What is a Marsupial?
Marsupials are a group of mammals that have an external pouch located on their mother's abdomen in which they carry their young. Most marsupials live in Australia, where they have evolved into some unique and unusual animals.
The key difference between marsupials and other mammals is that a marsupial has a very short gestation period. Gestation is the time between the fertilization of an egg until the the birth of an offspring.
Marsupials give birth to very immature and underdeveloped babies and then raise them to full maturity in a pouch outside their body. This is very different from placental mammals, such as humans, cats, dogs, elephants etc., that grow their babies inside their bodies and have a much longer gestation period. For example, a kangaroo which is a marsupial mammal has a gestation period of just 42 days. A deer, on the other hand, which is about the same size as a kangaroo but is a placental mammal has a gestation period of 201 days.
Marsupials range in size from the 1.8 meter tall red kangaroo which weighs 90kgs to the tiny, shrew-like Pilbara ningaui which is just 4.6 cm in length and weighs only 2 gms.
Marsupial Global Distribution
Most people think marsupials are only found in Australia but in actual fact they are also found in the Americas.
There are two main groups of marsupials in the world today. These are Australian marsupials and American marsupials. There are 235 species of Australian marsupial and 99 species of American ones. American marsupials are often called opossums and aren't as large and as varied as the Australian ones.
It was once thought that marsupials originated in Australia. Recent fossil evidence and genetic research, however, suggests that they may in fact have originated in what is now modern-day China on the ancient landmass known as Pangaea.
About 200-180 million years ago Pangaea broke up into two continents Laurasia with North America, Europe, China and parts of Asia and Gondwana with South America, Africa, Antarctica, Australia and India. (diagram left).
Marsupials are believed to have arrived in Australia around 50 million years ago via North America, South America and through Antarctica.
Once Australia separated from the other continents and started to drift southward the marsupials stranded on it didn't have any competition from placental mammals. Without competition the marsupials diverged into over 235 different species found in Australia today. Some descendants of those original marsupials even almost hopped their way back towards China reaching as far as Indonesia and Papua New Guinea.
Unfortunately the original marsupials in other parts of the world couldn't compete with placental mammals and became mostly extinct. A few marsupials still survive in North and South America.
Marsupials can live in habitats from trees to the forest floor to open bush and shrub drylands. Some like the wombat even burrow underground. The majority of Australian marsupials live in dry dessert and shrub habitats and generally avoid colder climates. Only the mountain pygmy-possum lives in in the snow country of the Australian Alps. Wombats too can tolerate some snow but usually only at the snowline.
Almost all Australian marsupials are nocturnal. This means they only come out at night. The reason for this behaviour is that the daytime temperature in Australia can get quite hot. So these animals have adapted to resting during the heat of the day and venturing out at night. The numbat and musky rat-kangaroo seem to be exceptions.
Marsupials are usually solitary animals, only coming together for short-lived pair bonds during mating. They exhibit little permanent social organisation. Some, such as kangaroos and wallabies, graze in groups called "mobs". However these are not considered true social groups because there are no leaders or elders that exercise cohesion between members of the mob. The one exception are the lesser gliders (Petaurus) that have loose social groupings.
Marsupials, just like their placental counterparts, eat a wide variety of food. Some such as kangaroos, koalas and wombats, are the herbivores, eating only plant matter. Others such as bandicoots and possums are omnivores, eating both plants and animals. The Tasmanian devil and the now extinct Tasmanian tiger are carnivorous, eating only meat. Others are insectivores, consuming only insects.
Marsupials usually have more teeth than other mammals . Their metabolic rates are also lower than that of comparable placental mammals. This means that they require less energy and as a result they require less food to survive. They also require less water than comparably sized placental mammal.
Marsupials use bipedal (two legs) and quadrupedal (four legs) locomotion. Macropods, a group of marsupials that include kangaroos, wallabies and bandicoots use hopping as their bipedal mode of locomotion. Quadrupedal marsupials such as the wombat and Tasmanian devil walk on four legs.
There are no aquatic marsupials, nor are there any flying marsupials even though some, such as the sugar glider, can glide from tree to tree. There are no bipedal walking marsupials either. (Humans are bipedal walking placental mammals).
Marsupials have many reproductive strategies which are suitable to the harsh Australian environment.
