[ 1 ] WHY ELEPHANTS HAVE BIG EARS C onsider the oddness of elephants. At between 4 and 7 metric tonnes (U.S. equivalents 4.4 and 7.7 tons) these creatures are fully twice as heavy as any other land animal on Earth. They have 3-meter-long noses (almost 10-feet). The African variety has the largest earflaps of any animal in history.
Nearly all land mammals are covered with hair, but not elephants. Their front teeth can grow to 3 meters in length and over 200 kg (440 pounds) in weight. Consider an animal capable of scratching its knees with its teeth without bending down. We have become so familiar with elephants from zoos, books, and television documentaries that we tend to take them for granted, which is something of an achivement, all things considered. The aim of this chapter is to explain elephants and why they evolved as they did, and not just because they are strange and fascinating animals in their own right. Elephants are the ideal starting point for an exploration of size and energy use in the animal kingdom because they are the largest creatures currently walking the planet and because their metabolic engines are among the most expensive to run. Once we appreciate the workings of these enormous gas-guzzlers, it will be but a short step to an understanding of why rats are furry, why there are no fly-sized or snake-shaped mammals, why the tiniest backboned animals are lizards and frogs, why King Kong could never have climbed the Empire State Building, and much else besides. Ultimately, a knowledge of how elephants work will lead us to the most profound disturbance to the Earth's biogeography in the last 65 million years, a man-made crisis that has left the biosphere teetering on the edge of global mass extinction.
But we are jumping ahead. Our exploration of patterns of life on our planet begins with the largest land animals on Earth. And to understand these magnificent creatures we must first explore some of the biological consequences of being big. Like most animals, elephants are an odd shape and thus rather difficult to measure; so, for the purposes of illustration, let us imagine that they are melons. A cantaloupe melon is around 16 cm (6.2 inches) in diameter or about twice the width of an orange. A linear measure such as width is one way of expressing the relative size of these two objects, but it is not the only comparison we could make. The melon, for example, has a surface area four times greater than that of the orange.
Cut the fruits, and the cross-sectional area of the melon will also be four times greater. The areas of spherical objects-surface or sectional-always scale in this way: double the width, quadruple the area. The volume of the melon, however, is eight times that of the orange, and because oranges and melons are mainly water, and water weighs the same no matter where it comes from, volumes and weights naturally scale in the same way. Although it is only twice as wide, the melon is eight times as heavy as the orange and contains eight times as much juice (Fig. 1.1). This general relationship between length, area, and weight always holds regardless of the objects involved, provided they are roughly the same shape. And the rules apply just as well to objects that grow from one size to another.
How much does a 12-cm (4.7-inch) fish have to grow to double in weight? Only about 3 cm (1.2 inches). An ostrich egg is only 2.5 times the width of a hen's egg, but it would make the equivalent of a twenty-egg omelette. Small increases in length result in large increases in area and enormous increases in volume and weight. FIGURE 1.1 As the radius of a fruit, ball, or any other roughly spherical object doubles, its surface area quadruples and its volume and weight increase by a factor of eight.
The factors of increase are less straightforward for irregularly shaped solids, but the general rule holds. How do these geometric principles affect animals? Figure 1.2 shows a close-up of an African elephant's remarkable cranial anatomy, and Figure 1.3 shows an elephant and a gazelle drawn to the same size. Many anatomic differences between the two creatures are obvious regardless of scale, but when they are standardized by height and length it is easier to make direct comparisons of the shapes and relative sizes of various body parts. Compared with the gazelle, the elephant has thicker, straighter legs, a shorter, more robust neck, a massively elongated nose, a distinct lack of hair, and, of course, those extravagant ears. Curiously, all these characteristic elephantine features are a consequence of the scaling relationship between areas and volumes. Working from the ground up, the strength of a leg bone depends mainly on its cross-sectional area, and legs do the job of supporting an animal's weight.
Imagine what would happen if a gazelle remained the same shape but grew to the size of an elephant. As it doubled in height, the cross-sectional area of its bones would quadruple, but the weight of the whole animal would increase by a factor of eight. The gazelle would not have to double in height many more times before its bones would snap under the influence of gravity (although the muscles and tendons in its legs would probably give out first). Over perhaps tens of thousands of years, elephants did evolve from animals the size of gazelles, so this problem of bone strength had to be resolved. As they grew larger over evolutionary time, their shank bones thickened disproportionately fast to cope with the increasing load, which is why an elephant's legs look stockier than a gazelle's. But bulking up bones was not the whole solution: an elephant's legs are also arranged in a very unusual way, which, among other things, explains why they are nowhere near as athletic as many smaller creatures. FIGURE 1.2 The head of an African bush elephant.
FIGURE 1.3 An elephant and a gazelle drawn the same size. Gazelles have the standard arrangement of leg bones characteristic of nearly all fast-moving mammals. Their front legs look like our own: straight up and down with a joint in the middle. This middle joint, however, is not a knee but the equivalent of our wrist. The shank below this joint corresponds to the bones in the palm of our hand, while the equivalent of our elbow is right up near the animal's chest. Regardless of the bones involved, straight legs are useful because they can be locked into position with little muscular effort. But a gazelle's back legs are different.
About halfway down is a backward-bending joint which is the equivalent of our ankle. The knee, again, is right up near the torso and often hidden by skin and fur. This arrangement of bones gives gazelles and many other running mammals the curious appearance of legs that bend forward at the front and backward at the rear. None of the joints in a gazelle's back legs is straight and locked like ours, which means that energy has to be expended to prevent them from collapsing.1 If the straight- u up-and-down arrangement is more economical, why do gazelles have bent back legs? FIGURE 1.4 A cheetah's enormous stride-length is a product of relatively long legs and a remarkably flexible back. At full stretch the whole animal is virtually parallel to the ground. Gazelles are magnificent runners.
They have to be because super-fast predators like cheetahs regard them as little more than mobile larders (Fig. 1.4). The architecture of a gazelle's back legs is best understood by appreciating that these creatures spend the most critical moments of their lives running away. Flat-out speed is important, but for the relatively short races between gazelles and cheetahs, acceleration and cornering ability are probably more critical. Human sprinters know that they can achieve the greatest rate of acceleration by assuming a crouched position and straightening their legs explosively when the gun fires. Tenths or even hundredths of a second are crucial for sprinters, but much less so for distance athletes, which is why sprinters get down on all fours to start a race and marathon runners don't bother. Needless to say, acceleration is even more crucial if the prize is to escape a predator's jaws.
The back legs of a gazelle are arranged in a permanent sprinter's crouch, allowing rapid acceleration from a standing start. The bent arrangement also means that the back legs are slightly longer than the front ones, which maximizes the length of the thrust stroke. (Other explosive accelerators like frogs and grasshoppers exploit the same principle.) In addition, when startled by a predator, a gazelle does not so much run as explode forward in a series of jumps. After each jump the back legs automatically recoil into their normal bent arrangement ready for the next one without having to be dragged all the way back into position by big heavy muscles. This system of elastic recoil saves energy, reduces leg weight, and shortens the time lag between each lifesaving thrust. A gazelle's legs are also exceptionally thin because the leg muscles are concentrated right up near the animal's torso. Imagine starting a 100-meter sprint or negotiating a sharp corner at speed with a 5-kg (11-pound) weight tied to each ankle, and the advantages of a gazelle's top-heavy arrangement of muscles should be obvious.
The legs are lightened as much as possible at the foot end, which moves through the greatest length of arc, allowing them to be quickly set in motion and maneuvered easily around corners. So a gazelle's legs are lightly built.