There is one very good reason why giant mice and rabbits will always remain the stuff of Hollywood: their bones couldn't take the strain. Christofer Clemente from Harvard University's Rowland Institute, USA, explains that small mammals' crouched skeletons would snap if they were scaled up to hippo size. This is why hippos and all other large animals opted for straight-legged postures: transmitting their weight to the ground through column-like leg bones to prevent them from snapping. But how do the largest lizards avoid bone-cracking stresses while effectively holding a press-up position as they walk? Do they become more upright like large mammals, or have they opted for an alternative strategy to protect the twisting upper-leg bones from fractures? Intrigued, Clemente and his PhD supervisor, Philip Withers from the University of Western Australia, decided to measure the posture of lizards ranging in size from tiny 50 g Varanus eremius up to a colossal 7.9 kg Varanus varius, to find out how sprawling lizards protect their legs from fractures (p. 3013).

Clemente searched the Australian bush with Graham Thompson and successfully collected 11 species. Taking the animals back to Perth, Clemente then tested their running performances in David Lloyd's human movement lab. Building an impromptu racetrack from tables and chairs and filming the lizards as they ran, Clemente admits, ‘The real trouble is that you have to put all these little stickers on the lizard and then you have to let it go in the lab filled with millions of dollars of equipment. That is the really scary part,’ he laughs.

Selecting only the movies where the lizards ran at 45–50% of their maximum speed, Clemente measured the total limb length of each lizard and then compared that with the height of the marker on the running lizard's hip to see whether the larger lizards were running with straighter legs than the smaller species. But they were not. ‘I went back and got more lizards and still didn't see anything,’ remembers Clemente. ‘No matter which variable I looked at for upright posture there was no change across lizards so they were clearly doing something different from mammals. The limb bones weren't breaking with stress so they were getting rid of it somehow and I had to figure out what that was,’ he adds.

Next, Clemente analysed the lizard's movements and found that the larger lizards modified their strides to relieve twisting stresses in their upper leg bones. The largest lizards' feet spent more time in contact with the ground during each stride than the smallest lizards: in other words, their duty factor increased. ‘This works to reduce stress by increasing the time you have to distribute the force over. You have a longer time so the peak of that force is going to be less,’ explains Clemente. He also found that the largest lizards rotated their thighs less than the smaller species to reduce the stress-inducing twisting force on their femur bones during each stride.

So mammals and lizards use different mechanisms to get rid of bone stress because sprawling lizards cannot walk in a more upright posture. However, the lizards' stress relief adjustments have cost them their speed. Larger lizards will never run as fast as mammals of a similar size because they cannot rotate their thighs to stride as far, and they have to keep their feet in contact with the ground for longer to prevent their limbs from fracturing.

C. J.
P. C.
Evolution of limb bone loading and body size in varanid lizards
J. Exp. Biol.