Some animals possess an extraordinary ability to move across the surface of water, seemingly defying gravity. This phenomenon results from a sophisticated interplay of biological adaptations and physical principles. This article explores the science behind this unique form of locomotion.
The Physics of Water Walking
The primary principle enabling water walking is surface tension, which causes the water’s surface to act like a flexible, elastic film. Water molecules are attracted to each other, forming strong cohesive bonds, particularly at the surface where they are exposed to air. This molecular attraction creates a delicate, membrane-like layer that can support lightweight objects.
Their limbs are often hydrophobic, meaning they repel water. This water-repellent property is typically achieved by microscopic hairs or structures on their legs that trap air, preventing the water from wetting the limb and breaking the surface film. Distributing weight over a large surface area also plays a role, reducing the pressure on the water’s surface and allowing the animal to remain afloat.
Insect Water Walkers
Water striders, belonging to the family Gerridae, are prominent examples of insects that master water walking. These small insects glide across ponds and slow-moving streams.
Their bodies are lightweight, and their long, slender legs are covered in thousands of tiny, water-repelling hairs called microsetae. These hairs trap air, making their legs highly buoyant and preventing them from sinking.
Water striders distribute their weight across their six legs, with the middle pair acting as oars for propulsion and the hind legs for steering. This specialized leg structure allows them to create dimples in the water’s surface without breaking through the tension. They can move at over 100 body lengths per second, an impressive speed that highlights their adaptation to the water’s surface.
Reptile Water Runners
Larger animals, such as the basilisk lizard, employ a different strategy to run on water, as their weight is too substantial to rely solely on surface tension. Often called the “Jesus Christ lizard,” the basilisk is native to Central and South American rainforests. They achieve this through rapid leg movements, specialized foot anatomy, and speed.
The basilisk lizard’s hind feet are notably large and feature fringes of skin along their toes. When running across water, these fringes unfurl, significantly increasing the surface area of the foot.
Their water-running gait involves three phases: the slap, the stroke, and the recovery. In the slap phase, the lizard forcefully strikes its foot flat against the water, pushing water downwards and creating an upward force. This action also traps temporary air pockets beneath the foot, which help support its weight.
Following the slap, the lizard pushes its foot backward through the water, propelling itself forward in the stroke phase. The foot is then quickly pulled out of the water during the recovery phase, often through the air cavity created, to minimize drag.
Basilisks can take up to 20 steps per second, maintaining the necessary speed to stay on the surface for short distances before gravity eventually causes them to transition to swimming. Smaller basilisk lizards can run further than adults, as their lighter weight makes it easier to generate sufficient force to remain above the water.
Survival and Ecological Role
The ability to move across water offers significant advantages to animals. For water striders, this locomotion allows them to hunt prey, such as insects and larvae, that fall onto the water’s surface. Their rapid movement also helps them escape from aquatic predators that might lurk beneath the surface. This adaptation positions them as effective surface predators.
For basilisk lizards, running on water is primarily an escape mechanism from predators. When threatened, they can drop from trees into the water and sprint to safety across the surface, surprising their pursuers. This specialized locomotion is a key survival strategy. The ability to utilize the water’s surface also aids in dispersal, allowing animals to access new areas for foraging or mating.