The familiar hopping gait of kangaroos probably evolved from the bounding gait that has been retained by the diminutive and primitive musky rat-kangaroo. The hindlimbs became increasingly larger than the forelimbs and the hindfoot became longer, providing a very effective means for fast. hopping locomotion. At equivalent speed, and making allowance for the differences in weight, hopping kangaroo uses less energy than a running dog or horse.
But such specialization is not without cost – a typical kangaroo is unable to walk. When moving slowly, it raises the hindquarters on a tripod formed by the forelimbs and the tail pressed down to the ground, then swings both hindlimbs forward and always together. Terrestrial kangaroos cannot move each hindleg independently while supporting the body, although they can and do kick them alternately when swimming.
Tree kangaroos, which have undergone a secondary shortening of the hindfeet, can also move their hindlegs alternately when walking along a branch. In typical kangaroos, the tail is not very flexible but is moved up and down to assist with balance during the hopping gait and acts as a fifth limb to support the body when moving slowly. In the more primitive rat-kangaroos the tail is moderately prehensile in the vertical plane, and is used to carry bundles of nesting material.
The ability to glide has arisen independently in three families of marsupials: the Pseudocheiridae, Petauridae, and Acrobatidae. In each case, this involves a membrane of skin between the forelimbs and hindlimbs, which, when the legs are extended, expands into a rectangular, kite-like airfoil.
Leaping from a high tree, a glider can volplane quite long distances, steering itself by altering the tension of the membrane on either side, balancing with the outstretched tail, and finally orienting the body vertically to land on the trunk of another tree with all four feet. No other gliding mammal has anything comparable to the tail of the feathertail glider: each side bears a thin row of stiff, closely packed hairs, all of the same length, forming a structure very similar to the vane of a feather.
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Although it seems that most marsupials are able to swim when necessary, only the yapok can be regarded as truly aquatic. With alternate strokes of its webbed hindfeet, this Central American species swims to the bottom of a pond or stream with its eyes closed, feeling with its long, spatulate fingers for living prey, which it grasps in its mouth and takes to the shore to be eaten. The rear-opening pouch of a female yapok is closed by a strong sphincter muscle and sealed with water-repellent secretions when the animal is swimming.
The most extreme locomotory specialization of any marsupial is seen in the small, sausage-shaped marsupial mole. It is blind, lacks external ears, and has a horny shield over the snout and surrounding the nostrils. The limbs are short, with very strong bones and powerful musculature, and the spade like forefeet have two immense, triangular claws on the third and fourth digits, with smaller claws on the other digits.
The feathertail glider, the honey possum and pygmy-possums, genus Cercartetus, are very small marsupials that climb by gripping with the expanded tips of their fingers and toes. Except on the conjoined second and third toes of the hindfoot, the claws are reduced and nail-like, lying above the tips of the digits. The pads on the fingers and toes of the feathertail glider are microscopically grooved, like those of geckos, enabling them to cling to a smooth surface such as a vertical sheet of glass and even to hold themselves, albeit briefly, to the underside of a horizontal sheet.
