Geckos are known to be experienced climbers, able to adhere to any surface thanks to the billions of small, hair-like structures at the bottom of their feet. Now it turns out that small lizards can also glide along the surface of high-speed water to evade predators. They can not do it for a long time; the energy expenditure required is too great. But it's incredible that they can do it to everyone. Scientists think they have identified the mechanisms underlying the prowess, described in a new article in Cell Biology .
The project began when co-author Ardian Jusufi, then a postdoc in the University of California laboratory, Berkeley biophysicist Robert Full, was on holiday in Singapore during the monsoon season. One day, after a big rainstorm, he caught a gecko that touches the water to escape a predator on video. The film astounded everyone in the laboratory when he showed it to him. "It was super strange and unexpected, so of course we had to test it," says co-author Jasmine Nirody, another complete ex-student who now splits her time between Rockefeller University and the University of Oxford.
There are several creatures in nature that can walk on water, but they employ different mechanisms depending on their size. The small and light water striders, for example, rely entirely on surface tension to stay afloat, while the larger and heavier basilisk lizards use a slap motion with their feet creating pockets of bubbles. air to avoid sinking. Standard theoretical calculations set very strict limits for how small an animal must be to use surface tension and how large it must be before the surface slapping mechanism is practicable.
The geckos fall somewhere in the middle. They are too large to rely solely on surface tension and too small to generate enough force to run along the surface of the water without sinking. Yet they can still make the enterprise somehow at the speed of light, almost a meter a second. That's why the Full team decided to investigate further.
They used laser cutters to create entry and exit holes in a large plastic box to create a water tank and then built two wooden ramps for their group of Asian geckos ( Hemidactylus platyurus ) could easily enter and exit the water. A pair of high-speed cameras have been positioned above and to the side at right angles to capture movement. The geckos would have been placed on the entrance ramp, and team members would have slightly touched the tail to scare them to swim.
The geckos have developed a series of complicated mechanisms for walking on water.
the geckos have developed a series of complicated mechanisms for walking on water. There are two objectives when you touch the water: keeping your body above the surface and propelling it forward. For the first time, the gecko combines the superficial slap and surface tension, aided by their unique hydrophobic skin that repels water, according to Nirody. A drop of water will sit over the skin of a gecko.
Raising their bodies above the water reduces resistance, making it easier for the geckos to push themselves forward than if they were completely submerged. They also use a flickering movement with their bodies and tails, just like swimming. "If you look at them from the top, it looks like they're swimming very fast," says Nirody. "And then you look at them from one side and you realize that the upper body and their legs are completely out of the water, even if they are still doing the swimming movement that helps them to push them forward."
To verify that the surface tension actually played a role in the skimming ability of the gecko surface, the researchers added a surfactant (dish soap) to the water. Surface tension occurs because water molecules tend to adhere to one another (molecular adhesion), forming a sort of supporting film to keep very light creatures afloat. The addition of soap causes the molecules to lose that viscosity. Put a rag of water in soapy water and it will sink because it depends entirely on surface tension. But a basilisk lizard is not affected because it is based entirely on the surface that crashes.
Once again, the geckos fall somewhere in the middle. They did not sink, but Nirody and others . he found that the addition of soap to water reduced gecko speed by half, most likely because their bodies were much lower in water due to the decrease in surface tension. "We knew they could not keep their full body weight by crushing the theoretical calculations on their own," Nirody said. This test has shown it.
The geckos showed some interesting reactions to soapy water. About half will double their efforts to swim as fast as possible, even if their speed has been severely limited. The other half, after the first blows, simply surrendered and plated, sinking to the bottom. The geckos can hold their breath for several minutes, so they are not in immediate danger, even if the team members saved them after about 30 seconds. "We hypothesized that if they can not dart away in time, rather than slowly brush against a predator, it's better for them to hide under the water and hold their breath," says Nirody.
One of the objectives of this research is to improve the design of bio-inspired robots. The authors point out that modeling a robot on the basilisk lizard would work, but it would require a lot of energy and some kind of active stabilization to be really functional. A wavy tail similar to the gecko could help with this last problem and strengthen forward propulsion, while coating the robot with a hydrophobic material similar to the gecko skin structure could significantly reduce the resistance. "Nature has so much to teach us," says Nirody. "He built all these amazing machines to watch and learn from."
DOI: Current Biology 2018. 10.1016 / j.cub.2018.10.064 (Information on DOI).