Marsupial
Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a relatively undeveloped state and then nurtured within a pouch on their mother's abdomen. Extant marsupials encompass many species, including kangaroos, koalas, opossums, possums, Tasmanian devils, wombats, wallabies, and bandicoots.
Marsupials constitute a clade stemming from the last common ancestor of extant Metatheria, which encompasses all mammals more closely related to marsupials than to placentals. The evolutionary split between placentals and marsupials occurred 125–160 million years ago in the Middle Jurassic–Early Cretaceous period.
Presently, close to 70% of the 334 extant marsupial species are concentrated on the Australian continent, including mainland Australia, Tasmania, New Guinea, and nearby islands. The remaining 30% are distributed across the Americas, primarily in South America, with thirteen species in Central America and a single species, the Virginia opossum, inhabiting North America north of Mexico. Marsupial sizes range from a few grams in the long-tailed planigale, to several tons in the extinct Diprotodon.
The word marsupial comes from marsupium, the technical term for the abdominal pouch. It, in turn, is borrowed from the Latin marsupium and ultimately from the ancient Greek μάρσιππος, meaning 'pouch'.
Anatomy
Marsupials have typical mammalian characteristics, e.g., mammary glands, three middle ear bones, true hair and bone structure. However, striking differences including anatomical features separate them from eutherians.Most female marsupials have a front pouch, which contains multiple nursing teats. Marsupials have other common structural features. Ossified patellae are absent in most modern marsupials and epipubic bones are present. Marsupials also lack a gross communication between the right and left brain hemispheres.
Skull and teeth
Marsupials exhibit distinct cranial features compared to placentals. Generally, their skulls are relatively small and compact. Notably, they possess frontal holes known as foramen lacrimale situated at the front of the orbit. Marsupials have enlarged cheekbones that extend further to the rear, and their lower jaw's angular extension is bent toward the center. The hard palate of marsupials contains more openings than that of placentals.Teeth differ significantly. Most Australian marsupials outside the order Diprotodontia have a varying number of incisors between their upper and lower jaws. Early marsupials had a dental formula of per quadrant, consisting of five or four incisors, one canine, three premolars, and four molars, totaling 50 teeth. While some taxa, like the opossum, retain this original tooth count, others have reduced numbers.
For instance, members of the Macropodidae family, including kangaroos and wallabies, have a dental formula of. Many marsupials typically have between 40 and 50 teeth, more than most placentals. In marsupials, the second set of teeth only grows in at the site of the third premolar and posteriorly; all teeth anterior to this erupt initially as permanent teeth.
Torso
Few general characteristics describe their skeleton. In addition to unique details in the construction of the ankle, epipubic bones are observed projecting forward from the pubic bone of the pelvis. Since these are present in males and pouchless species, it is believed that they originally had no function in reproduction, but served in the muscular approach to the movement of the hind limbs. This could be explained by an original feature of mammals, as these epipubic bones are also found in monotremes. Marsupial reproductive organs differ from placentals. For them, the reproductive tract is doubled. Females have two uteri and two vaginas, and before birth, a birth canal forms between them, the median vagina. In most species, males have a split or double penis lying in front of the scrotum, which is not homologous to the placental scrota.A pouch is present in most species. Many marsupials have a permanent bag, while in others such as the shrew opossum the pouch develops during gestation, where the young are hidden only by skin folds or in the maternal fur. The arrangement of the pouch is variable to allow the offspring to receive maximum protection. Locomotive kangaroos have a pouch opening at the front, while many others that walk or climb on all fours open in the back. Usually, only females have a pouch, but the male water opossum has a pouch that protects his genitalia while swimming or running.
General and convergences
Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the red kangaroo, grows up to in height and in weight. Extinct genera, such as Diprotodon, were significantly larger and heavier. The smallest marsupials are the marsupial mice, which reach only in body length.Some species resemble placentals and are examples of convergent evolution. This convergence is evident in both brain evolution and behaviour. The extinct thylacine strongly resembled the placental wolf, hence one of its nicknames "Tasmanian wolf". The ability to glide evolved in both marsupials and some placentals, which developed independently. Other groups such as the kangaroo, however, do not have clear placental counterparts, though they share similarities in lifestyle and ecological niches with ruminants.
