Marsupial - biology.

Marsupial - biology. I INTRODUCTION Marsupial, common name for a group of mammals typically distinguished by a pouch in the female, called a marsupium, where offspring are carried. The kangaroo, koala, wombat, and opossum are well-known marsupials, as are other less-familiar animals, such as the numbat, bandicoot, ringtail possum, and thylacine. These unusual creatures range in size from the tiny, mouselike ningaui, which measures about 5 cm (about 2 in) long and weighs a mere 2.8 g (0.1 oz), to the gray and red kangaroos, which can reach lengths of 2.5 m (8 ft), stand up to 1.8 m (6 ft) tall, and weigh 90 kg (200 lb). Depending on the classification system used, scientists have identified between 260 and 280 species of marsupials, which are further grouped into about 16 families. Most marsupial species are found in Australia and New Zealand, although many live in parts of South America, and one species--the Virginia opossum--is found in some regions of North America. II DISTINGUISHING CHARACTERISTICS All marsupials share reproductive traits that distinguish them from other mammals. For example, the uterus, the organ in which the female carries her young, is divided into left and right compartments. The female has two vaginas, or openings, one leading to each compartment. The male penis usually is two-pronged, which enables the male to place one prong in each of the female's vaginas during mating. One of the more significant reproductive traits is the lack of a true placenta in pregnant marsupials. The placenta is an organ that develops in most other mammals, called placentals, to provide nutrients and remove wastes from the developing embryo. Instead of a fully developed placenta, the pregnant marsupial forms a type of yolk sac in her uterus. The embryo absorbs nutrients from this yolk sac for four to five weeks, after which it emerges from the birth canal in an extremely undeveloped state. Immediately after birth, the tiny offspring, which weighs no more than 1 g (0.04 oz), crawls along its mother's body toward a nipple, which in most marsupials is located inside an abdominal pouch or within folds of skin. The newborn attaches firmly to a nipple to suckle its mother's milk. The nipple expands inside the mouth of the young, locking the offspring in place for several weeks until it is more fully developed and can move about on its own. The offspring may remain with the mother until it is more than a year old, climbing in and out of her pouch for nourishment or safety. The number of young vary among marsupial species, with, for example, kangaroos generally giving birth to only one baby at a time and opossums bearing as many as 12 young at a time. Some marsupials that give birth to one young at a time, notably kangaroos, are able to mate again as soon as offspring emerge from the uterus. After mating, the fertilized egg begins developing in the uterus, but stops at the blastocyst stage, when the embryo is a simple, hollow ball of cells. The blastocyst remains in the female's uterus while the newborn offspring in the pouch continues to grow. About six or seven months later, when the pouch young leaves the pouch, the blastocyst resumes its development. A new offspring emerges from the uterus and moves into the vacated pouch. This reproductive feature, called embryonic diapause, enables marsupials to produce offspring continuously, even if food supplies or environmental conditions cause the death of the pouch young. Several other features distinguish marsupials from other mammals. For example, the marsupial brain is smaller than the brain of placental mammals of similar size. In addition, marsupial brains lack a corpus callosum, a brain structure found in placental mammals that permits nerve communication between the left and right hemispheres of the brain. Marsupials have between 40 and 50 teeth; humans, in comparison, have only 32 teeth and elephants a mere 6, including their two tusks. Also different from other mammals are the marsupium bones, present in both male and female marsupials. Not found in other mammals, these bones project from the pelvic bones and may serve to strengthen the wall of the abdomen. III FEEDING AND SOCIAL BEHAVIOR Smell and hearing are important senses to marsupials because these creatures are primarily nocturnal, or active only at night. Depending on species and habitat, marsupials may be primarily plant eaters, insect eaters, or meat eaters. Some marsupials are omnivores, eating almost anything. Most marsupials are solitary animals. However, kangaroos and wallabies may be found in groups numbering from two to about ten members, depending on the size of the animals, their habitat, and other factors. The whiptail wallaby gathers into large mobs of up to 50 individuals. To mark their territory or communicate their desire to mate, marsupials primarily use scent from their skin glands, urine, or feces. Some arboreal (tree-dwelling) species also communicate vocally. Marsupials do not form male-female bonds that extend beyond the time necessary for mating, and the female cares exclusively for the young. IV EVOLUTION OF MARSUPIALS Scientists have long been interested in the evolutionary history of marsupials. A fossil found in China is thought to be an early ancestor of marsupials and dates from 125 million years ago. Called Sinodelphys, it was about the size of a chipmunk and climbed trees. This primitive marsupial shares many features with living marsupials and suggests that marsupials may have originated in Asia, spread to North America, and later migrated to present-day South America. This migration eventually took them to Antarctica and Australia, which were united on the ancient landmass Gondwanaland. In 1982 scientists identified the only fossil of a land-dwelling mammal ever found in Antarctica as a marsupial that lived there about 40 million years ago. This discovery confirmed the migration of marsupials from South America, where marsupial fossils date from 65 million years ago, to Antarctica, and to their final destination of Australia, where marsupial fossils date from about 25 million years ago. Marsupials have evolved into a great variety of creatures suited to a wide range of habitats. Some occupy arboreal habitats, while others burrow underground. Still others live in deep forests, and some inhabit open plains. Through a process called convergent evolution, the adaptation of marsupials has paralleled the adaptation of placental mammals to similar habitats, and these two distinct groups of mammals have developed remarkably similar physical features. For example, placental moles live underground and have tubular bodies and short legs with curved claws for digging; marsupial moles have these same features. Placental mice and rats have small bodies with pointed snouts and sharp teeth and claws, physical characteristics also seen in marsupial mice and rats. Unfortunately, similarities with placental mammals have sometimes caused problems for marsupials. When placental animals are introduced into areas where only marsupials are found, the species compete with each other for resources such as food, water, and habitat. In some cases the marsupials are displaced. In some instances, introduced placental mammals prey extensively on native marsupial species, putting them at risk for extinction. For example, foxes, dogs, and domestic cats have threatened some kangaroo species in Australia. A notable exception to this pattern is the Virginia opossum found in North America, which is thriving despite constant competition with a variety of placental mammals. Scientific classification: Marsupials make up the scientific order Marsupialia. The Virginia opossum is classified as Didelphis virginiana and the whiptail wallaby as Macropus parryi. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.