Why walk when you can jump?

Guest blog post by Donna Belder. She is a fourth year undergraduate science student at the University of Adelaide (South Australia). Her majors at uni are botany, ecology and environmental geoscience, but she is interested in many different aspects of science.

Until a recent zoology class, I had never really given much thought as to why different animals move the way they do. For instance, what is it that makes some animals amazing runners over long distances? Why are some fantastic sprinters but wear themselves out after a short time in action? The question that really got my attention, though, was this one: what on earth could make kangaroos abandon running altogether and start mimicking pogo sticks? Kangaroos (genus Macropus) are famous for being ridiculously good at jumping, and they do it so effortlessly that you have to wonder what their secret is. Try jumping around hurriedly for a bit and you'll soon find you suck at it (yeah, I've tried). Not only will you get tired after about 5 minutes, but the fact that you don't have a tail to balance yourself means that you will probably face-plant before the 5 minutes are even up. Jumping just doesn't work for us, or most other animals for that matter. So what is the big secret?

Firstly, you can tell a lot about how an animal moves just by looking at its size - you wouldn't expect to see an elephant galloping around like a horse, or a mouse slowly plodding about. Whether an animal moves over long distances also has a lot to do with the way it gets around. Kangaroos are reasonably large creatures; an adult male red kangaroo (Macropus rufus) can easily stand 2 metres tall and weigh up to 135 kilograms, so moving all that bulk around isn't easy. Kangaroos are also very mobile animals, and move large distances to forage for food. This is because our sunburnt country isn't the most reliable pantry in the world, and environmental fluctuations can force roos to travel hundreds of kilometres in search of greener pastures. Enter selective pressure - fitness coaching a la Mother Nature. Put simply, if kangaroos hadn't adapted to efficiently move their large bodies over long distances, they'd almost certainly be extinct by now.

Now to get technical. Elastic energy storage explains why animals are able to maintain a fluid gait once they start moving. It works like this: kinetic energy (energy possessed by an object in motion) is released during the first half of the ground-contact phase, i.e. as the animal steps forward. When the foot is lifted, some of this energy is then returned to the tendons in the walking limb and stored as ‘elastic strain energy'. The stored energy is then used to propel the body forward in the following step, so the animal doesn't have to input as much additional energy to keep itself going. All animals utilise elastic energy storage to some extent, but kangaroos have leapt ahead (see what I did there?) in the quest for efficient locomotion.

Through a combination of specialisations in tendon structure, tissue elasticity and skeletal alignment, the kangaroo's hind limbs have become jumping machines. Their elongated tendons have an extremely high tensile loading capacity, meaning that they are capable of storing huge amounts of elastic energy - around 7 times more than the limbs of similar sized quadrupedal animals. The angles at which the leg bones are arranged give kangaroos a mechanical advantage by maximising the ability of the limb to absorb kinetic energy during contact with the ground. Stride length is also increased. Together, the tendons and muscle tissues act as an elastic spring, meaning that with each hop energy is reclaimed. Kangaroos can therefore increase their hopping speed without increasing the total level of energy expenditure. Interestingly, this principle only works well for large hopping animals - smaller animals don't jump like kangaroos because the use of tissue elasticity for kinetic energy storage is mass-dependent.

So what does all this mean for a kangaroo bounding hundreds of kilometres across the Australian outback? Well, as it jumps at high speeds, its legs basically do become pogo sticks. It travels a great distance (up to 9 metres) with each jump, and the rebound energy stored in its hind limb tendons fuels each subsequent jump that it makes. Speed can therefore be increased without the need for the animal to raise its oxygen consumption. So, kangaroos can speed up on level ground independent of aerobic metabolism. Red kangaroos hopping at speeds of 22 km/h have been found to be operating at only 43% of their aerobic capacity - the equivalent energy expenditure of a briskly walking human going a pitiful 6 km/h. Provided they don't cross their tendon stress threshold (the maximum mechanical stress the tendons can support), they can pretty much keep at it as long as they like. Oh, and over short distances they can travel at a cool 65 km/h. Plus the cute factor. You should definitely be jealous.

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References:

Bennett, M. and Taylor, G. (1995). Scaling of elastic strain energy and the benefits of being big. Nature 378, 56-59.

Garland, T. (1983). The relation between maximal running speed and body mass in terrestrial mammals. Journal of Zoology 199, 157-170

Kram, R. and Dawson, T. (1998). Energetics and biomechanics of locomotion by red kangaroos (Macropus rufus). Comparative Biochemistry and Physiology (Part B), 120, 41-49.

Proske, U. (1980). Energy conservation by elastic storage in kangaroos. Endeavour (New Series) 4, 148-153.

Image source: http://www.guzer.com/pictures/baby_kangaroo.jpg