Why Elephants Have Big Ears

[ 1 ]
WHY ELEPHANTS HAVE BIG EARS
Consider 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. Smallincreases in length result in large increases in area and enormous increases in volume and weight.
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 ofsupporting 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.
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-up-and-downarrangement is more economical, why do gazelles have bent back legs?
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 andgrasshoppers 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 for agility and the back ones are coiled and ready-cocked for explosive acts of locomotion. The downside of this arrangement is that muscles have to work to keep the back end of the animal from collapsing when it is just wandering around. The cost is obviously minor compared with the benefits, at least for gazelles, but the situation becomes less clear as animals get bigger. The strength of a muscle, like that of a bone, depends on its cross-sectional area, but its job is to support, lift, lower, and otherwise shift weights. If an animal were to double in height while remaining the same shape, the weight of any anatomic structure would increase roughly eightfold, while the cross-sectional area of the muscles attached to it would increase only fourfold. In other words, scale up a gazelle to the size of an elephant, and its back end would probably collapse.
There are a number of adjustments that could be made to prevent this from happening. Muscles could be pumped up to heroic proportions in order to retain the locomotory benefits of a bent back end. Some dinosaurs, like Triceratops (Fig. 1.5), were as heavy as elephants but had bent back legs, and some paleontologists believe that they were able to gallop in much the same way that rhinos do today. Perhaps the fact that Triceratops shared its world with enormous predators like Tyrannosaurus (Fig. 1.6) explains why powerful, thrusting back legs were a distinct advantage. The solution that elephants adopted, however, was to do away with bent back legs altogether and replace them with the straight-up-and-down variety that they have at the front. This arrangement saves a lot of muscle power but at the expense of speed and acceleration: because of their straight legs and enormous bulk, elephantsare the only land mammals that cannot gallop.2 When they run, all four legs swing backward and forward like mobile columns, which is why they look a bit more ungainly in full flight than gazelles and cheetahs. But unlike gazelles, adult elephants are not pursued by predators (except humans), and unlike cheetahs they are vegetarians, so they don't need to run toward or away from anything with any great urgency. Their particular arrangement of leg bones is probably a good compromise.
The scaling relationship between areas and volumes underliesmany of the structural and locomotory problems associated with being a large animal of any kind. As animals grow, adjustments have to be made to account for the fact that weights naturally increase faster than the strengths of muscles, tendons, and bones. For this reason if no other, we should always be suspicious of the cinematic license often taken with animals. Godzilla looks like a scaled-up Tyrannosaurus and King Kong like a scaled-up gorilla, but something has to give if animals are to reach such monstrous sizes. Godzilla certainly would not be able to move very fast with all that weight supported on such spindly legs, and King Kong would have had great difficulty pulling his huge body up anything with such underpowered muscles, let alone all the way up the Empire State Building. If these fantasy monsters were to work reasonably within the laws of scaling, they would have to bulk up their muscles and bones so much that they would no longer look very much like the sleek and scary animals they are supposed to represent.
The same scale-related reasoning can be used to shed light on other anatomic differences between our two chosen animals. Relative to the gazelle, for instance, the elephant's head is attached to its torso via a much shorter neck (Fig. 1.3). Now if an animal is to grow large over evolutionary time and keep its head in proportion with the rest of its body, the laws of scaling dictate that it will inevitably have to cope with a very heavy head. And the problem of head weight for elephants is exacerbated by their extraordinary teeth: the four molars at the back of an adult elephant's jaw3 may measure 12 cm (4.7 inches) in width and 35 cm (14inches) in length and are immensely heavy. They are easily capable of pulverizing acacia branches complete with 10-cm (4-inch) thorns. The size and grinding efficiency of an elephant's teeth are crucial as these animals must consume enormous amounts of food to keep their huge bodies functioning. African bulls, for example, may consume 300 kg (660 pounds) of vegetation every day, and chewing gives an important head start to the digestive process by shredding the plant material into small bits that are more easily broken down by digestive juices in the stomach.
An elephant's back teeth may be unusual, but the front ones are quite bizarre. The upper incisors have become modified into tusks, which in the largest individuals may project 3 meters (10 feet) from the upper jaw. These impressive structures are used for sexual display, threatening rivals, fighting, or as tools working in concert with the trunk to collect and manipulate food. The largest tusks ever recorded belonged to an African bull shot near Mount Kilimanjaro in 1897 and are now on display in the Natural History Museum in London: they are each over 3 meters long and together weigh 200 kg (440 pounds).
