– Watch a lizard walk on water:
It’s one of the strangest sights in nature: lizards running upright across water. Watching their thin hind feet dip into the liquid, you expect them to sink or fall over, but they just keep going like a human sprinting for a bus.
Biologists refer to them as basilisks, but most people call them Jesus Christ lizards.
For a long time, George Lauder, a professor of biology at Harvard University, has wondered how they do it. He looked around for someone to work on the problem and he found Tonia Hsieh, a graduate student in biology who thinks lizards are cool. “I’ve caught small animals ever since I could walk, and I’ve been obsessed with lizards for years,” she says.
A big basilisk, 6 inches tall and weighing a hefty 5 ounces, can run across the surface of a pond at a speed of 3 to 4 miles an hour. Smaller, lighter creatures, like an insect known as a water strider, can also walk on water, as can some iguanas right after hatching. What’s unique about basilisks is that even the largest adults can run across water without getting more than their feet wet.
Hsieh hoped this project would get her to tropical Latin America where she could study the pitter-patter of little basilisk feet in the wild. But instead, she had to settle for five young lizards from a wholesale supplier. Hsieh housed them in aquaria and fed them crickets and mealworms dusted with vitamins and minerals.
The lizards were then prodded to run along a water track a few feet in length. The track was seeded with tiny, silver-coated glass beads. A sheet of laser shone on the beads, lighting them up and permitting Hsieh and Lauder to film the water movements with a high-speed video camera. This setup provided frame-by-frame images of fluid flow as the lizards ran across the water. What they show is lizard feet, with enlarged scales on their toes, slapping cavities into the surface, then pulling out before the water closes over them.
‘The babies are most fun to watch,because they are so light and run so quickly, they actually bounce across the water. Larger lizards sink deeper, so they flail a lot. It looks exhausting, but they manage to get across.’
– Tonia Hsieh,
“The babies are most fun to watch,” Hsieh comments. “Because they are so light and run so quickly, they actually bounce across the water. Larger lizards sink deeper, so they flail a lot. It looks exhausting, but they manage to get across.”
Slap and stroke
It was challenging to catch the details of a Jesus Christ lizard run. “The foot needed to fall within two or three inches of the transverse light sheet, something that was frequently missed,” Hsieh explains. The lizards run with their long, thin tails in the water, which probably helps to keep them upright. But its sweeping motion through the water would frequently obliterate the picture.
Enough persistence, however, yielded clear images of a slap and stroke gait that is the secret of successful water walking. The forward foot slaps vertically down into water with considerable force. The foot then sweeps back through the water, thrusting the lizard ahead. This S-shaped stroke also generates forces out to the side to keep the animal from toppling over. For the lizard to stay upright and move ahead, all this has to happen before the water cavity generated by the slap and stroke rushes in over the foot.
Before watching them so closely. Hsieh thought of these water runners as “graceful animals, padding across liquid surfaces with smooth clean strides.” But they’re actually “quite clumsy,” she discovered. As much as one out of five times, they tripped over the water, or ran into the sides of the tank. “That also made the experiments more challenging to do,” Hsieh recalls.
Jesus Christ lizards must fall over in nature, too. It’s not something that would bother them, however, since they’re good swimmers. The main reason they get up and run across water is to escape enemies.
Hsieh sees studying water walkers as part of a larger effort to understand how animals deal with specific surfaces when they run. For example, how do they balance stability and maneuverability when traveling rapidly over slippery, squishy, liquid, gravel, and other surfaces? She doesn’t think that an individual animal adopts completely different ways of locomotion when running across different surfaces, but that there is a continuum within which they operate.
Once this is understood, some practical value might come from the knowledge. It might, for example, be used to build new types of toys and robots. The latter may someday be used to travel over strange surfaces on different moons and planets.