Kangaroos – The 100 Days Project: Day 71 
Even the breath is involved. Kangaroos take one breath each hop: they breathe in as they leave the ground; their lungs are fullest at the height/centre of each bound, and emptiest as they hit the ground again. Amongst other things, the consequent shifts in abdominal mass contribute to the energy-efficiency of the hop. These shifts, and this breathing, has been called the kangaroo’s ‘visceral piston’. But I’m already into the technology. Let’s cover a few simpler things first.
It’s famously asserted that kangaroos cannot move backwards, and, apart from some back-stepping in its ‘boxing’ bouts, this is pretty much true. This is because they cannot move bi-pedally, that is, move one foot independently of the other, though even this has to be qualified. When swimming – yes, kangaroos swim (just last week a swamp wallaby was ‘rescued’ from a rather ambitious swim in Sydney Harbour) – they can paddle with their back feet, first one and then the other, like a dog. They can also foot-thump with one or both feet independently, as they will when warding off a suspected predator or warning other kangaroos of the same. And when hopping at some speed, they can change direction with remarkable agility by pivoting on one foot. But moving slowly on the ground, as when grazing, no: at such times, in fact, they move tetrapedally, which is to say with all five feet (the tail, in this instance, counted as a foot).
Moving tetrapedally, of course, is very energy-inefficient, and one can imagine that it’s hardly the roos’ favourite means of getting about. Hopping is much more energy efficient, particularly as the kangaroo approaches what we might call a cruising speed (18-20 kph). You could say their whole body has evolved to achieve this efficiency.
The larger macropods have supposedly been clocked at speeds between 65 and 70 kilometres per hour, though these perhaps push their upper limits. It might be safer to speak of a general top speed of around 50 kph. Likewise it has been asserted that red kangaroos can keep up their cruising speed all day, though one might wonder whether anyone has ever kept up with one that long – by what, motorcycle? car? helicopter? light plane? – to make sure. Safer to follow those who say they have kept up – or reported someone else who has kept up – with a kangaroo bounding at 20 kph for two hours. Still an extraordinary pace and endurance. By comparison, the fastest human marathon runner in the world keeps up a pace of just above 20 kph.
A great deal of it, of course, is in the feet and the tendons. A kangaroo’s Achilles tendons must be remarkable things, to take the immense stresses they do, but it is the larger (thicker, at the top) Lateral and Medial Gastrocnemius, that form what we might call the kangaroo’s calf muscles that perhaps do the most amazing work, extending as the kangaroos prepare their lift-off – most extended when the body is closest to the ground, and then, spring-like, snapping back into place when he/she extends his/her legs in the leap. It’s been calculated that, through this mechanism, the kangaroo recoups 70% of the energy expended with each bound.
And those long feet, that in one sense barely touch the ground as the kangaroo flies along. Who would have thought that one of their principle functions – a short foot would do nothing like it – is to ensure (leverage) the maximum extension of the aforementioned tendons?
I could write more. The way the head adjusts its angle in relation to the body in every leap, at once to ensure the best aerodynamics and to reduce the effect of each impact as the roo touches ground, or the way the tail acts as counterweight: how much energy is saved, after all, if the balance is perfect?
But enough. Far better to watch some bounding clips on YouTube than to read me any further. You can see a good one here.