What Bones Power a Kangaroo's Jumping?

Native to Australia, kangaroos are herbivores who sometimes approach 6 feet in height and 200 pounds in weight. Some species are social, forming mobs of up to 20 individuals. During the breeding season, males often “box” with each other by kicking with their powerful rear legs. As marsupials, the young are born early in the developmental process; they complete development inside a pouch on their mother’s belly. Eastern grey kangaroo adults regularly hop about 25 feet per bound and can leap higher than 6 feet. When necessary, they can travel up to 35 miles per hour; over long distances, they average about 15 miles per hour.

Kangaroos use the long, strong bones in their legs and feet as a scaffolding on which their muscles and tendons attach. The legs of kangaroos are similar to those of other mammals; the primary difference is the relative lengths of the bones. Their thighbones, or femurs, are short and thick; however, their tibia and fibula -- the paired shinbones -- are very long. Kangaroos have long and narrow bones of the toes and feet, called phalanges and metatarsals, respectively. The fourth toe is the longest and most important, as it transmits the force from the leg to the ground when moving.

Kangaroos adapted strong bones to power their jumps and developed joints that can handle landing. Every jump or hop kangaroos make imparts a great deal of force on his or her bones and joints. Most other animals would suffer damaged knees if they lived life constantly hopping about as kangaroos do. Kangaroos have coped by evolving thick pads of spongy cartilage in their knees. These pads work as shock absorbers; they help to minimize wear and tear on the kangaroos’ knees. Because of the structure of their ankle joints, kangaroos cannot rotate their ankles sideways -- this also help them to withstand the impacts from jumping. At the rear of their feet, structures termed tarsi act as shock absorbers.

Because of the combination of short femurs and extremely long bones in the lower legs, kangaroos face different locomotor challenges than other animals do. Kangaroos have two general modes of moving. When walking at slow speed, they use their tails and forelegs to balance as they swing their back legs forward. Once their rear legs gain purchase, they swing their arms and tail forward. By contrast, hopping provides an easy way for kangaroos to move at high speed. Kangaroos do so by repeatedly storing and releasing energy, stretching and relaxing the tendons at the rear of their legs, like springs. Up to about 20 miles per hour, it is actually more energetically efficient for kangaroos to move fast rather than slow. When hopping, kangaroos use their arms and tails for balance.