Thursday, May 11, 2006


Marsupials are animals that belong to the order Marsupiala and an infraclass Metatheria. Other members of the class include kangaroo, koala, Tasmanian devil and the Virginia opossum. Marsupials are also defined by biologists as a subdivision of mammals with several characteristics. They have more incisor teeth and in most of them the first toe of the hind foot is opposed to the other four and it also lack a claw. Their brain size is very small. They are generally accepted as non-placental mammal whose female carries her young in a pouch or marsupium, where young ones are provided with a proper environment, warmth, possess a placenta, although that placenta is non-invasive and functions in the transferring of nutrient and waste for a short period of time"(1)

"Placenta mammals are given this name because the placenta connects the growing embryo within the uterus to the circulatory system of the mother. It is the pathway provider for the nourishment of the fetus"(2). This enables the fetus to reach a higher level of maturity of the body and brain before birth, most of the placenta mammals invest most of their time in the stage of growth.

The major difference between the marsupial and placental mammals is the rate of gestation, or the time the offspring has taken in the uterus. Mating system vary in marsupials throughout the year. "Some species are solitary and they only come together to mate and this pattern of social behaviour reflect promiscuous mating systems. Male of some species defend their access to several females. In marsupials the developing embryo is separated from the body of its mother by amniotic membrane. After fertilisation the embryo will become a new organism and the immune system will attack it. The amniotic protects the embryo from this attack by isolating it from all biological interactions with the parent. The Marsupial embryo depends on the egg yolk for nutrients so it is limited to the quality of nutrients contained in the egg. The shortness of the gestation period is due to the type of yolk-type reproduction"(3). Birth in marsupials occur very fast compared in placental mammals including the helpless fetus journeys to the pouch where it becomes attached there for weeks or months depending on the species. The longest days a marsupial can spend in a reproductive tract are only twelve.

Placental mammals have a longer gestation period and this result in offspring that are born being fully developed. "The extended maturation of placental is caused by the placenta which, which allows nutrients to travel from the mother system to the embryo and waste to be carried away. The embryo and the mother do not share the same blood supply, but instead the placenta is composed of several layers which are richly supplied with blood vessels, and acts as a preferential immigration barrier letting nutrients and metabolites pass through, and preventing the transfer of immunity system components " (3).

When marsupials are pregnant it does not stop the oestrus cycle to take place but in placentals the oestrus cycle stop until the whole period of pregnancy is over and it will then re-start again. In marsupials the ovarian inhibition is mediated by lactation or suckling stimulus. This is important because the baby will no longer be carried internally so negative feedback stimulus from the presence of the babies must come from nursing activity.


1. Oppossaum Society Of The United States, 2003. Oppossaum Reproduction and Life Cycle [Internet] 2006 May 11; 08:45 UTC [Cited 2006 May 11] Available From:

2. Andrienne, L. 2001. The Human Evolution Coloring Book [Internet] 2006 May 11, 09:25 UTC [Cited 2006 May 11] Available From:

3. Culp, G. Richard. 1998. The Geographical Distribution of Animals and Plants [Internet] 2006 May 11; 10:30 UTC [Cited 2006 May 11] Available From:

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  • You have not always used quotes to indicate copying more than four words verbatim from a reference. This is plagiarism.

    By Blogger Gwen, at May 25, 2006 4:51 PM  

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