If you stroke a snake, its skin feels slick and slippery (ed: or smooth, at any rate). Yet according to a new study by scientists at New York University and Georgia Tech, snakes actually depend on friction to move.
Snakes crawl by contracting the muscles that run along their body and pushing against the ground. Recently David Hu and his colleagues took a close look at that snake-surface interface. They anesthetized snakes and lay them on a board. By tipping one end of the board, they could see how well a snake’s body could hold onto the surface thanks to friction alone, without any extra forces generated by the snake’s muscles.
Hu and his colleagues discovered that snake scales can actually create a lot of friction by catching on tiny bumps on the surface they’re lying on. (They only feel smooth if you stroke them tailward.) The scientists found that the scales can generate twice as much friction if a snake is sliding forwards than if it is sliding sideways.
To see if they were right, the researchers built a mathematical model of a snake on the basis of their observations. They then changed some of the variables, such as the smoothness of the surface on which their mathematical snake crawled to predict how a real snake would perform.
Here, for example, is what happens when a milk snake tries to slither across a smooth plastic surface. Without any bumps on which it can catch its scales, it crawls in place.
Hu and his colleagues then let their snakes crawl on a rough surface, but first put them in a cloth sleeve.The snakes could push against the surface, but because they couldn’t lock their scales onto it, they again slithered in place.
The model Hu and his colleagues created slithered a lot like real snakes do, as shown in this simulation (the red dot shows the center of mass).
But the scientists recognized how they could make the model match reality even more closely. In their original model, the snake lay completely flat against the ground. That’s not how snakes actually slither. They only make contact with the ground at a few spots along the length of their bodies. This picture shows a snake crawling across a plate of gelatin. The photo is lit by polarized light, which creates bright reflections where it hits places where the snake is pushing against the gelatin. Rather than creating a long, snake-shaped stretch of light, the snake creates just a few patches where it is pushing against the plate.
The researchers decided to see what happened if they let the snakes in their model lift up their bodies the way real snakes do. Hu and his colleagues found that their snakes slithered 35% faster and boosted their efficiency by 50%. In this movie, the body is colored red whe the snake has lifted its body, and blue where it is concentrating its weight on the ground. The model works better because the snakes can press their weight only on the spots where the force of friction is highest in the backwards direction. By continually redistributing their weight, the snakes can slither as quickly and efficiently as possible.
A snake may look a little silly trying to crawl while wearing a sleeve. But such humiliations can help us appreciate just how graceful snakes really are.
When the evenings get particularly thick with mosquitoes where I live, I sometimes sit out in the yard with my daughters and look up at the fading sky. Before too long, a single bat will usually flit out of the nearby trees and start flying circles around the house, scooping up bugs along the way. We can barely make out the bat’s wings as it takes its laps, a flicker of membranes. And so it was a revelation to spend some time earlier this week with two Brown University biologists, Dan Riskin and Sharon Swartz, watching slow-motion movies of bats in flight. There’s a lot going on up there.
Bats evolved about 50 million years ago from squirrel-like ancestors. They probably made their first forays into the air as gliders. Like living gliders, they used flaps of skin to increase their surface area, letting them glide further. Their hands evolved long spindly fingers that were joined by membranes. Some early bat fossils suggest that they may have shifted from gliding to alternating between gliding and bursts of fluttering. Eventually bats evolved sustained powered flight.
Bats evolved a way to take advantage of the same laws of physics birds use to fly. And many scientists who have studied bat flight in the past have basically treated bats like leathery birds. Yet there’s no reason to assume that this should be so. After all, it would not be surprising to find that the way the feathers on a bird’s wing react to air pushing against them are different from the way the stretchy membranes on a bat react. Birds don’t have wing surfaces connecting their front and back legs, like bats do. And while birds only have a couple joints in their wing skeleton, such as at the elbow and wrists, bats have lots of knuckles they could, in theory, bend selectively to alter their wing surface. Bats also have lots of sensitive hair cells on their wings that appear to track the speed and direction of the air flow, and the information they get from the hairs may help them make fine adjustments to their wings many times a second.
And when scientists like Swartz and Riskin study bats, they discover, in fact, that bats are not birds. Bats fly more slowly than birds, but they maneuver more effectively. Bats fly cheap compared to birds. A hovering bat use 60% less energy than a hovering hummingbird. These sorts of discoveries suggest that if you’d like to make an agile, efficient, and tiny flying robot (and who doesn’t?) it might be worth looking for some inspiration in bats.
The problem with looking to bats for inspiration is that scientists are only starting to figure out bat aerodynamics. What’s really challenging to figure out, however, is the difference between the aerodynamics of birds and bats. Riskin and Swartz use lots of tools to find the answer. They paint bright dots on bats and then film the animals as they fly in wind tunnels. The biologists can then use computers to create models of the bat wings and calculate the speed and direction of each dot at each instant of flight. They can spray mist into a tunnel and then film the swirls the bats leave in their wake. From this data on real bats, bat researchers can then test out simulations on computers to see if they produce the same forces and swirls of air as they see in their wind tunnels.
