Kangaroo bounce mechanics illuminated by infrared study

The precise details of how kangaroos bounce are now being laid bare, thanks to new technology.

Most animals adopt a more upright posture as their body mass increases, redistributing their weight to allow more efficient movement.

However, kangaroos do not appear to adjust their posture in this way.

Now an outdoor infrared motion-capture approach is showing how the kangaroos' bounds distribute forces along their legs and to the ground.

Motion-capture records movement, registering and analysing reflections from small plastic markers stuck on to the moving entity.

Golf coaches use it to analyse their pupils' swings, and it was used in the Lord of the Rings films to translate the movements of actor Andy Serkis into those of the creature Gollum.

Infrared lights illuminate the subject, and an array of cameras tracks the motion of the markers.

However, such studies are typically not undertaken outdoors, where a great deal of infrared light from the sun is bouncing around.

Now, a team of researchers from the Royal Veterinary College in London, the University of Idaho, the University of Queensland, and the University of Western Australia has been loaned a novel motion capture system from the firm Vicon that is capable of "looking past" the ambient infrared light and focusing on their study subjects: kangaroos.

Boundless energy

"The team is interested in trying to understand how the group of kangaroos change their body posture and hopping mechanics with body size," explained Craig McGowan of the University of Idaho.

"There are a number of species that, as they get larger, adopt more and more upright postures.

Motion capture relies on light bounced from a number of high-reflectivity balls

"That reduces the mechanical demands on the musculature - so it increases their 'mechanical advantage'."

In addition, the team is measuring the forces that the kangaroos' feet exert on the ground - and thus that are transmitted through their legs - using what are known as force plates.

The team also captured the kangaroos' movement using the traditional method of high-speed video - which in the past was analysed frame-by-frame to obtain the same kind of data that the motion-capture system provides automatically.

Taken together, said Alexis Wiktorowicz-Conroy, a researcher at the Royal Veterinary College, the studies will discover why kangaroos do not simply tear themselves to bits as they get up to speed.

"We want to know how are they able to hop fast - even when they are quite heavy - and not change posture," she told BBC News.

"That's important, because these animals get really big, and we can't really explain without this why their bones don't break at high speeds.

"People have started to look at that in ankle joints; we're looking more at joints in hind limbs."

The motion-capture setup is complemented by high-speed video cameras

Dr Wiktorowicz-Conroy thinks that information will shed light on another mystery of roo motion: how they do it in such an evidently efficient way.

"The kangaroos' movement is really neat - at low speeds, they use their tail like a fifth limb, inching along like an inchworm. As they move faster, they start to hop. Humans fatigue very easily when we do this, but the kangaroos don't; they don't expend much energy.

"Certain species of wallaby hop up large hills and don't seem to behave in the same way. There's quite a bit of variation (in the way the marsupials move), but all of them are more economical than you might predict."

The experiments, conducted in Alma Park Zoo in Brisbane, have garnered significant amounts of data that the team is still working to understand.

But Dr Wiktorowicz-Conroy said the outcome was certain to solve some of the biomechanical mysteries of the roo.

"There's a lot we don't know about them, and this is going to help study questions about hopping and animal locomotion in general.

"We hope in the end we can use this in veterinary medicine and for conservation."