelimbs and a homing instinct. The mother will lick down a track of fur from the opening of her womb to her pouch. Then, using its forelimbs, the embryo-like form (weighing less than I g) will drag itself to the pouch and latch onto a nipple, gathering nourishment and developing further until it has grown sufficiently to start letting go of the nipple for increasing amounts of time.
The young kangaroo will stay in the pouch for several months before tentatively venturing outside. It will return to the pouch for nourishment, safety and comfort for some considerable time after that, until it is finally 'evicted' by its mother. Some species of opossum lack a pouch, so the young are. left dangling from the nipple or grip onto the mother's back.
Why be a marsupial?
As wellas being able to use a pouch as a sort of 'second-stage womb', some marsupials are abe to practise an interesting form of family planning. Kangaroos living in semi-arid areas are able to delay egg release, and thus conception, untii the environmental conditions are right. When there is a plentiful supply of food and water, mating can safely be allowed to take place. Once a baby kangaroo has made it to the pouch, the kangaroo can mate agaen — and hold the development of the embryo at an early stage until pouch space is available. Once the more mature infant begins to leave the pouchithe embryo is allowed to continue developing. It is possible for a female kangaroo to have young at three stages of development at the same time: one weaned, but still likely to seek comfort in the pouch; one in the pouch, attached to a nipple; and one in a suspended state in the womb.
This mode of reproduction provides another benefit for marsupials that find themselves prey to other animals. On the African savannah, a heavily pregnant wildebeest would be relatively easy prey for a big cat, because it would not be able to run away. A kangaroo, on the other hand, may be able to eject its largest pouch infant and make a dash for it, in the knowledge that another baby in the pouch is less than a month away from emerging,
Marsupiai ecology
Until the arrival of placental mammals (humans in particular), the marsupials (together with some large, flightless birds) filled the niches that placental mammals filled elsewhere. In prehistoric times there were giant kangaroos over 3m tall feeding on the leaves of large bushes. more recent times (it officially became extinct in 1936), the thylacine (also known as the Tasmanian tiger orTasmanian wolf) preyed on smaller marsupials, There are kangaroos that live only in the canopy of rainforests, and wombats that live only on grassland.
This type of mammal is exotic enough to engage the interest of children, while providing an opportunity to explore an alternative evolution. Although not all marsupials are to be found in Australia, all the native mammals in Australia are marsupial — with two notable exceptions, the duck-billed platypus and the echidna or spiny anteater. These are both egg-laying monotremes. The breeding habits of marsupials provide an excellent example of an alternative mode of development.
In addition, marsupials are fascinating examples of parallel evolution: the forms of marsupial life in Australia resemble those of placental mammal life elsewhere, due to the evolution of species to fill a range of ecological niches.
Marsupial — a mammal that carries its young in a pouch, where it feeds and completes its development.
Monotreme — an egg-laying mammal.
An Australian eastern grey kangaroo can leap over 1 3m in a single bound and can reach speeds of up to 64km/h.
The Virginia opossum can have up to 56 offspring at any one time — but with only 13 nipples, the young have to race to survive.
The yapok (water opossum of South America) feeds by catching fish in rivers. The females have a waterproof seal to their pouches, so that their young won't drown,
Bandicoots spend only 12 to 13 days in the womb, then have to make for a nipple in their mother's pouch when they are only 10mm long.
The koala is a bear.
It looks a little like a bear (at least a teddy bear), but that is where the link ends. It does not fill the same ecological niche as a bear: it is much closer to the sloth in that sense. Most importantly it is a marsupial, which bears are not.
Why aren't there marsupials in other parts of the world?
Since placental mammals were able to migrate to the Americas, on the face of it there seems littie reason why the marsupials couldn't have travelled the other way, The key reason is that placental mammals are more effective survivors, and tend to drive the marsupials to extinction wherever they compete for food. The placentals were subject to greater competition as they evolved; so when they met the marsupials, they were able to feed more effectively and so force the marsupials into extinction, Because Australia was isolated by ocean from the rest of the world while the placental mammals were still deveioping, none made it that far until humans introduced them. Now rabbits, mice and rats that were introduced by settlers are driving some smaller marsupials to extinction — just as they did in many other parts of the world millions of years ago.
As this topic has been included to challenge more able children, the suggested tasks are research activities that can be performed using books, multimedia packages and the internet.
Children can research and record information on the dfferent stages in the life cyde of a marsupiai, then present it in a visual form.
Children can draw life-sized outiines of newborn and adult marsupials and placental mammals, noting similarities and differences bebween the two.
Niches
Children can find placental animals similar to various marsupials in the ecological niche they occupy (that is, the way that they live and use resources). For example, what is the African equivalent of Austraiia's red kangaroo? (Probably the antelope or deer.)
Marsupial humans?
After researching the life cycles and habitats of marsupials, ask the children to consider the idea of humans becoming marsupials. :What wouid be the advantages and disadvantages of this change?