PLACENTAL AND MARSUPIAL MODES OF REPRODUCTION

The placenta is a temporary organ which is present in females especially during pregnancy (2). This name is given to the mammal that have placenta, that connects the growing embryo within uterus to the mother. Placenta is composed of two parts the genetically and biologically part of the fetus and the other part of the mother. The fetus is implanted in the wall of the uterus where it receives nutrients and oxygen from the blood of the mother and passes out waste. (2)

While Marsupials can regard as non-placental mammal, they reproduce by lying eggs. In some marsupials there is rudimentary placenta that functions on for short period. Its reproduction is different to that of placental mammals; it develops a yolk sack in the womb which delivers nutrients to the embryo. The embryo of marsupials born at an early stage of development, it crawls up to the belly of its mother and attaches itself to a nipple. It stays at the nipple for a period. Then the offspring passes a stage where it can leave its mothers pouch temporarily and returns for warmth and nourishment (1).

Birth in marsupials occurs very fast depending on the egg yolk type, and also the type of reproduction speed up the birth process. It only takes twelve days for marsupial embryo in the reproductive track, whereas in placental mammals’ gestation period takes long, because the offspring will need to be fully developed before birth. It takes nine months for placental mammal to give birth.

During reproduction placental mammals develops a placenta that protect the fetus from the mothers’ immune system, while in marsupials they develop a complex fetus. In placental mammals, if they give birth before times the baby or fetus become prematurely and it will need care the same way the marsupial do to their embryo, carrying it to the belly for a period.

When the placental mammals are pregnant the process of oestrus cycle are likely to happen until the end of pregnancy. Whereas in marsupials the oestrus cycle does not stop it takes place. In marsupial the ovarian inhibition is mediated by lactation or suckling stimulus. These regulatory modifications are 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.

Differences in these two modes of reproductions are the time that the offspring take in the uterus. Also the development of the embryo is different to each other. As in marsupial the embryo is separated from the body of its mother. While in the placental is connected to the mother. Mating system in marsupial is different through out the year. Some species are single and they only come together to mate and this pattern of social behaviour reflect promiscuous mating systems.

In conclusion mode of reproduction differs with type of species and also the size of the species involve to the way in which species reproduce. Species that reproduce by placental must have big body as they have to carry the embryo for couple of months, while those with small bodies do not carry embryo in their belly they reproduce by marsupials process.

Reference:

1. Ashcraft.C.W. Marsupial Evolution And Post Flood Migration.[Internet]cited 2006-May-10] Available from: http://www.nwcreation.net/marsupials.html

2. Opossum Reproduction And Life Cycle [Internet][cited 2006-May-10]Available from: http://www.opossumsocietyus.org/opossum_reproduction_and_life_cycle.htm

3. Wikipedia contributors. Marsupials [Internet]. Wikipedia, the Free Encyclopedia; 2006 May 9, 17:16. PTA [cited 2006 May 10]. Available from: http://en.wikipedia.org/wiki/Marsupials

4. Wikipedia contributors. Placenta [Internet]. Wikipedia, the Free Encyclopedia; 2006 May 7, 18:21. PTA [cited 2006 May 10]. Available from: http://en.wikipedia.org/wiki/Placenta

5. Wikipedia contributors. Umbilical cord [Internet]. Wikipedia, the Free Encyclopedia; 2006 May 8, 18:02. PTA [cited 2006 May 10]. Available from: http://en.wikipedia.org/wiki/Umbilical_cord

EVIDENCE WHICH SHOW THAT DINOSAURS WERE WARM-BLOODED ANIMALS.

Dinosaurs fossils were found about 230 million years ago. It was suggested that they were vertebrate animals. They dominate the terrestrial environment for 160 years before extinction. This left many Paleothropologist with suggestion that relate modern bird to be the direct descendent of Dinosaurs. (2)


Most of the species found on land was suggested to be warm-blooded this include birds and mammals.paleothropologist also suggest that because of the body and characteristics that the dinosaurs have, can regarded it as warm-blooded animals. This because most mammal or birds with large body, they have high metabolic rate that can regulate their body temperature.


In warm-blooded animal it was suggested that their body can retain heat for long period and also the chest cavity can hold huge hearts that increase metabolism. The bone structure that the dinosaurs have also suggested that it play a part on heat retation, the parallel rows of plates on dinosaurs has been interpreted as additional temperature –control mechanism. This plate is made of bone-like sponge consists of many blood vessels that can absorb heat from the environment or disperse excess heat. (1)


The characteristics that dinosaurs have for living also indicate warm blooded animal, its character of migrating, instead of hibernation during cold periods shows that they can adapt any environmental change they encounter. Which is impossible for cold blooded animals? (2)


The anatomical analyses of dinosaurs suggest that they were active animal, moving fast which increase endothermic metabolism rate. Also the diet they have also increase the metabolic rate, this because the herbivorous dinosaurs require hundreds kilos of vegetation a day to sustain their enormous bulk and that they had a unique endothermic metabolism fuelled by the heat given off by non-stop digestion. The metabolism rate was the insulator of heat in dinosaurs because they do not have feathers and hairy skin; its skin is consisting of hides with scaly and bony bumps that do not insulate. (3)


Some paleothropologist suggest that dinosaurs maybe in between the warm blooded or cold blooded animal. But with the DNA test of the bones it shows that dinosaurs are warm blooded. “This because warm blooded animal grow fast than cold blooded ,this was proven by scientist Jack Horner through the bones of a tiny baby dinosaurs found in hadrosaur nests. But as scientist has conflicts other suggest that the heart of herbivores dinosaurs consist of four chambers instead of three which is common to warm blooded animal.”(2)

Evidence studied can reveal that dinosaurs were warm blooded animal, this because they can maintain their body temperature by increasing the metabolic process through eating lots of vegetation that will need non stop digestion. And also by moving fast which increase the endothermic metabolism. It was suggested that dinosaurs have large body that can regulate temperature, because they have high metabolic rate.


In general warm blooded animal maintain their body temperature at their constant level, this regulation of metabolic rate helps them to adapt any environment as they can increase or reduce their depending the surrounding environment.

Reference:


1. Were Dinosaurs Warm-Blooded .1999. [Internet] [Cited 2006-May-09] Available from: http://www.dinoruss.com/de_4/5c51d90.htm


2. Wikipedia contributors. Dinosaurs [internet] Wikipedia, The Free Encyclopedia [cited 2006-May-09] Available from: http://en.wikipedia.org/wiki/Dinosaurs


3. Dinosaurs [internet] [cited 2006-May-09] Available from: http://teacher.scholastic.com/researchtools/articlearchives/dinos/evolut.htm,