Kangaroos Can’t Walk Backwards. Their Tail Won’t Let Them.

There is a reason the kangaroo appears on the Australian coat of arms alongside the emu. Neither animal can walk backwards. It was chosen deliberately as a symbol of a nation that moves only forward. The symbolism is neat. The biology behind it is more interesting than the metaphor.

Macropus rufus, the red kangaroo, is the largest marsupial on Earth and the largest native land animal in Australia. Adult males can stand 1.8 meters tall and weigh up to 90 kilograms. They move through the Australian outback using a locomotion system that has no equivalent among large mammals, and the tail is central to everything.

The Mechanics of the Hop

A kangaroo at speed is not simply jumping repeatedly. It is operating a biological spring system in which the tendons of the hind legs store and release elastic energy with each hop, making high-speed travel energetically cheaper than walking would be for an animal of the same size.

Researcher Terence Dawson at the University of New South Wales conducted the foundational studies on kangaroo locomotion energetics in the 1970s and found something counterintuitive. At speeds above approximately 15 kilometers per hour, the energy cost of hopping does not increase proportionally with speed the way it does in running mammals. A kangaroo traveling at 25 kilometers per hour uses roughly the same energy per unit distance as one traveling at 15 kilometers per hour. The tendons are doing the work, not the muscles, and the tendons get more efficient as the animal moves faster.

This makes kangaroos extraordinarily economical travelers across large distances, which is essential in an environment where food and water can be separated by tens of kilometers. A red kangaroo can sustain speeds of 20 to 25 kilometers per hour for extended periods and reach burst speeds of 70 kilometers per hour over short distances.

The tail functions as a fifth limb during slow movement. When a kangaroo walks slowly, it uses its forelimbs and tail as a tripod, swinging its hind legs forward together. The tail bears a significant portion of the body weight during this gait and provides the stability that makes the movement possible. Remove the tail’s function and the animal cannot walk slowly at all. It can only hop, which is why the backwards walking question has a structural answer. The tail-based slow gait only works in one direction.

The Pouch and Developmental Biology

Kangaroo reproduction is one of the most unusual systems in any mammal. After a gestation period of approximately 33 days, a red kangaroo gives birth to a joey that weighs less than a gram and is roughly the size of a jellybean. The joey is essentially a fetus, with undeveloped hind limbs, closed eyes, and no fur. It crawls unaided through its mother’s fur to the pouch, guided by scent and an instinctive upward climbing motion, and attaches to a nipple.

It will remain attached to that nipple for approximately 70 days, during which the nipple swells inside the joey’s mouth, making detachment impossible. The joey then begins to detach and reattach voluntarily, spending increasing amounts of time off the nipple as it develops. It will continue to use the pouch for shelter and occasional nursing for up to a year after first emerging.

What makes this system particularly remarkable is that a female red kangaroo can simultaneously support three offspring at different developmental stages. A joey at foot, recently emerged from the pouch and still nursing occasionally. A joey in the pouch, developing. And a dormant embryo, held in a state of suspended development called embryonic diapause, waiting for the pouch to become available.

Researcher Marilyn Renfree at the University of Melbourne has studied marsupial reproductive biology for decades and her work on embryonic diapause has revealed the hormonal mechanisms that allow the embryo to pause its development and resume it when conditions are appropriate, typically when the pouch joey dies or is weaned. The system allows females to maintain a continuous reproductive pipeline with minimal gaps between offspring.

Mob Dynamics and the Outback

Red kangaroos live in groups called mobs, ranging from a few individuals to over 100 in areas of high resource concentration. Mob composition is fluid, with individuals moving between groups as conditions change. The social structure is not as tightly organized as that of primates or elephants, but it is not random either.

Males compete for mating access through boxing matches that are more serious than they appear in popular media. Two large males boxing can inflict significant injuries with their powerful hind legs, which are capable of disemboweling an opponent. The forelimbs are used for grappling and balance during these contests, while the tail provides the support that allows the animal to deliver kicks with both hind legs simultaneously.

Researcher Graeme Coulson at the University of Melbourne has studied red kangaroo population dynamics across the Australian outback for decades, documenting how populations respond to drought, flood, and the boom-bust cycles of rainfall that characterize the interior of the continent. Red kangaroo populations can fluctuate by an order of magnitude within a few years, crashing during severe drought and recovering rapidly when rains return.

The species is not endangered. With a population estimated at 50 million individuals, red kangaroos are one of the most numerous large mammals on Earth. They are harvested commercially for meat and leather under a management system that is among the most carefully monitored wildlife harvests in the world, with annual quotas set based on population surveys across each state.

An animal that stores energy in its tendons, raises three offspring simultaneously at different developmental stages, and cannot walk backwards because its tail won’t allow it, is doing well by any conservation measure. Not every wildlife story ends in crisis. Sometimes evolution just gets it right.