I drove a tractor for the first time a few weeks ago, when we were furiously collecting the last of the sap run for maple syrup. A small triumph for me since it seemed so terrifying at first. Trying to hide my confusion, I waited until the last moment to ask, “Which pedal is the brake, again?” Both of them? Right. No chance for a screw up, so I charged ahead. It only took until my second trip – with shouts of “Slow down!” from the trailer behind – for me to figure out why those two brakes weren’t working so well. Turns out that the hand throttle was the missing part of my pedal equation.
Locomotion does not come naturally to me. It does, however, for a huge variety of other living things. Powered flight evolved several times in the history of life: at least once in the ancestors of birds, and separately in insects, pterosaurs and bats. Human inventors have had a much harder time with it: unlike animals, we haven’t progressed much beyond our earliest working designs. Orgel’s second rule applies:
“Evolution is cleverer than you are.”
Thinking about this made me realize that the situation today, where most of us are more familiar with human-engineered forms of locomotion than we are with the natural examples, is kind of strange. For most of our history, the inspiration to look for new ways to get around probably came from seeing it done in nature.
So, I thought for today’s post I would describe some of my favourite examples of the way living creatures exploit physics for mobility.
- Powered flight may be the pinnacle, but the list of animals that have mastered gliding is even longer. It includes flying squirrels and other small mammals, flying fish, gliding ants, frogs, and lizards, as well as some other pleasant surprises: flying snakes, and, believe it or not, flying squid. In this video (from BBC Life), the flying fish sure look like they’re actively propelling themselves through the air – but they’re not. At least, not quite the way birds do it. In the taxiing phase before the flying fish is fully aloft, it beats its tail rapidly in the water to pick up enough speed for a sustained glide1. And while flying fish lack the flapping wings of birds, they can claim better wing design than a lot of true fliers. Aerodynamically speaking, their pectoral fins are about as good as duck wings, and better than the wings of a lot of insects2. How did scientists figure this out? By mounting a stuffed flying fish in a wind tunnel.
- Spiders balloon. This part of Charlotte’s Web was not fiction, although paragliding might be a more accurate description. In many species, babies (and small adults) climb to a high point and release a length of fine silk into the air. These “parachutes” can generate enough lift to keep a spider aloft for many miles, even in a slight breeze. Much like balloonists, the spiders can determine lift by adjusting their initial release of silk, but they they lack streerage – and have no control over where they end up.
- A lot of arctic mammals, like snowshoe hares, have big, furry feet to keep from sinking into the snow. It turns out that snowshoes can be useful in warm places, too: dune crickets of Asia and Africa have flattened, paddle-like feet for efficient running on desert sand. Apparently, it’s a strategy that works quite well, since fossils of an ancient predatory insect with similar paddle feet date back at least 100 million years.
- L. Frank Baum’s Wheelers aside – the race of people in Oz that had four wheels made of fingernail material instead of hands and feet – it’s probably a safe bet that no animal has ever, or will ever, evolve wheels. But plenty of organisms do get around by rolling. When tumbleweeds mature, they dry out and separate from their roots, and, well, you’ve seen the rest. A spherical shape is an effective vehicle for dispersing seeds – so much so that the tumbleweed lifestyle has evolved several time in different desert plants all over the globe. Incredibly, there is at least one animal that also finds rolling to be a useful mode of travel. South American pebble toads turns themselves into tumbling spheres to get away from predators (video from BBC Life). This mimicry strategy is so effective, the frogs sometimes stay balled up in order to blend in with their surroundings after they land. When it comes to maneuverability, spheres have the wheel-and-axle design beat. For instance, James Dyson has shown that vacuum cleaners can be greatly improved by using a freely-rotating sphere instead of the standard set of wheels.
- Rockets and space shuttles use jet propulsion for forward thrust. So do a lot of marine organisms. Octopuses and squid can move forward in rapid bursts by taking water into their bodies and forcing it out in a jet behind. Incidentally, this is what flying squid do to get airborne (photos here).
- Other aquatic animals row and paddle their way along. The webbed feet of ducks and swans are used in this way. Water boatmen and backswimmers are even closer to our man-powered vessels. Water striders do something a bit different: they push back on the surface of the water with special water-repellant legs. And while sculling along, the striders can use their other appendages as rudders.
- Here’s my favourite: some marine organisms actually sail. Siphonophores, like Portuguese Man o’ War and Velella, are not like typical jellyfish. These things are actually made up of thousands of tiny organisms stuck together in a group. It starts with a single individual, with new colony members forming by budding off of the ones that are already there. As the siphonophore grows, individual members become specialized to act as different body parts, like feeding apparatus or the long venomous tentacles below. Portuguese Men o’ War are something in between a colony and a true multicellular organism3. And the comparison to a warship is apt: Men o’ War float by means of a gas filled bladder or “sail” that extends several inches above the water. This structure is so effective at catching wind, the species has spread over all of the warmer oceans of the world. I saw some while racing my sailboat in Miami. But to be a true sailor, you have to harness the flow of the water as well. At first glance, it seemed like this was possible: that cluster of short tentacles just below the sail looks like it could act as a keel. So I dug a bit further. It turns out that Portuguese Men o’ War don’t just drift along with the wind – they move at an angle of about 45° from the downwind direction4,5. These things are broad reaching!
Even cooler: Portuguese Men o’ War come in two asymmetrical forms, sort of a left-handed and right-handed version – and if you set them off side by side, they’ll take opposite tacks. Some people think this might an adaptation to disperse widely or to spread to new food sources6. Perhaps, but there’s no disputing this fact: evolution came up with starboard and port before we did.
One by one, we’re knocking down inventions long thought unique to humans, like agriculture and the world’s oldest profession7. Leslie Orgel, a chemist who dabbled in the origins of life, came up with his “Evolution is cleverer” rule as a response to the creationist argument from lack of imagination. Example? Wheels can’t possibly evolve because there’s just no obvious way to get there. Response: Just because you can’t think of one doesn’t mean it does not exist. Maybe there should be an originality sub-clause added to the rule: If you can come up with it, there’s a good chance evolution already has.
- Mills, C. A. 1936. Science 80: 83.
- Park, H. and Choi, H. 2010. Journal of Experimental Biology 213: 3269-3279.
- Dunn, C. 2009. Current Biology 19: R233-234.
- Mackie, G. O. 1962. International Review of Hydrobiology 47: 26-32.
- Azuma, A. 2006. In: Flow Phenomena in Nature (ed. R. Liebe). WIT Press.
- Woodcock, A. H. 1997. Pacific Science 51: 12-17.
- Gumert, M. D. 2007. Animal Behaviour 74: 1655-1667.