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Koalas have developed the ability to eat the leaves of gum trees Eucalypts which are not only low in nutrition, but are packed with toxins. This ability to survive on a very widespread food source source that no other mammal can eat, and that is highly adapted to the arid climates of many parts of Australia, frees them from the pressures of competition from other mammals. Gum trees have adapted to survive drought better than most other trees, so koalas have a constant food source that also provides their water needs in dry spells when ground-dwelling animals have much more difficulty finding enough water, either free or in vegetation, to survive.

Reproductive anatomy of marsupials The female reproductive structures of the 3 groups of living mammals are unequivocally distinct. When the foetus of a Tammar wallaby 4 days before birth is compared with a human embryo at 5 weeks of gestation, the arrangement of kidney ducts, genital ducts and gonads are the same. Kidney ducts, ureters, and both genital ducts combine into the urogenital sinus, a common tube, on its dorsal side and the future bladder on its ventral side.

The ureters migrate to the ventral side to enter the bladder, in both groups, later in development, the genital ducts remaining dorsal. The ureter migration occurs in placental and marsupials, but not in monotremes. This is believed to be an adaptation for more efficient urine storage in the bladder. In monotremes, the urine discharges into the top of the sinus, passing across the sinus to enter the bladder. The exact mechanism for the this is not known.

The ureters migrate outside and below the genital ducts in placentals, including humans, but in marsupials the ureters migrate inside and above the genital ducts. In the ancestral mammals, it is believed the initial adaptation was associated mostly with excretion, the route taken to the bladder by the ureters distinguishes the placentals from the marsupials, with profound consequences for reproduction.

The most obvious difference between placentals is the female reproductive tract. In marsupials there are 2 full sets of structures, 2 fallopian tubes, 2 uteruses, 2 cervixes and 2 vaginas. The union of the 2 sets of reproductive structures in marsupials is prevented by the path of the ureters that pass between them, which results in 2 lateral vaginas, both of which arise from the posterior of the common urogenital sinus.

The 2 vaginas loop back to the midline above the ureters, becoming partially fused. At the onset of birth in most marsupials, a canal forms that passes from the lateral vaginas directly to the urogenital sinus, between the 2 ureters, through which the very small, immature baby passes. This pseudovaginal canal, or birth canal, reforms at each birth in almost all marsupials, the few exceptions include Macropus, the kangaroo and Tarsipes rostratus the honey possum. In these species it remains after the first birth and is called the median vagina.

The 2 lateral vaginas receive sperm and the baby passes through the median vagina at birth. It has long been believed that the arrangement of the birth canal and the small size of the 2 uteri are the reasons marsupial young are so small at birth. This is yet to be proven. In monotremes the yolk content of the egg is greatly reduced compared to that of reptiles, to the point where it is not sufficient to maintain development.

Nutrients secreted by the endometrial gland are absorbed, probably being absorbed by the yolk sac, as occurs in the pre-attachment marsupial. In the monotremes there are structural similarities to the progestational condition as seen in viviparous mammals in the luteal phase of the oestrus cycle.

The allantois enlarges in the egg of Tachyglossus after the egg is laid, makes contact with the chorion then becomes highly vascular See Griffiths, It has been shown to cover half the inner surface of the shell, the vascularised yolk sac covering the remainder of the shell Semon, After the egg leaves the uterus of Tachyglossus there is no need for a nutritional route but the entire surface becomes vascularised as it essential for respiratory exchange, probably the reason for the vascularisation.

Monotremes were excluded from the character 'placenta' in the development of characters for a cladistic analysis Marshall, , which increases the apparent separation between the 2 groups. It has been suggested Gregory, there is a possibility that there is not a lot of difference between organogenesis continuing for 10 days in the uterus or in an externally held egg, as the products are so similar.

The definition of oviparity is not consistent with the egg accumulating so much nutrient material after the shell has been laid down. It has been suggested that the evolution of the eutherian villous allantoic placenta allowed a greatly increased exchange to take place thereby allowing the retention of the fetus during its major phase of growth Luckett, It has been suggested that as the marsupials didn't evolve a trophoblast that was able to mask histocompatibility antigens on its surface there could be only a brief attachment period in the gestation of marsupials, intolerance of the mother's immune system for the fetal tissues would cause any attachment that was longer to fail Moors, It was suggested that this is the reason for the very short-lived chorioallentoic placenta attachment, that is very intimate in Peramelidae Tyndale-Biscoe, The development of the thesis that a trophoblast layer differentiated that was able to mask the histocompatibility antigens on its surface was a major adaptation enabling gestation to be greatly lengthened in eutherian mammals, and that this was the main dichotomy with marsupials Lillegraven, ; Cox, There have been 2 attempts to test the hypothesis that the trophoblast of marsupials does not have the ability to mask histocompatibility antigens failed to support the hypothesis.

