Kangaroo

The ancestors of kangaroos differentiated from small, tree-dwelling, possumlike marsupials fifty million years before the present. About thirty million years ago, they came down from the trees of the rain forest that covered most of Australia at the time. Around five million years ago, the continent had started to dry out, and species closely related to modern kangaroos appeared. When the Aborigines first came to Australia forty thousand years ago, the continent included some larger marsupials. The latter apparently were not as fast and mobile as kangaroos, were easier prey, were overhunted, and as a result became extinct. The kangaroos were smaller and swifter, permitting them to survive. The six living species of true, large kangaroos differ in their habitats (temperate woodlands, tropical grasslands, arid outback) and size; the red kangaroo is the largest, with some males weighing two hundred pounds and standing six feet tall; the smallest, the black wallaroo, can weigh sixty pounds. Smaller relatives of kangaroos include wallabies (up to sixty pounds), pademelons (up to fifteen pounds), and rat kangaroos (less than one pound). While kangaroos are only found in Australia, some smaller relatives are also found in New Guinea, which was contiguous with Australia in the distant past.

Physical Characteristics of Kangaroos
Kangaroos stand on large rear legs, using their long tail for added support. They have small front legs, with handlike paws that lack an opposable thumb. Hopping is their most unusual characteristic. Besides kangaroos and their relatives, no vertebrate bigger than ten pounds hops. At slow speeds, kangaroos walk awkwardly and inefficiently, using their front legs and tail. However, at speeds over fifteen miles per hour, they hop upright in a graceful motion that can be more energetically efficient than running by quadrupeds, whose energy use is proportional to their speed. Kangaroos increase their speed by lengthening their stride, while keeping their hop frequency constant, at little increased energy expenditure. They propel themselves by virtue of highly elastic legs which move in unison and use their long tails to provide balance. As marsupials, kangaroos nurse their young (called joeys) in a pouch. Female kangaroos, half the size of males, have one-month gestations, which can be interrupted if a young is still suckling in the pouch or under adverse nutritional conditions. In these cases, the embryo goes into diapause, a form of "suspended animation", until hormonal signals permit development to resume. The newborn is highly immature, pink and naked, resembles a slug, and weighs less than 0.03 ounces. Using its front legs and a good sense of smell, it crawls fromthe birth canal into the pouch and attaches itself to one of four teats of the mammary gland. Over the next three months, it remains permanently attached to that teat and becomes fully developed. Depending on the species, joeys leave the pouch for the first time at six to ten months, permanently leave the pouch at eight to eleven months, and are weaned at eleven to eighteen months. Females are sexually mature at eighteen months to two years, although some males do not become so until they are four years old. Kangaroos are herbivores, and all six species are grazing animals. Their teeth are suited to grasses rather than shrubs and trees. They are also very efficient in their use of water, making them suitable for the arid regions of Australia. Some species consume less than 10 percent the water sheep do under the same conditions. When temperatures are moderate, they can get all of their water from the plants that they eat. They are inactive in the heat of the day and cool themselves by panting, sweating, and licking; the latter refers to the fact that they cover their front legs with saliva, which by evaporation cools not only their extremities but also their bodies via a dense network of blood vessels close to the surface. Kangaroos are among the most heat-tolerant of mammals. In addition, they have large, padded feet that compact the soil less than domesticated livestock.

Future of Kangaroos
Totaling over twenty million, the six species of large kangaroos are not presently endangered. This is in contrast with the risk to survival faced bysome wallabies and smaller marsupials that are preyed upon by introduced wild animals, such as foxes, or by feral cats. The habitats of most kangaroos have been reduced by human activities, housing, industry, and agriculture, although the range of some of the less arid-tolerant species (eastern gray and western gray) has been increased when water is provided for livestock in remote regions. In addition, areas where sheep are protected from dingoes, as with the patrolled fence that stretches across Australia, have increased numbers of kangaroos. While they do compete with sheep and cattle for food and water, the extent of competition is limited, except during drought. Farming kangaroos for meat and hides remains a possibility but has not been developed. Some extensive preserves would be desirable not only to display these large animals in their natural environment but also to conserve their smaller, endangered relatives.