A 1 tonne head necessarily has to be supported against the pull of gravity by very robust muscles, which is why elephants have such thick necks. To understand why their necks are also short, imagine sitting with your elbow planted firmly on a tabletop holding a heavy object like a cannonball. In this position, the full weight of the cannonball is supported mainly by the muscle that runs down the front of the upper arm and attaches to the forearmnear the elbow (the bicep). Next, imagine the cannonball fixed to a long stick and held in the same position as before. The stick effectively lengthens the forearm and puts the weight of the cannonball much further from the pivot at the elbow. Shifting the weight forward like this would increase the strain on the bicep ruinously. In general, if heavy weights have to be carried around and manipulated, it is best not to suspend them on the end of long levers. Animals with heavy heads, therefore, tend to have short necks. This is true wherever we look in the animal kingdom. Elephants, rhinos, buffaloes, and many extinct animals like mammoths and the dinosaur Triceratops all converged on the same principle of keeping their heavy heads on the end of short levers. Similarly, all the really huge dinosaurs that are famous for their long necks are equally famous for their relatively tiny heads.
We are now in a position to understand what is probably the strangest feature of this very unusual animal. Why do elephants have trunks? The most widely accepted answer is almost comical, and no less so because it stands a good chance of being true. Elephants probably have trunks for the simple reason that without them their heads would not reach the ground. An elephant may be 3 or 4 meters (10 to 13 feet) tall, so if it were to forage for food at or near ground level without a trunk, it would need a neck 3 to 4 meters long. It probably isn't feasible to lighten an elephant's head all that much because of the large braincase and huge jaws and teeth that it needs to process food, and we have already established why no animal in the history of life has carried a 1-tonne head on the end of a 3-meter lever. It is likely that elephants evolved from much smaller animals with prototrunks such as some tapirs have today (Fig. 1.7). As they increased in size over many thousands of years, their trunks simply lengthened to stay in contact with the ground.
Alternatively, it has long been suspected that elephants are closely related to sirenians (manatees and dugongs) and that thecommon ancestor of both groups may have been a fully aquatic animal of some kind. This raises the possibility that trunks developed during an earlier aquatic phase of elephant evolution and may originally have been used as snorkels.
The laws of geometry have also left other marks on the anatomy of elephants, baldness being one of the more subtle. Like you and I, elephants are mammals, and mammals are characterized by a number of traits that set us apart from the rest of our tetrapod cousins (tetrapods are four-legged animals with backbones; that is, amphibians, birds, mammals, and reptiles). We are warm-blooded, suckle our young on milk, have mouths with teeth, and bodies covered with hair. Birds are also warm-blooded, but they lay eggs, have no teeth, and are covered with feathers. Reptiles are cold-blooded with scaly skin, and amphibians are cold-blooded with smooth, moist skin. Mammalian features distinguish us from all other types of tetrapods, but this is not to say that all defining mammalian characteristics are equally obvious in every species. And hair is a good case in point. Over 99 percent of all nonaquatic mammals are furry, but elephants are all but bald. To understand why, we need to know a bit more about the nature of themammalian metabolic engine and the way in which body size influences how it works.
Of all the tetrapods on Earth, only mammals and birds are classically warm-blooded. The members of these two great classes maintain a relatively high and constant body temperature between 30°C and 42°C (86°F and 108°F) by generating heat internally and regulating the rate at which it escapes into the environment. The heat is produced by chemical reactions that occur inside cells, and these proceed at a furious rate inside warm-blooded animals even when we are at rest or asleep. There are various ways in which mammals and birds regulate the heat produced by their tissues in order to stay at their particular set temperature. If the environment gets too hot, they can hide in cool places like burrows and divert blood to the skin where heat can escape more easily into the environment. Larger animals find it difficult to hide, but they contain a relatively large amount of water, just as melons contain a lot of juice, so they can afford to operate an evaporative cooling system--sweating or panting--to rid themselves of excess heat. (One gram of water carries away 2.4 kilojoules of heat as it evaporates. Some mammals sweat, some pant, and some are capable of both. Birds do not sweat, but most of them pant.) If the temperature of the environment drops, mammals and birds can divert blood to the center of their bodies to keep it warm and heat themselves by shivering. Shivering is not an annoying side effect of being cold, it is the body's most effective way of producing heat, because when muscles work--voluntarily or otherwise--they draw on energy supplied by elevated rates of heat-producing chemical reactions within cells. The muscular effort of shivering can raise the internal heat production of a warm-blooded animal to five times its resting level.