It has been acknowledged Lillegraven, but he suggests that as a result of the species used, Macropus eugenii being derived from an island population the animals may have been closely related so would not provide a good test of the hypothesis. Birth size The largest living marsupials are Macropus giganteus, the eastern grey kangaroo, and Macropus rufus, the red kangaroo. Females of these large kangaroos weigh about 28 kg, but at birth their single young weigh about mg, 0.

Among the marsupials, the birth weight is about mg, some of the smaller dasyurids weigh as little as 10 mg, though the smallest newborn marsupial, the honey possum, is about 4 mg. As with the placental mammals, where the young are much more advanced at birth, the young marsupials control the onset of their own birth.

It has been suggested that the small size is the result of the short gestation of marsupials. Some marsupials do have a short gestation period, in some cases less than 2 weeks, but others have gestation periods that are longer than in placentals of similar size. The main difference between the reproductive strategies of marsupials and placentals is the advanced stage of development at birth in placentals, most of the development having taken place in utero, which can occur because of the well-developed placenta.

As a result, the young of many placentals are 'ready to roll' a very short time after birth, especially in the case of prey species, where they need to be ready to escape predators as soon as possible, or keep up with the herd. In marsupials, most development takes place after birth. The females of both marsupials and placentals make investments in their young, the placental before birth, the marsupial after birth. In marsupials, the gestation is often short, but the lactation is long and complex, requiring large changes in the quantity and composition of the milk before the young can reach the stage of development attained by placental newborns.

This difference has been exploited by kangaroos, that inhabit some of the driest parts of the driest vegetated continent, with the most erratic climate, the land of 'drought and flooding rain'. When times are bad enough for the milk supply to dry up, they lose any young in the pouch, or out of the pouch but still depending on milk, or the pouch young leaves the pouch, the young that are still at an early stage of development in the uterus that has stopped growing, embryo diapause , resumes development.

Because of their method of reproduction, the females of kangaroo species can have one young out of the pouch but suckling, another still-developing young attached to the other teat and the a third still in the uterus that stops development, embryonic diapause, until the more advanced of its larger siblings is weaned, when it resumes development.

Each teat produces milk with the composition and volume suitable for the stage of development of the young that feeds from it. When the 'joey' is weaned, the teat it has fed from since its birth 'resets', producing milk with the appropriate composition, and of the appropriate quantity, for the developing young in the uterus to attach to after it is born.

This allows the kangaroo population to quickly rebound after it has been reduced by severe conditions, such as a prolonged drought, during which reproduction ceases, but with an embryo that resumes development when required. Sexual Differentiation - placentals vs marsupials The external genitalia of marsupials and placentals are superficially similar, but there are differences in development.

Sexual differentiation occurs during gestation in the foetus of placentals, at which point the external appearance of both sexes is the same, hence the term 'the indifferent stage'. In male foetuses, the production of testosterone by the developing testes causes the genital tubercle to develop into a penis, and behind it, a scrotum. The same structures develop into a clitoris and the outer lips of the vulva respectively in female foetuses, where testosterone is normally absent.

The nipples and mammary glands are formed in both sexes and retained throughout life. The processes that occur after the indifferent stage is controlled by the 'sex determining region', the SRY gene, on the Y chromosome. When this gene is present, as in normal males, it causes the testes to develop from the gonads, the testosterone produced by the testes taking on the orchestration of the changes that occur to produce the rest of the male reproductive structures.

Marsupial mammals are one of the three major mammalian groups with about almost extant species. Predominantly, marsupials are found in Australia; they are also found in the American continent. Marsupials give birth to an undeveloped young called Joey, following a small gestation period. The Joey comes out of mother, and its development takes place inside an external body pouch that has milk-secreting mammary glands.

Joeys do not have hairs on their body when they are newly born. In addition, Joeys are tiny as the size of a jellybean, and they cannot open their eyes; in other words, they are blind at birth. But, the completed development has to take place inside the pouch. However, during the short gestation period, there is a placenta between fetus and mother, but it is a very simple structure.

One of the noticeable absences in marsupials is the lack of corpus callosum or the bridge of neurons between the left and right hemispheres of the brain. Kangaroo, wallaby , and Tasmanian devil are few of the most well-known marsupials. What are the Similarities Between Placental and Marsupial? Placental and marsupial are two of the three groups of mammals. Also, both are vertebrates. Furthermore, both are warm-blooded animals too.

Besides, they have four-chambered hearts. What is the Difference Between Placental and Marsupial? Placental and marsupial are two groups of mammals. Placental mammals have a placenta to nourish the fetus while marsupials have a simple placenta that lasts for a short time period.

So, this is a difference between placental and marsupial. However, the key difference between placental and marsupial is that the placental mammals give birth to developed young ones while marsupial mammals give birth to undeveloped young ones. Hence, they keep their young ones in a pouch and nourish them till they become mature. Moreover, placental mammals are more diversified and inhabit a wide range of habitats while marsupial mammals are less diversified and are predominantly found in Australia.

So, we can consider this also as a difference between placental and marsupial. The below infographic on the difference between placental and marsupial provides a detailed comparison. Summary — Placental vs Marsupial Among the three groups of mammals, placentals and marsupials are two common groups.

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