Bipedal reptiles caught on film hint at locomotion mysteries
By Alla Katsnelson | June 13, 2008
Darting lizards swing from four-legged to two-legged locomotion like a road bike popping a wheelie, a study published today in Journal of Experimental Biology suggests.
"It's a very interesting paper," said Duncan Irschick, a functional morphologist at the University of Massachusetts who did not participate in the research. "People have never understood why lizards run bipedally."
For most of us animals who regularly move about on two limbs, this capacity harkens back to some advantageous adaptation; primates make tools, for example, and birds" "forelimbs" have become wings. But scientists have long wondered why lizards -- about half of which can run bipedally -- will temporarily abandon the use of their front legs, because their forelimbs don't seem to have evolved any other functional abilities.
Back in the 1950's, one researcher proposed some theories for what kind of evolutionary advantage lizards' bipedalism might have. Having half as many limbs to trip over may help the animals run faster, he suggested, or it may be more energetically efficient. Then, in 2003, a Belgian researcher modeled lizard running at high speeds and proposed a different option: Maybe bipedalism in these reptiles isn't adaptive at all, but simply a passive trait that emerged as a consequence of the way they move. "He showed that you can have a torque on the hip joint that causes the front of the body to pop up," said Christofer Clemente of the University of Cambridge, who led the present study.
Clemente and colleagues filmed lizards running on a treadmill. "We showed that there tends to be this acceleration threshold -- there's this one acceleration where once the lizard hits it, it really has no choice but to go bipedal because the torque starts to move its head up," he said.
Video courtesy of Christofer Clemente
That threshold differed between the 16 species they looked at, and generally correlated to the animals' body-shape. The closer their bodies' center of mass was to their hip, Clemente explained, the lower the acceleration required to drive their posture vertical.
Strangely, the creatures tended to flip into upright mode at a lower speed than the 2003 model predicted. "The only way it can do that is by actively changing its body posture," Clemente said. "That sort of implies that there is some sort of advantage." He suspects that wheelie popping first evolved as a consequence of body posture at high speeds, like the model predicts, but that it eventually did become advantageous in some types of lizards, such as the Australian agamids his group studied, which commonly run bipedally. What's still unclear, though, is what that advantage could be. By analyzing their videos, the researchers found that bipedal runners tired quicker, so the trick probably wasn't giving them extra endurance; neither did it seem to increase their running speed.
Recently, Clemente said, while working on a documentary with the BBC, he was watching a high-speed video of the animals running and noticed the animals maneuver around a stick in their path. "Bipedally, they moved over the stick as if it wasn't even there," he said, while the quadrupeds had more difficulty. Perhaps, he thought, the trait helped the animals avoid obstacles in their path.
"It fits the data we have so far," he said. "We just have to come up with an experiment to see whether it's likely or not." Another possibility, though, might be the boost in visibility the animals get when they're upright, he added.
But while the Clemente's data is intriguing, it doesn't entirely solve the mystery of lizard bipedalism, Irschick said. "What a lot of lizards will do," he explained, "is they start at a standstill, then you spook them and they take off." After swinging into bipedal mode, some stay upright, while others settle back into a four-legged run. But the study tested not how the animals take off from a full stop, but while they were already moving. "Locomotion is very complicated," he said. "It's not exactly the same thing."