Body temperature
Marsupials, along with monotremes, typically have lower body temperatures than similarly sized placentals, with the averages being for marsupials and for placentals. Some species will bask to conserve energyReproductive system
Marsupials' reproductive systems differ markedly from those of placentals. During embryonic development, a choriovitelline placenta forms in all marsupials. In bandicoots, an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas.Both sexes possess a cloaca, although modified by connecting to a urogenital sac and having a separate anal region in most species. The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.
Males
Most male marsupials, except for macropods and marsupial moles, have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas. The penis is used only during copulation, and is separate from the urinary tract. It curves forward when erect, and when not erect, it is retracted into the body in an S-shaped curve. Neither marsupials nor monotremes possess a baculum. The shape of the glans penis varies among marsupial species.The shape of the urethral grooves of the males' genitalia is used to distinguish between opossum species Monodelphis brevicaudata, M. domestica, and M. americana. The grooves form two channels that form the ventral and dorsal folds of the erectile tissue. Several species of dasyurid marsupials can also be distinguished by their penis morphology. Marsupials' only accessory sex glands are the prostate and bulbourethral glands. Male marsupials have one to three pairs of bulbourethral glands. Ampullae of vas deferens, seminal vesicles or coagulating glands are not present. The prostate is proportionally larger in marsupials than in placentals. During the breeding season, the male tammar wallaby's prostate and bulbourethral gland enlarge. However, the weight of the testes does not vary seasonally.
Females
Female marsupials have two lateral vaginas, which lead to separate uteri, both accessed through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent. Some marsupial species store sperm in the oviduct after mating.Marsupials give birth very early in gestation; after birth, newborns crawl up their mothers' bodies and attach themselves to a teat, which is located on the underside of the mother, either inside a pouch called the marsupium, or externally. Mothers often lick their fur to leave a trail of scent for the newborn to follow to increase their chances of reaching the marsupium. There they remain for several weeks. Offspring eventually leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.
Early development
Gestation differs between marsupials and placentals. Key aspects of the first stages of placental embryo development, such as the inner cell mass and the process of compaction, are not found in marsupials. The cleavage stages of marsupial development vary among groups and aspects of marsupial early development are not yet fully understood.Marsupials have a short gestation period – typically between 12 and 33 days, but as low as 10 days in the case of the stripe-faced dunnart and as long as 38 days for the long-nosed potoroo. The baby is born in a fetal state, equivalent to an 8–12 week human fetus, blind, furless, and small in comparison to placental newborns: sizes range from 4–800g+. A newborn can be categorized in one of three grades of development. The least developed are found in dasyurids, intermediates are found in didelphids and peramelids, and the most developed are macropods. The newborn crawls across its mother's fur to reach the pouch, where it latches onto a teat. It does not emerge for several months, during which time it relies on its mother's milk for essential nutrients, growth factors and immunological defence. Genes expressed in the eutherian placenta needed for the later stages of fetal development are expressed in females in their mammary glands during lactation. After this period, the joey spends increasing periods out of the pouch, feeding and learning survival skills. However, it returns to the pouch to sleep, and if danger threatens, it seeks refuge in its mother's pouch.
An early birth removes a developing marsupial from its mother's body much sooner than in placentals; thus marsupials lack a complex placenta to protect the embryo from its mother's immune system. Though early birth puts the newborn at greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as the fetus cannot compromise the mother in bad seasons. Marsupials are altricial animals, needing intensive care following birth. Newborns lack histologically mature immune tissues and are highly reliant on their mother's immune system for immunological protection.
Newborns front limbs and facial structures are much more developed than the rest of their bodies at birth. This requirement has been argued to have limited the range of locomotor adaptations in marsupials compared to placentals. Marsupials must develop grasping forepaws early, complicating the evolutive transition from these limbs into hooves, wings, or flippers. However, several marsupials do possess atypical forelimb morphologies, such as the hooved forelimbs of the pig-footed bandicoot, suggesting that the range of forelimb specialization is not as limited as assumed.
Joeys stay in the pouch for up to a year or until the next joey arrives. Joeys are unable to regulate their body temperature and rely upon an external heat source. Until the joey is well-furred and old enough to leave the pouch, a pouch temperature of must be constantly maintained.
Joeys are born with "oral shields", soft tissue that reduces the mouth opening to a round hole just large enough to accept the teat. Once inside the mouth, a bulbous swelling on the end of the teat attaches it to the offspring till it has grown large enough to let go. In species without pouches or with rudimentary pouches these are more developed than in forms with well-developed pouches, implying an increased role in ensuring that the young remain attached to the teat.