Many groups of animals, from humans to dinosaurs to kangaroos, have evolved the ability to stand and walk on two legs. In all of these cases, the move from four legs to two has provided clear advantages. Kangaroos save energy because their hopping is so efficient, birds have freed their front pair of limbs for use as wings, and humans use ours for manipulating tools and objects.
But some species of lizards also rear up on their hind legs from time to time, particularly when running. The Australian frilled lizard does it so readily that it has earned the nickname of 'bicycle lizard'. And the South American basilisk can move so quickly on its hind legs that it can even run over water. In both these cases, running helps the lizards to escape from danger, but unlike humans or birds, their front legs haven't been put to any special purpose and remain adapted only for walking. So why run on two legs when you have four perfectly good ones?
Christofer Clemente from the University of Cambridge a team has the answer - it's just a fluke of physics. Together with a team from Western Australia, Clemente filmed 16 species of Australian dragon lizards running on a treadmill. The videos revealed that as some species accelerate, they shift their centre of gravity backwards. This small change is enough to make their front legs lose traction, and they automatically rear up on their hind legs. The dragons are inadvertently doing a wheelie.
Run, lizard, run
Clemente captured his runners with the help of expert lizard-tracker Graham Thompson, who even managed to acquire three reddening dragons (Ctenophorus rubens), a species so rare that it's only known to live in a single Western Australian cattle station. The lizards were filmed with a high-speed camera as they ran on a steady treadmill until they were too exhausted to carry on.
To his surprise, the lizards fell along a neat continuum in terms of their preference for two-legged running. Species like C.rubens only spent about 2% of the time on two legs. Others, like the frilled lizard (Chlamydosaurus kingii) and the Tata lizard (Lophognathus gilberti) couldn't get enough of it, and reared up on 72% and 83% of their strides respectively. But why were they doing it? It certainly wasn't a trait that that evolved in a particular lineage, for the lizards' preferences for two-legged running didn't match their positions on an evolutionary family tree.
Previously, other scientists had suggested that lizards might be able to run faster or more economically on two legs. Bipedal lizards tend to have longer hind-limbs which gives them a longer stride; that in turn, could make them speedier. And with their front limbs off the ground and doing no work, they could potentially save energy.
But the dragon runners disproved both ideas. Neither the speed nor their stamina of the two-legged runners outclassed those of the four-legged ones. In fact, the bipedal runners tended to give up earlier, and Clemente thinks that the extra burden placed on the hind legs outweighs the fact that the front pair are idle.
Need for [strike]speed [/strike]acceleration
The videos showed that bipedalism had nothing to do with speed, but Clemente found that it had everything to do with acceleration. Many of the species were accelerating at a greater pace when they reared onto their hind legs than when they were running on all fours. In fact, some of the lizards showed a distinct threshold of acceleration when they quickly shifted from four legs to two.
Clemente thinks that their front legs lift off the ground as a natural consequence of their acceleration. He noticed that lizards whose centre of gravity was closer to their hips ran on two legs more often than those whose weight was balanced further forward. This shift in weight makes them more manoeuvrable as runners, but as a side effect it causes their front legs to lose contact with the ground. At high accelerations, the lizards have no choice but to run on two legs.
However, some species appear to have taken this side effect and... er... run with it. Clemente found that some species started rearing up at speeds much lower than predicted and he thinks that they may have developed techniques that allow them to start running bipedally at lower speeds than normal. These include tucking their arms to their sides, or lifting their tails up early. This suggests that some lizards are actively opting for a two-legged gait, presumably because it affords them some sort of advantage. But if that's not speed or endurance, what is it? For the moment, no one knows.
Reference: Clemente, C.J., Withers, P.C., Thompson, G., Lloyd, D. (2008). Why go bipedal? Locomotion and morphology in Australian agamid lizards. Journal of Experimental Biology, 211(13), 2058-2065. DOI: 10.1242/jeb.018044