Tuesday, May 09, 2006


Mammals are animals of the class of Vertebratata whose females have milk producing mammary glands for feeding their young; they also have fur (hair) on their endothermic bodies (warm-blooded) 1. The placental and marsupials are mammals that feed their young using mammary glands. So how do these groups of mammals modes of reproduction vary from each another?

Both placental and marsupial animals reproduce through internal fertilisation. Marsupials have an external pouch (marsupium) on the underside where it rears its young2. After fertilisation the young marsupials grow inside a york sac that is formed inside the females womb which provides nutrition to the embryo2. The young marsupials are born in their early developmental stages and crawl to the pouch where they are nurtured till they develop to independent young ones. In the pouch they attach to the nipples to feed on the milk from the mother.

On the other the placentals have their young through internal fertilisation and the young are enclosed by a placenta in the female’s womb. The placenta allows the young to remain within the uterus for a very long time. The placenta is attached to the uterus and is joined to the foetus by an umbilical cord4. It has a high amount of blood vessels and at this point there is a lot contact between the placenta and the uterus to allow for an interchange between the mother and foetus. The mother’s blood and the foetus don’t mix but through diffusion the waste products from the foetus are exchanged in the mother’s circulatory system. The mother removes them via the kidney4. The same happens when they need to exchange of oxygen from mother’s lungs and nutrients from her food4.

The mammalian sexual cycle involves the regular production of a new egg. For the marsupials before the next egg is produced the neonate in the womb is born but for the placentals’ the foetus has to remain the uterus for a long time4. The foetus of placentals secretes a hormone that stops the production of any more eggs during this period. Also the foetus’ tissue has genetic material from the father so immunological rejection can occur by coming into contact with the female. However this does not happen as the placenta takes care of it and the process by which is stopped from happening is not completely understood4. The marsupials don’t have to have such a complex mechanism as their neonates are ejected early enough2. Marsupials are able to avoid gestation complications that are encountered by the placentals.

The red Kangaroo of central Australia is an example of a marsupial. After fertilisation the neonate is ejected after thirty three days, after which the neonate which has more developed front legs crawls to the mother’s pouch and attaches to the teats on the mother’s womb to suckle4. The sexual cycle of the mother starts again and another egg descends into the uterus and she mates and the egg is fertilised. Then the egg’s development is stopped for a while as the neonate in the pouch continues to grow and adapts really fast. After one hundred and ninety the neonate has grown and can get out of the pouch to go venture outside the pouch4. Afterwards it continues to venture while returning to the pouch before eventually venturing out completely after two hundred and thirty five days. In case there is drought the fertilised egg in the uterus remains dormant till there’s rain and good pasture. This begins the cycle once again. A female red Kangaroo can have three young dependent with her each at a different stage of development. One active young that suckles and ventures out, one tiny neonate suckling in the pouch and the fertilised dormant egg, awaiting further development4.

Marsupilals give birth to neonates that are not fully developed as compared to the placental which carries its foetus till they are mature. The neonates of the marsupials are prone to all the hazards in the environment as compare the placentals foetus which is born fully mobile and well developed outside the mother’s womb. This has made the placental method of reproduction a success as compared to the marsupials and made them successful “in colonising the whole earth”4.


1. Wikipedia contributors. Mammal [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 8, 11:25 UTC [cited 2006 May 9]. Available from:

2. Wikipedia contributors. Eutheria [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 18:31 UTC [cited 2006 May 9]. Available from:

3. Wikipedia contributors. Marsupial [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 22:57 UTC [cited 2006 May 9]. Available from:

4. Knight, R. BCB Biodiversity chapter 2 Eggs, Pouches and Placentas

(Cited 2006 May)


Biodiversity and Conservation Biology

University of the Western Cape

Private Bag X17 Bellville

TEL: +27825103190




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