COMPARISON BETWEEN PLACENTAL AND MARSUPIAL MODES OF REPRODUCTION

Placental mammals “are highly developed mammals which give birth to young at a relatively advanced stage” (1). There are “almost 4,000 known species of placental mammals” (1). They are more advanced than the young of the other mammals, the monotremes and marsupials. Before birth, the young are nurtured through a placenta. The placenta is attached to the mother uterus and it is a specialised embryonic organ. These help to deliver oxygen and nutrients to the young placental mammals.

Marsupials are mammals in which the female typically has a pouch in which it rears its young through early infancy. Marsupials are “animals that belong to the order Marsupial and an infraclass Metatheria” (2). “The member of the class includes kangaroo, koala, Tasmanian devil and the Virginia opossum” (3). Marsupials are known have a lot of characteristics and they are not considered to be non-placental mammal. “The female marsupials “carries the young in the marsupial or pouch” (3). This is where the young are provided with warmth and proper environment. They are characterised by non-invasive placenta which is used in the transferring of nutrient and waste for limited period of time. (3)

“Placental mammals differ from the marsupials in that their young develop to a relatively mature stage within a uterus attached to the mother by an allantoic placenta. The placenta allows the foetus to plug into its mother's circulation and share her respiratory and excretory systems, and the nutrients carried in her blood. Mammals, like marsupials, feed their young with milk and care for them for prolonged periods until they can look after themselves” (4).

The marsupials differ from placental mammals in the way in which reproduction take place. The “female has two vaginas and both of which open externally through one orifice but lead to different compartments within the uterus” (3). The “male marsupials have two penis which only passes sperm” (3) and these two penis corresponds to the females two vaginas. ”The pregnant female develops a kind of yolk sack in her womb” (3) which delivers nutrients to the embryo. “The embryo is born at a very early stage of development (at about 4-5 weeks), upon which it crawls up its mothers belly and attaches itself to a nipple” (3). It remains attached to the nipple for a number of weeks. The offspring later goes through a stage where it temporarily leaves the pouch and then return for warmth and nourishment.

The “early birth of marsupials removes the developing young much sooner than in placental mammals, and marsupials have not needed to develop a complex placenta to protect the young from its mother's immune system” (5). “The early birth places the tiny new-born marsupial at greater risk, but significantly reduces the risks associated with pregnancy, as there is no need to carry a large fetus to full-term in bad seasons” (5).

The offspring of the placental mammals are born fully developed. This is one of the characters that distinguish this two. The placental mammals have a longer gestation period which results to the situation where off springs are able to develop fully while they are still inside their mothers. The extended maturation of placental is caused by the placenta which allows nutrients to travel from the mothers system to the embryo and waste to be carried away. The placenta is composed of several layers and these layers are supplied with blood and serve as immigration barrier letting nutrients to pass through. The embryo and the mother do not share the same blood supply.

Reference:

1. Enchated Learning.com, Placental Mammals. 1998 [Internet] [Cited 2006 May 24] Available from:
http://www.enchantedlearning.com/subjects/mammals/Placental.shtml

2. Wikipedia contributors. Marsupial [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 19, 22:51 UTC [cited 2006 May 24]. Available from:

http://en.wikipedia.org/w/index.php?title=Marsupial&oldid=54105926.

3. Opossum society of the United States, Opossum reproduction and life cycle, 2003 [Internet] [cited 2006 may 24] Available from:
http://www.opossumsocietyus.org/opossum_reproduction_and_life_cycle.htm

4. Matthews B.E and Preston, M.P. The placental or true mammals, Joint museum curators, Edinburgh, 2001 [Internet] [Cited 2006 May 24] Available from: http://www.nhc.ed.ac.uk/index.php?page=24.134.166

5. WGBH education foundation, convergence, marsupials and placental, 2001 [Internet] [cited 2006 may 11] Available:
http://www.pbs.org/wgbh/evolution/library/01/4/pdf/l_014_02.pdf

Mr Lufuno Mukwevho
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