Oge Otiji
Bio 1108-04
Monotremes, Marsupials, and Placental mammals are the 3 major groups of modern mammals. These mammals have a few things in common, for example, they are all warm blooded vertebrates with a covering of fur, skin or hair. They all have four-chambered hearts. They also have full circulatory, respiratory and digestive systems. Their limbs are oriented vertically and they produce sound via the larynx. They also feed on their young mother’s milk. Although they share many similarities there are a few characteristics that differentiate them. The major difference between these three mammals are their gestation and lactation methods. Monotremes are mammals that lay egg unlike the marsupials and placental mammals that give birth to live young offspring. Monotremes have one opening for excretion and reproduction called the cloaca. They fertilize internally after the sperm of the male enters the cloaca of the female. They hold the eggs internally for several weeks, providing nutrients and when they lay them, they cover them up like birds. After a gestation period of 23 days, they usually lay 1 egg into a temporary pouch formed by abdominal muscles, developed only during breeding season. The egg hatches in 9 days by tearing the shell with a temporary egg tooth on its snout. Monotremes also lack nipple to feed their young ones, instead they have glands that secrete milk on their skin and the younger ones suck it off their bodies. The young then continues suckling on their mother's milk, for about 3 or 4 months. They stay attached to the mothers for a long time due to short gestation period. To recompense for their lack of nipples, their milk contains a highly expressed antibacterial protein not found in any other mammal. There are only two types of monotremes in the world, the platypus and the echidna. Marsupials on the other hand are pouched mammal that give birth to live offspring. Their offspring are usually underdeveloped because they are born early. They are known to have a very short gestational period due to having a yolk-like placenta in the mother marsupial. Pregnancy for marsupials usually last for 4 to 5 weeks unlike the placental mammals and the monotremes. This puts the newborn marsupial at risk but it also reduces dangers that come with long pregnancies to the mother. The female has two vaginas, both of which open externally through one orifice but lead to different compartments within the uterus. The males only pass their sperm. The female develops a yolk sac in her womb which delivers nutrients to the embryo. The females also develop a pouch where their young grow and develop, this pouch is also known as a marsupium and it is just a flap of skin covering the nipples. They give birth very early and the embryo climbs from the mother’s birth canal to the nipples, they have strong limbs to help with this, it continues to develop for weeks or sometimes months. Unlike the monotreme, the marsupial poses nipples which is used for lactation. Unlike the other mammals, Marsupials are known to be able to stop or pause their pregnancies, this process is known as a diapause. They are able to have multiple kids at the same time period and when they don’t want to have more they pause the pregnancy. They also poses teeth unlike the monotremes. Marsupial species include Kangaroos, wallabies and wombats which are usually a native of Australia. Unlike the monotreme or marsupials, placental mammals give birth to live young offspring, which are nourished before birth in the mother’s uterus with the help of the placenta thus the name placental mammals. Although the marsupial also has a placenta, it is short-lived and does not make much of a contribution to fetal nourishment as it does in placental mammals. The placental mammals have a much longer gestation period than monotremes and marsupial, which contributes to them being born in a more advanced state than non-placental mammals. The gestation period could last from 3 weeks in the house mice to 22 months in elephants. The longer gestation period is due to the placenta, it allows nutrients to travel from the mother’s system to the embryos, and for waste products to leave the embryo’s system so they can be disposed of by the mothers. The placenta is composed of several layers of material. It is very richly supplied with blood vessels and acts as an immigration barrier between the mother's system and the developing embryo's. Once born, like the marsupial but unlike the monotreme, the placental mammal possess nipples used to feed the young ones. They give birth to big babies, so they have big nipples to feed the baby. And they are protected by one or more of their parents until they are able to fend for themselves. Carrying and giving birth to a large fetus is risky for the mother, it requires her to eat more food. These are the major differences that set each mammal apart from the other. Apart from the major differences, there are also some minor differences, for example: monotremes are warm blooded and have a fast metabolism. Placental mammals have a single vagina, while the marsupials own 2. Marsupials and placental mammals are able to adjust to different environments whether warm or cold, open or forested. Each mammal has characteristics that make them unique and fascinating.