A close look at these movies reveals that bat flight is just too complex for simple labels, like upstroke and downstroke. The shoulder of a bat starts rotating upwards before the wrist, which move up before the fingers. The fingers on each hand don’t move in sync with each other. A joint on the left wing is often out of sync with the corresponding joint on the right wing.
Physicists like to treat wings as rigid surfaces because the math involved causes fewer headaches. But that’s a gross simplification when it comes to bats. The bones in a bat’s hands are surprisingly flexible, and the skin of the bat wing is never fully stretched out during its stroke. In fact, the region of the wing close to its body actually balloons out to double its surface area during each flight stroke. Bats probably use this ever-changing wing surface to control their lift and drag, so that they can make tight maneuvers without stalling.
Bats have clearly evolved a sophisticated flight system, but they face some awkward challenges when they’re not flying. Birds only need two limbs for flying, leaving their remaining two relatively free to land and walk around on the ground. Bats, on the other hand, make their hind legs part of their wings, and so natural selection has to strike a compromise between several different functions. And while birds can stop flying by using their feet to land on the ground, most bats have to use their feet to hang upside down.
To figure out how bats manage this feat, Riskin turned the typical biomechanics lab upside down. Scientists can measure the forces of a running animal by putting a force-sensitive plate on the floor; Riskin put his on the ceiling. In his recordings of landing bats, he’s discovered two strategies. In one species that lives in caves, the bats make an elegant backwards flip combined with an upside-down cartwheel, so that they can land with just two feet.
In a species that hangs from trees branches, the bats use a very different technique. They swoop in without a cartwheel, and bring both feet and both hands upward to grab onto the tree. And they hit the tree hard. The cave bats land with a force that’s twice their body weight; the tree bats generate forces as high as eleven times their body weight.
This discovery (published this week in the Journal of Experimental Biology) illustrates an important fact about bats–a bat is not a bat is not a bat. Bats live in many environments and are adapted to eating many different kinds of food, from moths to fruit to cow blood. They’ve adapted to these different ways of making a living, in part by evolving different ways of moving around. If you’re a bat flying towards a wall of rock, you don’t want to hit it too hard. But if you can grab a branch that can absorb the shock, you can skip the fancy acrobatics.
That same lesson emerges from how bats behave on the ground. With their delicate legs yoked together by their wings, you might expect that bats don’t do very well on the ground. And indeed, most species won’t win any track and field medals. When Riskin puts a typical bat on a treadmill, they stumble around. If the treadmill goes too fast they start to lose all control. It’s likely, then, that the ability to walk efficiently and to run was lost in the early evolution of bats. But millions of years later, that ability evolved once more in at least two species.
One place where bats have taken to the ground again is New Zealand. The remarkable isolation of New Zealand left it without big predators and without any mice or other ground-dwelling mammals. One species, the New Zealand short-tailed bat, has adapted to this niche. While it can still fly, it now moves around comfortably on the ground in search of bugs, nectar, fruit, and pollen.
Riskin found that New Zealand short-tailed bats walk comfortably on a treadmill, using the same pendulum-like movements that other walking mammals use to save on energy. But when other mammals have to move faster, they break into a run so that they can store extra energy in their tendons as they hit the ground. The New Zealand short-tailed bat can’t make the transition from walking to running.
But another species of bat can make that switch. A vampire bat will walk on the ground to sneak up on its victim. If its victim tries to get away, it can scramble in pursuit. Riskin found that if he put vampire bats on treadmills, they can walk like New Zealand short-tailed bats. But when he speeds up the treadmill, they suddenly switch to a bizarre form of running. Instead of pushing off with their hind legs, like a squirrel, they use their long, heavily muscled arms. It’s a mammal version of front-wheel drive versus rear-wheel drive.
The difference between the two species of ground-moving bats is not surprising when you consider where they live. Bats on New Zealand didn’t pay any cost for evoling into slow walkers, because life was pretty easy (at least before humans showed up with their rats and other assorted camp followers). But vampire bats evolved in a more competitive environment where they had to adapt to moving prey.
Once bats evolved flight, in other words, they did not stop evolving. Their movements have been changing in astonishing ways for millions of years, and will continue to change as long as bats fly, walk, or run across the Earth.
Our bodies are bunches of atoms, and like any rock or star or other bunch of atoms, we have to obey the laws of physics as we move. But each species obeys those same laws in its own way. My cat leaps onto my desk most mornings, his grace unblemished by the paper clips and computer cables he kicks onto the floor. A maple tree outside bends in the wind, a happy medium between flopping over and snapping in two. A hawk arrives at the tree and lands precisely on a branch. On their own, our eyes cannot tell us much about the different ways in which living things move. We can’t see the invisible vortices of air spiraling behind a hawk, the stresses experienced by different parts of the leaning maple, the thrust and torque generated by my cat as he rises into the air.
The first glimpse into this invisible world came in 1872. Leland Stanford, a railroad tycoon and the founder of Stanford University, spent a lot of time watching his race horses run. He was sure that when they trotted, there were moments when all four legs left the ground. Legend has it that he even bet $25,000 that they did.
