The lizards range in size from less than 0.01 ounces (2 grams) upon hatching to more than 7 ounces (200 grams) as adults. Throughout their size range, they can run across water on their hind limbs at about 5 feet (1.5 meters) a second for a distance of approximately 15 feet (4.5 meters) before they sink to all fours and swim. The lizard is often called the Jesus Christ lizard in Central America because of its seeming ability to walk on water.
Most animals that attempt to walkor runacross water immediately sink toward their supporting limb because water, unlike solid ground, offers little support or resistance. "It's as if you were not paying attention and thought you were putting your elbow on the table top and missed," Hsieh said.
To understand how the Jesus lizards move across the surface of the water without tipping over, Hsieh and Lauder filmed them with a high-speed video camera as they ran across a water tank. The water was sprinkled with tiny, reflective glass beads that sparkle when lit.
Using computer software, the researchers are able to overlay adjacent video frames to see how the beads move. This allowed them to calculate the forces generated throughout the lizards' stride.
The stride is divided into three phases: the slap, the stroke, and the recovery. During the slap the foot moves primarily vertically downward. During the stroke it moves primarily backward. And during the recovery the foot moves up and out of the water, returning to the start position of the next step.
According to Hsieh, the lizards need to generate forces during these phases sufficient to overcome the challenges of staying on top of the water and upright. "It sounds obviousyou can't run if you're falling overbut it's even a greater challenge for the lizards, because they are dealing with a squishy surface," she said.
According to analysis of their video, the lizards generate force for support during the slap phase, in which the lizards' foot plunges straight down, pushing water down and away from the foot while creating a pocket of air surrounding the foot.
The support force generated by the slap is sufficient to keep the lizards' bodies above the water's surface during the stroke phase in which they propel themselves forward by kicking their leg back through the water.
To stay upright, the lizards also create forces off to the sides called lateral reaction forces.
"If you trip and fall off to the right, you instinctively reach out and push off something," Hsieh said. "It's the same thing with lizards, but they do it all with their feet. When their body moves off to the right, they want to produce a force with their feet to push back left."
To create these forces, the lizards push outward during the slap phase as they fall toward their supporting limb. Then they pull toward their body during the stroke as their center of mass shifts away from the supporting limb. The effect keeps the lizards upright as long as they are moving.
"Think of it as riding a bike. The minute you stop pedaling and the bike comes to a halt, you will fall over. If the lizards stop running, they will fall over. What they need are forces to maintain stability while they are moving," Hsieh said.
Earlier research conducted in Full's lab at the University of California at Berkeley showed how legged insects generate lateral forces for stability while running on solid surfaces. He said it is novel to see that these forces also play a pivotal role when running on water.
According to Hsieh and Lauder, the mechanics of lizard water-running challenge a major established rule of legged locomotion known as the spring-mass model.
In the model, energy is stored in an animal's limb during the start of the contact, or slap, phase and then released to help propel the animal forward.
"This does not occur during basilisk water-running, which suggests that the hindlimb no longer functions like a spring but instead acts more like a piston, limited to only generating force during a step," Hsieh and Lauder conclude in their study.
Now that the researchers have shown how the mechanics of locomotion differ between walking on a solid surface and water, they hope to tease out the subtle differences of how locomotion works on various other types of surfaces.
Shi-Tong Tonia Hsieh has received funding from the National Geographic Society's Committee for Research and Exploration to study the behavior and locomotion of leaping blennies of the tropical Pacific.
John Roach is a freelance science journalist based in Ketchum, Idaho
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