Macropods are marsupials belonging to the family Macropodidae, which includes kangaroos, wallabies, tree-kangaroos, pademelons, and several others. Before European settlement, there were about 53 species of Macropods. Six species have since become extinct. Another 11 species have been greatly reduced in numbers. Other species (e.g. Simosthenurus, Propleopus, Macropus titan) went extinct after the Australian Aborigines arrived and before Europeans arrived.
Like the eutherian ruminants of the northern hemisphere (sheep, cattle, and so on), macropods have specialised digestive systems that use a high concentration of bacteria, protozoans, and fungi in the first chamber of a complex stomach to digest plant material. The details of organisation are quite different, but the end result is somewhat similar.
Macropods vary in size considerably but most have very large hind legs and a long, powerfully muscled tail. The term macropod comes from the Greek for "long foot" and is appropriate: most have a very long, narrow hind foot with a distinctive arrangement of toes: the fourth toe is very large and strong, the fifth toe moderately so, the second and third are fused and the first toe is usually missing. The short front legs have five separate digits. Some macropods have 7 carpal bones instead of the usual 8 in mammals All have relatively small heads and most have large ears, except for tree-kangaroos, which must move quickly between tight branches. The young are born very small and the pouch opens forward.
The unusual development of the hind legs is optimised for economical long distance travel at fairly high speed. The greatly elongated feet provide enormous leverage for the strong legs. But there is more to the famous kangaroo hop: kangaroos and wallabies have a unique ability to store elastic strain energy in their tendons. In consequence, most of the energy required for each hop is provided "free" by the spring action of the tendons (rather than by muscular effort). The main limitation on a macropod's ability to leap is not the strength of the muscles in the hindquarters. It is the ability of the joints and tendons to withstand the strain of hopping.
In addition, there is a linkage between the hopping action and breathing. As the feet leave the ground, air is expelled from the lungs by what amounts to an internal piston; bringing the feet forward ready for landing fills the lungs again, providing further energy efficiency. Studies of kangaroos and wallabies have demonstrated that, beyond the minimum energy expenditure required to hop at all, increased speed requires very little extra effort (much less than the same speed increase in, say, a horse, a dog, or a human), and also that little extra energy is required to carry extra weight — something that is of obvious importance to females carrying large pouch young.
The ability of larger macropods to survive on poor-quality, low-energy feed, and to travel long distances at high speed without great energy expenditure (to reach fresh food supplies or waterholes, and to escape predators) has been crucial to their evolutionary success on a continent that, because of soil fertility and low, unpredictable average rainfall, offers only very limited primary plant productivity.
Gestation in macropods lasts about a month, being slightly longer in the largest species. Typically, only a single young is born, weighing less than a gram at birth. They soon attach themselves to one of four teats inside the mother's pouch. The young leave the pouch after 5-11 months, and are weaned after a further 2-6 months. Macropods reach sexual maturity at 1-3 years of age, depending on species.
There are two subfamilies in the Macropodidae family: the Sthenurinae was highly successful in the Pleistocene but is now represented by just a single species, and a vulnerable one at that, the Banded Hare-wallaby; the remainder, about 60 species, makes up the subfamily Macropodinae.