Cold-blooded animals, in contrast, have lower metabolic rates, produce much less heat, and cannot shiver. Warm muscles contractmore efficiently and powerfully than cold ones regardless of what sort of animal they are in, so cold-bloods like lizards frequently wish to attain body temperatures in the mid-to-high 30s (95°F to 102°F) too. But because of their low metabolic rates, they have no choice but to warm themselves behaviorally, by basking in the sun, for example. Voluntary muscular exercise also produces heat, so cold-blooded animals could, in theory, heat themselves by running around all the time, but the energetic cost of such a strategy is obviously prohibitive. Pythons are a possible exception to this rule as they can elevate the temperature of their bodies in the early stages of digestion or when brooding eggs by rhythmically contracting their skeletal muscles, but this is the only known example of a cold-blooded tetrapod using muscular exercise specifically for the purpose of controlling its body temperature. (Even though snakes are legless, they are classed as tetrapods because they evolved from animals with legs. The same is true of legless mammalian tetrapods like whales and dolphins.) Cold-blooded animals also have a number of ways of cooling off if they get too hot. They can seek shade, orient their bodies to minimize the area of skin illuminated by the sun, escape to water, gape (which allows cool air to circulate around the mouth and tongue), and some can even change the color of their skins. Some lizards stand on their hind legs to catch the breeze, and a few can create their own air movements in an emergency by running around. Some arboreal species do the same by jumping or gliding between trees.
The most important difference between warm-blooded and cold-blooded animals is the primary source of heat: warm-bloods generate large amounts of heat internally, while cold-bloods rely primarily on external sources like the sun. There are some interesting cases (which we will examine later) where this dividing line becomes a little blurred, but basically mammals and birds are theonly tetrapods on Earth capable of producing enough metabolic heat to raise their bodies to a high temperature without engaging in muscular exercise.
Now, maintaining a high body temperature at all times is costly. To keep a house at 38°C during a temperate winter would require the central-heating system to be turned up high and fed with a lot of fuel. So it is with warm-blooded animals. To keep their metabolic fires raging, mammals and birds consume about ten times more food than reptiles of similar size. To keep a house at 38°C but cut down on fuel bills, the only option is insulation. Investing in a vacuum-sealed boiler, installing loft insulation, and pumping foam into cavity walls are all measures that prevent heat from escaping from our houses. Warm-blooded animals work on the same principle except that they are covered with hairs or feathers that trap an insulating layer of air next to the skin. Mammals living in cold climates tend to have very dense fur to guard against heat loss. The denser the hair, the narrower each strand tends to be, and the softer the fur feels to the touch. Baby harp seals, mink, lynx, snowshoe hares, and Arctic foxes have all suffered mightily at the hands of human hunters precisely because of their luxurious insulation.
So mammals and birds produce a lot of heat, and a layer of insulation stops it from leaking out. If, say, 1000 metabolic reactions occur inside a cell every minute, then the total amount of heat produced by a large animal will be greater than that produced by a small one simply because the larger animal has more cells. But animals lose heat to the environment through their skins, and the laws of scaling dictate that volumes increase much faster than surfaces as animals get bigger. So as animals increase in size, the amount of heat-producing flesh inside them increases rapidly, but the amount of skin through which the heat can escape increases more slowly. Clearly some adjustment has to be made if large animals are to avoid cooking themselves.
As animals evolve toward large size, they cope with this problemby reducing the rate at which their cells produce heat--the larger the animal, the greater the reduction. But there is something extremely curious about this adjustment when looked at in detail. Elephant cells do produce less heat than gazelle cells, but this difference is not enough to balance an elephant's overall heat budget. And it turns out that this mismatch is characteristic of creatures of all kinds: large animals within any particular taxonomic group tend to produce more heat than expected on the basis of the scaling relationship between areas and volumes, and the larger the animal, the greater the discrepancy. Generations of biologists have banged their heads against this particular brick wall, and we still do not have an explanation. There are a number of theories, and new ones come along every few years, but none is very satisfactory. There are ways of making the mathematics work so that the mismatch can be accounted for, but there is little agreement on which way is best. Clearly, as animals evolve toward large size, they adjust their metabolic rates to keep pace not with their dwindling surfaces, but according to some other rule or rules of which we remain stubbornly ignorant.
Whatever the reason for this state of affairs, the fact is that large animals produce more heat than seems sensible. It should be clear now where this is leading. African elephants, the largest land animals on Earth, also happen to live in one of our hottest climates. Their bodies produce an enormous amount of heat, and the sun beating down on their backs doesn't help. Even though they have adjusted their thermostats to reduce the heat output of their cells, this countermeasure has fallen some way short of compensating for their relatively small surfaces. So the baldness of elephants makes sense. The last thing that such a large animal needs, especially one that lives under a baking sun most of the time, is a fur coat. White rhinos are the second-largest land animals on Earth, they also live in hot climates, and they are also bald. It is likely that such large mammals living in hot places must bebald in order to lose heat through their skins at an adequate rate when temperatures soar.
