Rat-kangaroos are four-legged marsupial mammals that are smaller than most cats. Marsupial mammals are different from most familiar mammals such as cats, dogs, and horses, which are eutherian (yoo-THEER-ee-an) mammals, meaning they use a placenta in reproduction. A placenta is an organ that grows in the mother's uterus and lets the mother and developing baby share food and oxygen. Marsupial mammals do not use a well-developed placenta. Because of this, they give birth to tiny young that are not physically mature enough to survive on their own. Instead, the young are carried for several months after birth in their mother's pouch, or they are attached to the mother's teats, or nipples, on her underbelly. While they are carried this way, they continue to grow until they have matured enough to fend for themselves.
Rat-kangaroos usually have a head and body length that ranges from about 6 to 16 inches (15 to 42 centimeters). The tails of rat-kangaroos can be nearly as long as their bodies, and range in length from about 5 to 15 inches (12 to 39 centimeters). Rat-kangaroos range in weight from about 0.8 to 8 pounds
(0.4 to 3.5 kilograms). Male and female rat-kangaroos are usually about the same size.
Rat-kangaroos have heads that are long and are usually tapered, with small ears that are either round or slightly pointed. Like all kangaroos, their hind legs are longer and stronger than their front legs. This is because rat-kangaroos use their hind legs to move by hopping. Rat-kangaroos have four toes on each of their back feet, but the second and third toes actually grow together although the claws remain separate. Each of the front feet has five toes, each with a claw. Their second, third, and fourth toes on their front paws are longer than their other front toes, and these longer claws help them dig for food.
The fur of the rat-kangaroo ranges in color from dark brown to gray or light brown. The fur is lighter on the underbelly than on the rest of the body. These animals can use their tails, which usually have fur on them, to curl around objects and hold onto them. This type of tail that can be used to grasp is called a prehensile tail. Female rat-kangaroos have a pouch containing four nipples.