Why move at all?
Animals have a problem. They need to eat. To do that, at some point in their lives, they’ve got to move about and find food. Plants never have to worry about this, they can make all the chemicals they need to live and grow by the process of photosynthesis. By capturing the energy of sunlight, they are able to turn the simple chemicals around them: Carbon Dioxide, Oxygen and mineral compounds, into the complex organic chemicals: Carbohydrates, Proteins and Fats that they need. If animals could photosynthesise it would be a very different world indeed, however, they can’t because they’re too dense and opaque. The only way animals can get those essential energy-giving and bodybuilding chemicals is to eat either plants, or other animals.
There are of course some animals that move very little throughout their lives: corals, sponges, barnacles and sea anemones, to name but a few. But even the world’s most sedentary animals often have a surprising double life, with a larval stage that travels far and wide before it attempts to settle in a suitable location.
The need to travel about in order to: find food, escape predation by bigger animals, find shelter and find a mate, is central to the lives of all animals. So where do you start if you’re going to look at animal locomotion? Well, how about walking? We do it all the time and we do it for most of our lives. It can’t be that complex? Can it?
What’s so special about walking?
There are many land animals that don’t bother with walking (or legs) at all. Nematode worms manage to burrow through the soil in much the same way as an eel swims through water. Molluscs (slugs and snails) are swimmers too, travelling with the aid of ordered ripples of muscle contraction and a specially created sheet of watery slime. The Annelids (earthworms) have a clever way of pulling themselves along by alternately anchoring and stretching their body segments. You will find countless thousands of these animals living in the soil of every continent on Earth except Antarctica. They are essential to the proper functioning of every ecosystem on the planet. However, it’s the animals with legs that have really colonised the surface of the Earth and dominate to this day.
Why don’t animals have wheels?
Although legs and feet aren’t as efficient as wheels and an engine, they can cope with hills, rough tracks, ditches, stairs and all manner of obstacles that wheeled vehicles would find impassable. But even animals that live in flat places don’t have wheels. This is because there is no such thing in nature as an axle with a bearing which can rotate freely around it, an essential component of wheeled vehicles. An animal’s limb can only go so far in any direction before it has to be “reset” by returning it to its starting position. It’s attached to the rest of the animal by tendons, muscles, blood vessels and nerves that would all tear and snap if the limb rotated freely. Humans and the other apes come close with their special “ball and socket” shoulder joint that allows us to reach all round with our arms and swing under branches (we still can’t spin them round in a true circle however).
A look at legs.
The first many-legged beasties to walk on earth were exactly that; many legged. They were the arthropods: millipedes, centipedes, scorpions, spiders, woodlice, springtails, and insects (in roughly that order). Their strong, chitin based exoskeletons protected them from damage by both the sun’s rays, and the harsh terrestrial environment. But as well as protection, their body armour, made of segments that could move freely, gave them another great advantage: jointed legs. These were animals really built for colonising the surface of the planet, and they did just that from around 420 million years ago. Today, they are still the most successful group of animals on Earth. 80% of all the animals known to science are arthropods and you find them everywhere, including Antarctica. The arthropods don’t win all the design awards however, as their exoskeletons cause them a few tricky problems. Firstly, they can’t grow without shedding their skins, a dangerous thing to do as it leaves them temporarily soft-bodied, unable to move and at risk from predation, water loss and physical damage, whilst they wait for the new bigger skin underneath to dry. Secondly, the skeletons, though strong for their weight, need to become so bulky and heavy to support the weight of a large animal that they become impractical. This is one of the factors that creates an upper limit on the size of terrestrial arthropods (the other is breathing).
The next big step in leg design happened around 340 million years ago when some particularly muscle-bound fish started to take advantage of tasty terrestrial titbits of food. Their strong “walking fins” formed the blueprint for the pentadactyl limb design that all tetrapod vertebrates (animals with four legs and backbones) share. Their descendants were the amphibians, reptiles and eventually the mammals. Unlike the arthropods, the internal skeletons of vertebrates are made of living tissue and grow as the animal does. Although bony skeletons are much heavier than the skeletons of arthropods, vertebrate skeletons are incredibly strong and some terrestrial vertebrates, notably the Dinosaurs grew incredibly large.
