Kangaroos jump by launching off both hind legs simultaneously, using massive tendons in their ankles that work like biological pogo sticks. Each landing stores energy in those tendons, which then snaps back to power the next leap, recycling force so efficiently that hopping faster barely costs the animal any extra energy. A single bound can cover 25 feet horizontally and reach 6 feet in height.
The Tendon Spring System
The real secret to kangaroo locomotion isn’t muscle power. It’s elastic energy stored in their tendons. The major ankle extensor muscles, the same group that connects to the Achilles tendon in humans, have unusually short muscle fibers attached to long, thin tendons. When a kangaroo lands, its body weight compresses those tendons like a spring, converting the kinetic energy of the fall into elastic potential energy. As the tendon snaps back to its original shape, it launches the kangaroo into its next hop.
What makes this system remarkable is how little the muscles themselves have to do. Research on wallabies (close relatives that use the same hopping mechanics) shows that as hopping speed increases, the tendons absorb and return more energy, but the muscles perform roughly the same amount of work per hop regardless of speed. The tendons do the heavy lifting. This is why kangaroos can increase their speed without burning significantly more calories, a trick no quadruped can match.
Why Faster Doesn’t Mean Harder
Most animals burn more energy the faster they run. Kangaroos break this rule. When red kangaroos hop faster over flat ground, their oxygen consumption stays nearly the same. The energy cost per distance actually drops at higher speeds, because each hop covers more ground while the tendon spring system recycles roughly the same amount of force.
This flat metabolic curve has a practical limit, though. Kangaroos typically prefer moderate cruising speeds rather than pushing to their maximum. Higher speeds mean more stress on those tendons, and there’s a real tradeoff: the same design that makes tendons great at storing energy (long and thin) also makes them more vulnerable to rupture under extreme loads. Larger kangaroos store more elastic energy but operate with a smaller safety margin before tendon failure, which likely sets an upper limit on how big hopping mammals can get.
Built for Impact
A kangaroo’s hind limbs are structurally different from those of other mammals of similar size. Rather than relying on thicker bones to handle the repeated shock of landing, kangaroos have evolved larger muscle cross-sections and longer lever arms at the joints. This combination lets the muscles and tendons absorb more force before it reaches the skeleton. Bone dimensions scale roughly the same way they do in other mammals, meaning larger kangaroos do experience greater bone stress. But increased bone cortical thickness (the dense outer layer) appears to partially compensate, keeping the skeleton functional despite thousands of high-impact landings per day.
The hind feet themselves are elongated, which extends the moment arm of the ankle joint and gives the tendons more mechanical advantage. Think of it like a longer lever: more distance for the tendon to stretch means more energy stored per landing.
The Tail as a Fifth Leg
At high speeds, the tail streams behind for balance. But at slow speeds, it plays an entirely different role. When kangaroos move slowly, they can’t hop efficiently, so they shift to an unusual walking gait where they plant their tail on the ground in sequence with their front and hind legs. This “pentapedal” locomotion (five points of contact) looks awkward, but it’s surprisingly effective.
The tail generates as much forward propulsive force as the front and hind legs combined during this slow walk. It pushes against the ground with almost no braking force, functioning biomechanically like a true leg: supporting body weight, driving the animal forward, and delivering mechanical power. The tail’s musculature is substantial, making up a significant portion of the animal’s total body mass, which is part of why it can do so much work during ground contact.
Top Speed and Jump Distance
Red kangaroos, the largest living marsupials, can clear 25 feet in a single horizontal leap and jump 6 feet vertically. The fastest recorded kangaroo was a mature female eastern grey that hit 64 km/h (40 mph) in a burst. The highest sustained speed on record belongs to a large male red kangaroo that maintained 56 km/h (35 mph) over a full mile, though that effort proved fatal.
These peak numbers represent extremes. Day-to-day movement is far more conservative. Kangaroos typically cruise at speeds that keep tendon stress well within safe limits, saving their explosive capacity for escaping predators or covering open ground quickly.
How Joeys Learn to Hop
Joeys take their first hops around 8 to 10 months of age, and the early attempts are visibly uncoordinated. Mastering the elastic recoil technique takes practice because it requires precise timing. The tendon spring only works efficiently when the animal loads and unloads its ankles at the right moment in each stride cycle. Young kangaroos have to develop both the muscle coordination and the tendon stiffness to make hopping viable, which is why they rely on crawling and pentapedal movement for months before they can bounce reliably.
Interestingly, smaller kangaroo species and juveniles don’t benefit as much from elastic energy storage. The physics of tendon springs favor larger bodies, since heavier animals compress tendons more forcefully and store proportionally more energy. This is one reason why the largest kangaroo species are the most efficient hoppers, and why hopping as a locomotion strategy only really pays off above a certain body size.