Stanford paid a famed landscape photographer named Eadweard Muybridge to find out if he was right. Muybridge had horses trot down a path strung with threads connected to a row of cameras; when the horses snapped the threads, the cameras snapped the pictures. It took Muybridge years to perfect a shutter fast enough and film sensitive enough to capture the images (he also needed some time off to defend himself–successfully– against the charge that he had murdered his wife’s lover). But in 1877 he was finally able to give Stanford his answer. Horses do indeed bring all their legs off the ground during each cycle of a gallop. Later, Muybridge built contraptions that could display his pictures in quick succession. His moving pictures brought the horses back to life.
Eventually Muybridge made his way to the University of Pennsylvania, where he photographed many other four-legged animals. He found that whenever they ran, they lifted all their legs off the ground at once. Even two-legged humans did. That complete lack of contact with the ground, in fact, came to define the act of running. Muybridge’s photographs also revealed other rules. When four-legged animals walk rather than run, their feet usually hit the ground in the same pattern: hind left, front left, hind right, front right. Here’s a diagram of the cycle in a walking horse.
Muybridge opened the way to the scientific study of life in motion. These days, biologists can film animals with high definition video cameras and use computers to calculate the speed and direction in which different body parts move. They can put sensors on animals or have them run over force-sensitive plates to measure the thrust they generate with their muscles. Instead of Muybridge’s flickering photographs of horses, we can enjoy their glacial grace in movies like this, from researchers at the Royal Veterinary College:
These superior tools have allowed scientists to discover some of reasons that animals move the way they do. The cycle of footfalls in a walking dog or a walking elephant, for example, is the best way to keep a four-legged animal stable. Walking is not just stable, but also efficient, because it turns animals, in effect, into pendulums. A pendulum can swing for such a long time because it continually recovers some of its energy. On its downward stroke, it’s powered by the force of gravity; when it reaches the lowest point of its arc it has so much energy that it can counteract gravity and swing upward. When you walk, your body behaves like an upside-down pendulum: the foot you plant in front of you is the pendulum’s axis, your center of mass the hanging weight. In the beginning of your stride you work against gravity, vaulting your center of mass upward with your leg until you reach your highest point. Gravity then takes over, and your body swings downward until your other leg hits the ground. The next stride is even easier. You can use the energy given to you by gravity to vault yourself into your second and all successive steps, just as a pendulum reclaims its energy in each swing.When you run, however, you stop behaving so much like a pendulum and begin behaving more like a pogo stick. Now when you first plant your leg, your body sinks down on it instead of rising up. Your leg actually acts as a brake for your body, and so your center of mass is at its lowest point when your acceleration is lowest. Meanwhile your tendons are acting as springs. As they stretch and snap back, they store and release energy, just like the spring in a pogo stick, and propel you upward and forward.There are many other ways to move around, of course. If you’re a cockroach or a centipede, you can use more than four legs. It turns out that many of the same rules that govern walking and running among us vertebrates also apply to invertebrates. Meanwhile, other researchers are discovering the rules behind other kinds of motion, like flying, jumping, and swimming.
For all the advances in biomechanics, however, it turns out that a lot of people still live in a pre-Muybridge universe. A team of biologists, biophysicists, and a veterinarian in Hungary recently did a survey of the depictions of animals in museum displays and other places. In each case, the researchers determined whether the poses of the animals followed the basic rules for how four-legged creatures move.The grades they handed out were pretty dismal. Museum displays were wrong 41% of the time. Taxidermy catalogs were wrong 43% of the time. Animal toys were wrong half the time. And, incredibly, coming in dead last were animal anatomy books–63.6% wrong.
Here, for example, is an illustration of a horse not being a horse. B is a diagram showing its limbs. C and D show two real poses it could have taken.
And here’s a picture of an aardwolf in a museum display doing what no self-respecting aardwolf would do.
I was surprised that there are so many biomechanical mistakes out there, especially on such a simple matter of how to position an animal’s legs. To be fair, a lot of the biomechanical misktakes in museums are baggage they carry from the past. Today museums are following Hollywood’s lead and are working with biomechanics experts.
John Hutchinson of the Royal Veterinary College has done some pioneering work on how dinosaurs walked, and his research is the basis of an exhibit called Be The Dinosaur. Here’s a sample of the computer simulations the exhibit has to offer.
I first became fascinated with biomechanics in the mid-1990s, and I often dreamed of embedding movies on the pages of my articles. Words could only go so far, and photographs couldn’t go that much further. Most of my futuristic dreams have not come to pass, or have proven to be banal disappointments. But when it comes to writing about biomechanics, the future is here, and it is good. This will be the first of what I hope is a long line of blog posts about life in motion, illustrated with moving images that Muybridge could not imagine.
But the scientists recognized how they could make the model match reality even more closely. In their original model, the snake lay completely flat against the ground. That’s not how snakes actually slither. They only make contact with the ground at a few spots along the length of their bodies. This picture shows a snake crawling across a plate of gelatin. The photo is lit by polarized light, which creates bright reflections where it hits places where the snake is pushing against the gelatin. Rather than creating a long, snake-shaped stretch of light, the snake creates just a few patches where it is pushing against the plate.
