13-Jun-2008 00:15 Eastern US Time
Why bother running on two legs when the four you've got work perfectly well? This is the question that puzzled Christofer Clemente of the University of Western Australia.
For birds and primates there's a perfectly good answer. Birds have converted their forelimbs into wings and primates have better things to do with their hands.
But why have some lizards gone bipedal? Have they evolved to trot on two feet? Or is their upright posture a fluke of physics? Curious to find the answer, Clemente and his colleagues Philip Withers, Graham Thompson and David Lloyd decided to study why dragon lizards run on two legs.
First Clemente had to catch his lizards. Fortunately Thompson is a lizard-tracking expert. Driving all over the Australian outback, Clemente and Thompson eventually collected 16 dragon lizard species, ranging from frilled neck lizards to the very rare C. rubens - which is found only on a remote Western Australian cattle station. Returning to the Perth lab, Clemente and Withers set the lizards running on a treadmill and filmed them.
Clemente says when he started, he thought the lizards would fall into one of two groups - lizards that mostly ran on two legs, occasionally returning to four, and lizards that never reared up at all. But that's not what he found.
Even the lizards he'd never seen on two legs in the wild managed an occasional few steps on their hind legs. In fact there seemed to be a continuum of running on two legs among lizards. C. rubens and P. minor spent only 5% of the time on their hind legs, while L. gilberti was up there for 95% of the time.
Curious to know if bipedalism had been selected by evolution, Clemente drew up the lizards' family tree. He plotted a graph of the fraction of time each species spent on their hind legs. He could find no correlation. So it wasn't a case of lizards that evolved later using two legs to get some kind of advantage. Something else must driving them off their front legs. But what?
But when Clemente analysed the films of lizards running on two legs, he realised that it actually used more energy. What's more the bipedal runners were no faster than the quadrupeds.
Peter Aerts had suggested that lizards whose centre of mass is further back are more manoeuvrable. So Clemente teamed up with David Lloyd to model the running lizards' movements as they accelerated, and look at the effect of the centre of mass.
What they found was that the lizards whose centre of mass was closer to their hips ran on two legs more often than those whose weight was further forward.
They were 'pulling a wheelie'. Having heavier back ends made them more manoeuvrable runners. But it also made their front rise up as they accelerated - just like the front wheel of a bike when the rider's weight is shifted back and the power applied. It's simple physics. At high accelerations the lizards have no choice but to run on two legs.
But it's not all simple physics, Clemente found. Some species reared up at much lower accelerations than expected. They were doing this deliberately, by tucking their arms into their sides, for instance, or lifting their tails up to shift their centre of mass.
Some lizards are choosing to run on two legs, it seems. But why? It must be giving them some kind of advantage, says Clemente. "But we have no idea what that advantage is."
The value of this research goes beyond gaining scientific knowledge, says Clemente. "It has great conservation importance since it raises awareness of the diversity of lizard species we have in Australia."
The continent's deserts have the highest diversity of reptile species in the world, he adds. "Yet we know relatively little about them.
"Moreover many of the tropical species in this study are under threat from the relentless westward march of the cane toads. Very few people are aware that we stand to lose many species of large tropical varanids."
The data from his research will also improve our knowledge of the biology of Australia's extinct varanids, such as the giant goanna Megalania prisca, which roamed Australia less than 30,000 years ago. It was about 4 metres long and weighed 150kg.
"By using the relationships between morphology, locomotion and habitat formulated in this study, we may be able to make predictions about the locomotory ability and habitat of this magnificent lizard."
Clemente, C. J., Withers, P. Why go bipedal? Locomotion and morphology in Australian agamid lizards. 2058-2065
Full text of the article available on request. To obtain a copy contact Kathryn Phillips, The Journal Of Experimental Biology, Cambridge, UK. Tel: +44 (0)1223 425525 or email firstname.lastname@example.org