Ants are not known for their swimming skills, but a new study suggests many are surprisingly adept in the water. Out of 35 species of tropical ant studied, over half could “swim” on the water’s surface, which helps them escape predators and drowning, according to the research, recently published in the Journal of Experimental Biology.
Walking on water isn’t magical, though—it all comes down to physics.
Tiny animals can rest on the water’s surface because their weight is supported by the water’s surface tension, the force created when water molecules cling to each other. (See “How Snails Walk on Water Is a Small Miracle.”)
“Surface tension is a property of the air-water interface that makes it behave, roughly speaking, like a trampoline,” said John Bush, a professor of applied mathematics at the Massachusetts Institute of Technology who studies fluid dynamics.
There are more than 340 species of water striders, insects that are specialized to spend nearly their entire lives at the water’s surface. They belong to a category of water walkers called gliders, which also includes fishing spiders and pygmy geckos.
When the insects press their legs down on the water, they make tiny dents in the surface but don’t break through it. The water’s surface tension pushes the water strider forward.
Their stride also helps: Water striders glide over the water’s surface by sweeping their middle pair of legs backward in a sculling motion, like the oars of a rowboat. This creates swirls beneath the water’s surface that propel them.
Finally, water striders’ legs are coated in a layer of waxy hairs that renders them water repellent. (See “Hairy Legs Help Bugs Walk on Water.”)
Fishing spiders are relatively large spiders found throughout North America. They tend to live on the edges of streams and ponds, where they feed mostly on insects (although they are capable of catching small frogs, tadpoles, and minnows). They hunt by detecting the vibrations in the water made by their prey.
Like water striders, fishing spiders’ legs are covered with water-repellent hairs that help keep them afloat.
Fishing spiders have several ways of getting around on the water’s surface. When they’re not in a rush, they row in a manner similar to that of water striders. (Also see “Walking Began Underwater, Strolling-Fish Discovery Suggests.”)
To go after prey or get away from predators, fishing spiders can speed up into a gallop.
“They sort of bounce along the water’s surface,” said Robert Suter, professor emeritus of biology at Vassar College. “They take their first, second, and third pairs of legs and push down and backwards, and that wafts them into the air. They’re airborne for a few centimeters and then they land and push backwards again with the same legs.”
Fishing spiders can also sail: Taking advantage of the wind and the slipperiness of the water’s surface, the arachnids stand with two or three pairs of legs up in the air and allow the wind to catch them and propel them along the water’s surface.
Suter said this might be a form of cheap locomotion for the spiders: A way to travel a long distance while expending almost no energy.
The Brazilian pygmy gecko, at less than one and a half inches (about 4 centimeters) long, could drown in even the smallest puddle. But it has evolved some tricks to stay safe in its rain forest home.
Because the reptiles are so tiny, they can rest on the surface of water like a water strider or fishing spider. Pygmy geckos also have water-repellent skin that keeps them from breaking the surface tension. (Also see “Watery Gecko Grip Could Lead to Stickier Tape.”)
The BBC natural history series Life captured footage of a pygmy gecko surviving a rainstorm by walking across a puddle without breaking the water’s surface tension.
Basilisk lizards, a group of tree-dwelling reptiles found in Central America, earned the nickname the Jesus Christ lizard for their ability to run on water (see above picture). When startled, the animals can run on their hind legs for about 15 feet (4.5 meters). (See “How ‘Jesus Lizards’ Walk on Water.”)
The basilisk is considered a “slapper,” or a group of water walkers whose weight is too great to be supported by the water’s surface tension.
“Large animals cannot reside at rest on the water’s surface, but must be in a constant state of motion,” said MIT’s Bush.
As the lizards’ feet push down on, and break through, the water’s surface, the water beneath their feet pushes back. This keeps them up just long enough to take their next step and stops them from sinking.
But to stabilize themselves and stay upright, the reptiles also produce huge sideways forces to catch their weight and keep their bodies upright.
Western grebes are North American water birds that spend nearly their entire lives in the water. Grebes have short wings and powerful legs that are set far back on their bodies, making them ungainly on land.
These birds are best known for an elaborate and beautiful mating ritual known as “rushing.” The male and female turn to one side, lunge forward synchronously, and rise up on their feet with their wings beating furiously. With their bodies completely out of the water, the birds race across the water’s surface side by side in perfect unison for up to 30 feet (9 meters).
When scientists used high-speed film to analyze western grebe rushing, they found the birds made as many as 22 steps per second.
The researchers also discovered that grebes use their unique feet during rushing. Grebe feet are not webbed like a duck’s, but are lobed, with toes that flatten out like paddles to help them stay afloat.
Even animals as big as dolphins can “walk” on water sometimes.
Whale and Dolphin Conservation Society scientist Mike Bossley has studied the bottlenose dolphins in the Port River of Adelaide, Australia, for over 25 years. Recently, he documented a behavior called tail walking.
By vigorously beating their tail flukes back and forth, dolphins can lift their bodies vertically out of the water and propel themselves along the surface with only their tails submerged. (Watch a video of tail walking here).
Dolphins in captivity can be trained to tail walk, but in the wild, the behavior is extremely rare. Bossley originally observed two wild female dolphins tail walking. Over time, the behavior spread throughout the Port River dolphin community, and he has observed four other dolphins practicing the technique. (Also see “Dolphins Have Longest Memories in Animal Kingdom.”)
Walking on water doesn’t always have any obvious benefits: The scientists believe the Port River dolphins are simply doing it for fun.