A kangaroo landing at speed is doing something that looks wasteful. Its body drops, its feet hit the ground, and its forward and downward motion has to be redirected into another bound.
But much of that energy is not lost. Long tendons in the lower hind legs stretch under the force of landing, storing mechanical energy as elastic strain. A fraction of a second later they recoil, helping lift and propel the animal into its next flight phase.
That spring-like exchange helps explain one of the strangest curves in animal locomotion. Large kangaroos can increase their hopping speed with little change in the rate at which they consume oxygen. Because they cover more ground in the same time, the energy cost of moving each meter can fall as they go faster.
The headline comparison with a four-legged mammal comes from several decades of biomechanics, not a literal side-by-side race between a kangaroo and a perfectly matched runner. Researchers measured oxygen consumption in kangaroos and wallabies, then compared the results with the established relationship between body mass, speed, and metabolic cost in quadrupedal mammals.
The original result looked almost backwards
In a short 1973 Nature paper, Terence Dawson and C. Richard Taylor measured red kangaroos moving on a treadmill. From roughly 2 to 6 meters per second, equivalent to 7.2 to 21.6 kilometers per hour, their rate of oxygen consumption remained nearly constant.
That does not mean hopping costs no energy, or that a kangaroo can accelerate indefinitely for free. It means the metabolic power required during steady, level hopping changed remarkably little across the tested range. Most terrestrial runners show a much clearer increase in oxygen use as speed rises.
A later treadmill study of tammar wallabies found a similar pattern. Oxygen consumption rose at speeds below about 2 meters per second, then became effectively independent of hopping speed up to 9.4 meters per second, or nearly 34 kilometers per hour.
At travel speeds observed in the field, that study estimated the cost per distance for large macropods at less than one-third the predicted cost for a quadruped of equivalent body mass. “Dramatically less” is therefore defensible for fast, steady travel by large kangaroos and wallabies. It should not be generalized to every species, speed, slope, or maneuver.
The tendons are springs, not motors
When the feet meet the ground, a kangaroo’s center of mass loses kinetic and gravitational potential energy. If all of that energy were dissipated, its muscles would have to replace it before the next takeoff. Muscle work is powered by chemical energy from metabolism.
The gastrocnemius and plantaris muscle-tendon units around the ankle change that accounting. Their muscles have relatively short, angled fibers capable of producing large forces without shortening very far. Their long, compliant tendons take most of the stretch.
Stretching loads the tendon much as compressing or extending a spring stores energy. Recoil returns part of it during takeoff. A major 2018 review of mammalian hopping concluded that tendon return may supply as much as half of the mechanical energy required for steady hopping. Earlier calculations reached as high as 70 percent at 6 meters per second.
Those percentages are estimates, and they refer to mechanical energy rather than a simple percentage of calories saved. Tendons cannot create energy. Muscles still support body weight, stabilize joints, replace unavoidable losses, and power acceleration. Uphill hopping is much more expensive because elastic recoil cannot do the net work of lifting the animal against gravity.
Why speed changes the cost per meter
Two measurements are easy to confuse. Metabolic rate describes energy used per unit time. Cost of transport describes energy used to move a unit of body mass over a unit of distance.
If a kangaroo uses oxygen at almost the same rate while covering ground faster, every minute takes it farther. Its energy cost per meter therefore declines even though its total expenditure is not zero.
Large kangaroos also increase speed mainly by lengthening each bound. Their stride frequency changes much less than it does in many running quadrupeds. That reduces how often muscles must cycle the limbs and generate support force in a given interval.
The result is not perpetual motion. Each contact still loses some energy to tissues, the ground, air resistance, and movements that tendon recoil cannot reverse. The muscles continually top up the system. The difference is that they are topping up a bouncing mechanism rather than rebuilding the entire next bound from nothing.
“Enormous tendons” means long and highly loadable
A useful biological spring is not simply the thickest possible cable. Kangaroo ankle extensor tendons are long and relatively slender. Under high force, that geometry allows them to experience enough stress and strain to store substantial elastic energy.
Body size strengthens the effect. A 1995 Nature study of elastic-energy scaling found that the capacity for storage increases disproportionately as macropods become larger. Larger muscles can apply greater force to the tendons, while tendon architecture permits greater elastic return.
That benefit comes with a cost. High tendon stress means a narrower safety margin before damage. Large red kangaroos can operate their ankle tendons at safety factors far below those typical of many mammalian tendons. They may prefer slower hopping speeds than pure energy economy would predict because tissue stress also matters.
Small hoppers expose another limit. Kangaroo rats, small wallabies, and bettongs share the general hopping plan, but their thicker tendons are not loaded in the same proportion. Studies disagree about the size of their energetic advantage, and the nearly speed-independent oxygen curve is most reliable in larger macropods.
Posture helps the spring take a heavier load
Tendons explain much of the energy return, but they do not explain by themselves how a kangaroo increases tendon stress without making its muscles do much more work.
A 2025 eLife study combined three-dimensional motion capture, force-plate measurements, and a musculoskeletal model of red and eastern gray kangaroos. The researchers found that faster animals adopted a more crouched posture, especially at the ankle and the joint where the long foot meets the toes.
That posture changed the leverage between the ground force, the joints, and the ankle extensor muscles. The muscles had to generate high force, which increased tendon stress and elastic storage, but the model did not infer a matching increase in their net mechanical work.
This is a proposed part of the explanation, not a closed case. The study’s models depend on estimates of muscle and tendon properties, and the authors said further work is needed across the whole body. Breathing synchronized with hopping and the near-constant stride rhythm may also contribute.
Fast hopping is not the kangaroo’s cheap gait at every speed
Slow hopping is energetically awkward. Large kangaroos commonly switch instead to pentapedal walking: the forelimbs touch down, the powerful tail supports and propels the body like a fifth limb, and the hind feet swing forward together.
This makes the animal’s life history unusually asymmetric. A newborn joey reaches the pouch with relatively developed forelimbs while its hind limbs are barely formed, as an earlier Natural History account of the pouch crawl described. The adult’s fastest travel later depends on the enormous muscles and elastic structures of those once-undeveloped hind limbs.
Kangaroos are not alone in recycling energy. Humans, horses, and other running mammals also store and return substantial energy in tendons. The distinctive feature is the combination of large body size, long distal tendons, joint posture, and a hopping gait that gives large kangaroos an unusually flat relationship between speed and metabolic rate.
The efficiency probably did not invent hopping
It is tempting to imagine hopping evolving on Australia’s open, dry plains as a way to travel cheaply between scattered food and water. Comparative and fossil evidence points to a less tidy history.
The ancestors of modern macropods were small animals living in more wooded environments. The 2018 review argued that bipedal hopping arose before the large-bodied species that receive its most dramatic energetic benefits. Efficient long-distance travel may therefore be a later consequence of increasing size, not the original reason the gait appeared.
For a large kangaroo moving quickly across level ground today, however, the mechanical advantage is real. Each landing stretches living tissue that preserves part of the last bound. Each recoil returns that energy to the next one. The animal still pays to move, but it does not have to buy the whole stride again.