When Darwin made his historic HMS Beagle voyage, one animal in particular caught his eye: the giant Galápagos tortoise, plodding along at a leisurely 0.16 metres per second. Intrigued by the hefty reptile's painfully slow pace, Peter Zani and colleagues at the University of Colorado,Boulder, decided to put giant tortoises through their paces(p. 1489).

Zani wondered whether tortoises, just like many other animals, use a mechanism known as the inverted pendulum to conserve mechanical energy as they amble along. He explains that as you walk, the potential energy that is associated with the height of your centre of mass changes into kinetic energy. When your foot touches the ground, lifting your centre of mass, this kinetic energy is converted back to potential energy. Throughout this energy exchange, your centre of mass rises and falls in the shape of an inverted pendulum,' Zani says. It's an efficient mechanism to recover energy, but do giant Galápagos tortoises use it?

According to Zani, two conditions have to be met to show that the inverted pendulum mechanism is at work.' First, kinetic and potential energy should be out of phase – as kinetic energy rises, potential energy falls. Second,fluctuations in kinetic and potential energy must be of equal magnitude and shape. Since kinetic energy is the energy of motion, and giant tortoises are so slow, they are likely to have very small kinetic energy fluctuations. Teaming up with Rodger Kram, Zani calculated that if tortoises use the inverted pendulum mechanism, their modest kinetic energy should raise their centre of mass vertically by only 0.5 mm. But tortoises sway as they walk, so Zani and Kram suspected that they don't use the inverted pendulum mechanism.

To test this, they needed to analyse the mechanical energy patterns of travelling tortoises. Zani widened and strengthened a traditional force plate walkway to cope with the bulk of the giant creatures. If he could convince tortoises to walk across it, he'd be able to convert its force measurements into mechanical energy values. Jinger Gottschall joined Zani and Kram as they loaded the modified walkway into a trailer and drove to the Oklahoma City Zoo. Once there, they were faced with the challenge of luring five giant Galápagos tortoises up a ramp onto the walkway. Zani soon found thatyou can't force a 200 kg animal to do what you want!' But tempting the creatures with juicy apples and carrots seemed to do the trick, and the team videotaped them as they lumbered across the walkway to get their treat.

Analysing the force plate measurements and video footage with Gottschall,Zani could see that the tortoises didn't meet the two conditions. Their kinetic and potential energies fluctuated randomly – sometimes they were in phase, sometimes they were out of phase. There was certainly no systematic exchange of energy,' he says.

They also saw that a giant tortoise's centre of mass rises about 1.5 cm as it walks, much more than the 0.5 mm predicted if they use the energy-conserving mechanism. As the team had suspected, giant Galápagos tortoises do not use the inverted pendulum mechanism. Yet despite their inefficient pace, Zani notes that giant tortoises don't have to work harder than other animals to travel the same distance.

Zani, P. A., Gottschall, J. S. and Kram, R.(
2005
). Giant Galápagos tortoises walk without inverted pendulum mechanical-energy exchange.
J. Exp. Biol.
208
,
1489
-1494.