Despite the differences between the skeletons of arthropods and vertebrates, when it comes to walking, both skeletons do the same job in much the same way. They are strong, rigid, and act as levers to which the animal’s muscles are attached. When the muscles contract, they exert a force against the skeleton, which is transferred to the ground in order to propel the animal along.
How many legs are best?
This is something robot engineers and biologists alike are keen to sort out, as it has wide implications. Does an animal (or robot) use less energy swinging a large number of small legs than it would were it to swing fewer, larger legs? Recent research by biomechanics specialists at the University of California has produced some surprising results. They claim that it actually makes little difference to the overall efficiency of the animal. According to them, animals bounce as they move using two alternating sets of legs as springs. This means that one human leg ends up being equivalent to two dog legs, three cockroach legs or four spider legs, five crab legs, etc. in terms of energy output per kilogram of body mass.
The claims are startling, as traditionally the splayed leg arrangement that you see in many animals, both arthropods (scorpions and spiders) and tetrapod vertebrates (newts and toads), was always considered to be inefficient. This was because this particular limb arrangement causes the animal to rock at each step so that any energy spent moving forward is also “wasted” moving from side to side. Animals like dinosaurs and mammals, which have shoulders, positioned close together, narrow pelvises and limbs which extend down, almost vertically, from the body were thought to be more efficient because all their energy was directed into forward movement. Also, the position of their legs allowed them to take longer strides than would otherwise be possible. This research actually suggests that the rocking action, far from being a hindrance, is a good thing as it causes the animal to behave like a pendulum, aiding movement and making it more, not less efficient.
The scientists’ work continues, and soon we should have a better idea as to the real reason for the long straight strides that mammals take as they walk. Until then, I’m inclined to stick to the old theory that it’s a more efficient way of getting from A to B than waddling!
There are, of course, many reasons why having lots of legs can be useful. An animal needs a lot less brainpower to control and balance on four, six or eight legs, as it’s an arrangement that provides the animal with a wide base of support and a low centre of gravity. It’s no accident that all the successful robots and automatic walking machines humans have created have six or eight legs, to allow them to move about with the minimum of computer power.
Two Legs is company, more’s a crowd!
Because humans are bipedal (we walk our hind two limbs), we tend to think of that as the normal way to be. In actual fact, it’s unique. No other animal can do it for any length of time.
Walking and balancing on two legs is a lot harder than it looks, it requires a huge amount of brainpower. Even when you’re standing still, your brain has to constantly monitor the signals coming in from: your eyes, the pressure sensors on the soles of your feet, the tension sensors in your muscles and the balance sensors in your inner ear. Based on all that information, your brain then sends out a constant stream of instructions to your muscles to tense or relax as necessary, to ensure that your centre of gravity stays “central” and doesn’t pull you over. If you want to see how this combination of senses and fast reactions works, try this test: Stand up, hold your arms straight out in front of you, keep your arms out, stand on tiptoe, hold it for twenty seconds. That shouldn’t have given you any trouble, but now I want you to do it again – and when you’re on tiptoe, close your eyes! It’s much harder to balance without your eyes providing that extra information. Animals with four or more legs don’t have to try so hard as they are much more stable.
Being bipedal creates some other interesting problems for humans. Because we need a big brain we naturally have babies with big brains. However, babies with big brains have to have big heads. Big headed babies need mothers with a wide pelvis and large birth canal through which they can fit, and be born. All fine so far, except that in order to walk efficiently, us mammals need to have a narrow pelvis. Humans have evolved a remarkable solution to the problem, they give birth prematurely! At the point when they’re born, human babies are completely unable to do that most basic of human activities, walking. After nine months in their mother’s womb, long enough for their bodies to fully form, babies’ brains are far from fully grown. In fact, human babies; brains grow at the pre-birth (foetal) rate for a whole year after birth. No other animal does this.