Hippos are also bald, but the reason in their case is less obvious. Semiaquatic animals can retreat to water if the air gets uncomfortably hot, so bald skin may not be as important for hippos as for elephants and rhinos. Some smaller artiodactyls (the taxonomic group to which hippos belong), e.g., warthogs, bison, and babirusas, are also bald or partly so, and many of the piglike members use fat for insulation rather than hair, so baldness in this group of animals is obviously not a simple function of size. The closest relative of the common hippo is the pygmy hippo, a bald animal weighing no more than 275 kg (600 pounds) and spending most of its time on land. Simple theories based on scaling arguments have to be evaluated carefully in the complex world of living things.
We are now, at last, in a position to answer the title question of this book: Why do elephants have big ears? Or, to put it in its comparative form for those who already know the answer but not the reason, why do elephants have big ears when rhinos, giraffes, and all the other warm-blooded mammals toiling under the African sun do not? The crucial difference between elephants and all other land animals is probably size: the mismatch between the amount of metabolic heat produced and the amount of skin through which it can escape tends to increase as animals get bigger, and elephants are fully twice as heavy as any of their savanna neighbors. So being bald like a rhino is not enough. Elephants need some additional way of ridding themselves of excess heat, and large flat ears densely packed with tubes through which hot blood can be pumped are a rather neat solution.
Of course, heat will not flow out of an elephant's ears if the surrounding air is warmer than 38°C unless the cooling process is aided by the evaporation of water. Unlike most other large mammals, elephants do not have the ability to sweat--possibly becausetheir aquatic ancestors didn't need to--so they must either seek a favorable thermal gradient in the shade or find some other way of wetting their ears (and, just for good measure, other parts of their bodies too). Despite their enormous bulk, elephants regularly manage to find shady spots, and they endeavor never to stray far from water. When caught short, elephants have even been known to draw water from their throats with their trunks and spray it over their ears. One way or another, an elephant's magnificent flaps represent a rather elegant solution to the problem of being a 5-tonne mammal with an overactive metabolism in a hot climate.
Both the present-day ecology and the evolutionary history of elephant-like creatures is consistent with this interpretation of their ears. Modern elephants, for example, are classified into African and Indian species, and the African ones are further divided into the familiar savanna or bush elephant from eastern Africa and the little-known forest elephant from the equatorial woodlands of central and western Africa. Indian and African forest elephants inhabit shaded woodland where temperatures tend to be much lower than on the savannas where bush elephants live. If the temperature-regulation theory of elephant ears is correct, then bush elephants should have larger ears (or at least ears capable of holding more blood, either by being larger or by having a more dense network of blood vessels). And sure enough, though Indian and African forest elephants have ears that are huge by normal mammalian standards, they are considerably smaller than the enormous flaps that grace the heads of their bush-dwelling cousins.
Extant elephants of all kinds live in relatively warm climates, but some extinct species of mammoth lived in very cold places indeed. We know most about the anatomy of mammoths from animals frozen in the tundra regions of Siberia. Fur and thick layers of fat are preserved in some specimens, a sure sign that mammoths were vulnerable to cold. But what about their ears? If mammoth ears were for sexual display or intimidating rivals, thenit is possible that they could have been large but poorly supplied with blood, or covered with fur, or tucked in close to the body most of the time. Or perhaps heat loss through their ears could have been compensated for by thicker fur or an extra layer of fat. But if the ears of elephant-like creatures are primarily devices for letting out heat, then it would be very odd for those living in bitterly cold places to evolve thick layers of fur and fat yet have big, fleshy, blood-filled ears sticking out. And indeed, though the mammoths of Siberia were majestic animals in many ways, there is one anatomic department in which they were distinctly lacking compared with their living relatives: they had tiny, furry ears. It all fits together nicely. The magnificent ears of elephants are, quite literally, radiators.
While elephants are odd creatures in many respects, their bodies conform to a relatively simple set of design criteria. Their thick legs, stately carriage, short necks, trunks, heat production, bald skin, and big ears are all consequences, one way or another, of the inescapable scaling relationship between areas and volumes. Elephants make satisfying scientific subjects because the discovery of basic principles that explain diverse and seemingly unrelated phenomena is what science is all about. But the anatomy of elephants is just the beginning. The ways in which animals of different sizes and types consume and manipulate energy exerts a fundamental control over numerous patterns of life on our planet. We have figured out elephants and their ears. Next, shrews and their extraordinary hearts.
WHY ELEPHANTS HAVE BIG EARS: AND OTHER RIDDLES FROM THE NATURAL WORLD. Copyright © 2000 by Chris Lavers. Illustrations copyright © 2000 by Chris Lavers. All rights reserved. No part of this book may be used or reproduced in any manner whatsoever without written permission except in the case of brief quotations embodied in critical articles or reviews. For information, address St. Martin's Press, 175 Fifth Avenue, New York, N.Y. 10010.