Female Marsupial Reproductive Anatomy
The female marsupial has three vaginas and two uteruses (uteri). The two outermost lateral sexual vaginas are used for sperm transportation to the two uteruses. Babies are born through the middle birth vagina. (See photo). By contrast, female placental mammals have only one uterus and one vagina.
With this unusual reproductive system a female marsupial can be in a continuous state of pregnancy, with a fertilised egg in one uterus waiting to be released, a baby growing in the second uterus, one in her pouch and another hopping outside but coming to its mother for milk. Another unique feature of these animals is that during times of extreme drought and starvation the female marsupial can can practice birth control by putting the babies growing in her uteruses "on hold", stopping their future development until conditions improve. This is called embryonic diapause. When the mother's pouch becomes free the next baby will be born and the fertilised egg will start developing into a new foetus.
Because of this multiple-offspring strategy and other adaptabilities unique to the marsupial, populations can increase rapidly when food is plentiful.
Male Marsupial Reproductive Anatomy
Most male marsupials have a bifurcated penis. That is, the penial shaft splits into two prongs at the end in order to enter the two separate lateral sexual vaginas of the female. Macropods, such as the kangaroo and wallaby, however have a slender tapering single-shaft penis which enters only one of the lateral sexual vaginas. A marsupial's penis is located behind its scrotum. (Most animals have the penis located in front). When flaccid, the penis is withdrawn into the body. The Tasmanian tiger (thylacine) had a scrotum pouch in which it stored its scrotum and testicles for safe keeping.
Marsupial males have also adapted their sperm production Australia's harsh environment. Because sperm production is energy intensive, marsupial males have adapted three strategies. Adult kangaroos, wallabies and rat-kangaroos produce sperm continuously and are therefore continuously fertile. However in periods of extreme stress such as a drought when food is in short supply their bodies shut down sperm production to conserve energy. Greater gliders only produce sperm during the mating season and at other times sperm production is shut down. An extreme strategy is that of the Antechinus where sperm is produced only once in the animal's lifetime, synchronised with the female's oestrus period (fertility period, comes into heat). After copulating as many times as possible during this time males dies in a few days.
Why Marsupials have Eggs with Shells
The reason for the marsupial egg shell has long been a puzzle. One suggestion is that it prevents the young embryo from being attacked and rejected by its mother's immune system. Placental mammals on the other hand have evolved a way to prevent maternal rejection of the foetus thereby enabling them to the have prolonged gestation of their young within their bodies.
The marsupial egg descends from the female's ovary into an uterus where it is fertilized. Once fertilised the eggs is encased in a very thin shell similar to that of birds and reptiles. This shell is just a few microns thick and disintegrates when the egg reaches the third phase of gestation. A remnant from the evolutionary past this unusual characteristic is common amongst marsupial mammals. The gestation period for a marsupial is between 12 to 30 days and varies amongst the different types of marsupials.
As the time approaches for the young marsupial to be born the female marsupial cleans out its pouch by sticking her head into her pouch licking the inside of it clean. It then takes up a "birthing position" by sitting on its back with its tail between its legs and the hind legs extended straight forward. It also leans the trunk of its body forward. It then licks its birth canal opening possibly to stimulate the birth.
The young marsupial, ranging in size from no larger than a grain of rice to about the size of a jelly-bean, soon emerges from the birth canal. It is born blind, hairless, with stumpy forelimb and hardly any trace of its hind legs. Even though it is still so underdeveloped the young newborn has an excellent sense of direction, knowing which way is up and down, and also an acute sense of smell. Using its little forelimbs in a swimming motion, the young joey crawls laborious to its mother's fur to the pouch. This journey takes it about three minutes. The joey's journey is made entirely by itself. The mothers does not assist it in any way. Once inside its mother's pouch the joey quickly attaches itself firmly to a nipples in the pouch.
Once it has attached itself to its mother's nipple the young joey will stay hidden for up to six and a half months. Then it will start to tentatively pop its head out of its mother's pouch and
observe the world around it. About two weeks later it will have gained enough confidence to venture out of the pouch and hop about close to its mother. However if frightened it will immediately jump
back into the safety of the pouch. By the time it is about 8 months old the joey no longer uses its mother's pouch.
Unusual Weaning (Milk Production)
The female marsupial can produce two types of milk depending on the joey it is feeding. The milk produced in the nipple on which an embryonic joey is attached will be different from the milk produced to feed a joey which has already left the pouch and only comes back to be weaned.
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