A red kangaroo crossing the Australian outback can clear nine meters in a single bound, and it does so with a stride that looks almost weightless. That impression isn’t just aesthetic. Kangaroos have solved a problem that most running animals never do: they’ve made speed nearly free.
The energy paradox of hopping
In humans, horses, and dogs, oxygen consumption climbs more or less in step with speed — the faster the muscles work, the more fuel they burn. Kangaroos don’t follow that curve. Treadmill studies going back to the 1970s found that red kangaroos hold a nearly flat metabolic rate across a wide range of hopping speeds. Part of the explanation is well established: their Achilles tendons act as springs, storing and releasing elastic energy with each bound instead of relying purely on muscle contraction. But tendons only tell half the story. The other half is respiratory.
A mechanical model: the guts as a piston
In 2010, physiologist Mauricio Giuliodori and colleagues published a paper in Advances in Physiology Education that laid out a simple, elegant mechanism for how hopping and breathing lock together. As a kangaroo leaves the ground, its abdominal organs lag slightly behind the rest of the body due to inertia, tugging the diaphragm downward and drawing air into the lungs — the same principle as pulling back a plunger. On landing, the organs surge forward again, pushing against the diaphragm and helping force air back out. The upshot is counterintuitive: hopping actually takes less respiratory muscle effort than standing still. Breathing isn’t interrupted by movement here — it’s driven by it.
What the 2025 research adds
The 2010 model explained the basic piston action, but it didn’t account for how that action might change with speed or posture. A study published in eLife in December 2025, led by Lauren Thornton and colleagues at the University of the Sunshine Coast and the Royal Veterinary College, filled in that gap. Using motion capture, force plates, and a detailed 3D musculoskeletal model built from red and grey kangaroo anatomy, the team tracked how hindlimb posture shifts as hopping speed and body mass increase.
Faster hopping brings a more crouched stance, with greater ankle dorsiflexion and more flex through the toes. That posture reduces the ankle’s mechanical leverage, which sounds like a disadvantage — except it raises stress on the tendons, and higher tendon stress means more elastic energy stored and returned on the next bound. The paper also points to a ceiling on this system: tendon stress can only climb so far before it approaches material failure, which may explain why very large kangaroos face real biomechanical limits on how fast and how efficiently they can move.
The postural piece matters for breathing too. How the torso is angled changes how the viscera sit relative to the diaphragm, which means the respiratory assist isn’t a fixed mechanical trick — it’s tuned, stride by stride, to the animal’s speed.
Why this matters beyond the outback
None of this is only a curiosity for physiologists. Efficient, low-cost locomotion is what lets kangaroos travel long distances across an arid landscape without overheating or running out of energy reserves — a direct survival advantage. Loose versions of the same coupling show up elsewhere in nature: horses often synchronize breath to stride at a gallop, and some birds time wingbeats to breathing. Kangaroos simply take the principle further, helped by a large gut mass and an upright hopping gait that amplifies the effect.
It’s also why engineers building legged robots and prosthetics keep coming back to kangaroo biomechanics, and why physiologists are now going further still — teams using MRI and custom breathing masks on live kangaroos are trying to capture actual airflow in real time, to see the piston in action rather than just model it.
A rhythm worth borrowing
For practitioners of the Wim Hof Method, the kangaroo’s trick points to a principle that sits at the heart of structured breathwork: breath and movement are rarely two separate systems running in parallel — they’re meant to work as one. A kangaroo never has to think about synchronizing its breathing with its stride; evolution wired the two together so tightly that effort simply disappears into rhythm. Humans don’t have a visceral piston to borrow, but conscious breath-movement coordination — the kind practiced in WHM breathing sessions paired with cold exposure or physical exertion — works toward a similar goal through different means: reducing the perceived cost of effort by bringing breath into deliberate alignment with the body’s rhythm, rather than leaving it to catch up on its own. The kangaroo simply reminds us, in the most literal way nature offers, that efficient movement and efficient breathing were never meant to be separate disciplines.