杏吧原创

THE SIZE of an organism places many limits on its shape, physiology and behaviour, often in surprising ways. For instance, if we were scaled down to the size of mice by the absent-minded inventor from Honey, I Shrunk the Kids, we could happily fall from a cliff safe in the knowledge that after a bump, we could get up and walk away. Bigger animals would certainly be seriously hurt or killed in such falls.

But suppose we were scaled up to the size of the giant in Jack and the Beanstalk, our leg bones would no longer be thick enough to support our weight, and would snap.

If we were shrunk even smaller than a mouse, however, we would live in fear of water. The weight of the thin film of water which covers the surface of fully-grown humans when they get out of the swimming pool is only a small portion of their body weight. Not so for a wet housefly, which must stagger around with a burden several times its own weight.

Size also has implications for the way organisms obtain their energy and grow. All organisms must provide themselves with nutrients and gases, remove waste products and coordinate their own activity. A tiny single-celled animal can obtain oxygen by simple diffusion from the surrounding water into the centre of the cell. Special respiratory structures are unnecessary.

But for a large organism, simple diffusion is not enough. Fishes have increased the effective surface area available for oxygen absorption by developing numerous gill filaments. The lungs of reptiles, birds and mammals are almost sponge-like, offering a relatively enormous surface area across which gases may diffuse. Most of the more complex animals have evolved a circulatory system, which, among other things, is used to rush oxygen away from the respiratory surfaces to the body tissues and to move carbon dioxide in the opposite direction.

All these examples are biological consequences of simple geometrical relationships between length, surface area and volume.

Consider a one-centimetre high wooden cube. It has a surface area of 6 cm² and a volume of 1 cm³. Glue 27 of these together to form a new larger cube; this object has a height of 3 cm, a surface area of 54 cm² and a volume of 27 cm³. The threefold increase in height results in a ninefold increase in surface area and a 27-fold increase in volume or mass. In the small cube, one cubic centimetre of wood is surrounded by a surface of 6 cm², a ratio of volume to surface area of 1:6. In the bigger cube, each cubic centimetre of wood has on average only 2 cm² of surface, a ratio of volume to surface area of 1:2. The smaller cube has a greater relative surface area. The same is true, of course, for any objects of the same shape, but differing in size. Inevitably, smaller organisms tend to have a greater amount of surface to their volume than do larger organisms. This is important because some processes depend on the area of the body surface, while other processes depend on the body鈥檚 volume.

So if we take our examples in turn, we can see how these relationships operate. For the falling mouse or inadvertently shrunken kids, air resistance is proportional to their surface area, which is relatively large compared to their mass. Air resistance slows the fall of small animals so much that falling may often be adopted as part of normal locomotion, or escape.

In the case of our blown-up giant, the strength of his leg bones is proportional to their cross-sectional area. Suppose our scaled-up giant was three times as tall as normal humans. The cross-sectional area of the giant鈥檚 leg bones would be nine times those of ours, but, and here lies the problem, the giant鈥檚 mass would be 27 times greater than ours. The moral of this story is that large terrestrial animals must have proportionately thicker leg bones 鈥� as does the elephant in comparison to the mouse.

Many of the different paths along which animals evolve are limited by size. For instance, the large dinosaurs were at the very upper limit for support and locomotion for quadrupeds on land. Flapping flight becomes increasingly difficult as birds reach about 12 kilograms in weight (see Diagram). The same argument suggests that the giant flying dinosaurs did not flap their wings to stay aloft, but soared like gliders on warm air currents 鈥� though a recent hypothesis suggests that perhaps the atmosphere was more dense during the Cretaceous period, making it easier to explain how such large flying creatures as the 12-metre wingspan Quetzalcoatlus could have stayed airborne.

Water, of course, is much denser still 鈥� around the density of most organisms 鈥� and there is, in theory, no gravitational limit to the maximum size of an aquatic animal. Nevertheless, the size of a fish does determine the most efficient speed of its tail movements. The smaller the fish, the faster its tail must waggle if it is not to waste energy. The same would apply to a mechanical robot fish. The energy of cells is provided by specialised organelles called mitochondria, which are often called the 鈥減owerhouse鈥� of the cell. The speed of contraction of muscle fibres is associated with the number of mitochondria in each muscle cell. The tail muscle cells of smaller fish turn out to have more mitochondria than those of larger fish, which suggests they are well adapted to limits imposed by the overall size of the fish.

A curious consequence of these simple principles is that animals built to the same design can jump the same height, regardless of their size. This is because the power of the muscles is proportional to mass, allowing, in theory, a jumping elephant to raise its centre of gravity by only the same distance as can a mouse (see Diagram). In general, smaller animals can jump several times their own height, while larger animals may only be able to jump a portion of their own height Humans come somewhere in the middle of the scale in this respect. The best high-jumpers can raise their centre of gravity from a bit over a metre or so off the ground, to nearly 2.4 metres.

The maximum size of insects appears to be limited by their type of respiration, which relies on the diffusion of oxygen into the tracheae, fine tubes which enter the body from holes on the outside (spiracles). Once the distance of the tube down which oxygen must diffuse exceeds a little over half a centimetre, getting oxygen into body tissues becomes difficult. Insects tend not to be thicker than about 1.5 centimetres at any point, which limits their overall size.

Running hot and cold

Life鈥檚 vital element

THE vertebrates have overcome this problem by evolving the complex surfaces of gills or lungs and a sophisticated blood transport system. The human lung, for instance, has 350 million air pockets, alveoli, with a total absorbing area of about 90 square metres 鈥� about the surface area of half a tennis court. Across this surface, oxygen passes into the blood, where it binds with haemoglobin, displacing carbon dioxide, and turning haemoglobin from blue to red. This must be pumped around the body at sufficient speed to maintain the normal activity of the organism 鈥� a rate which, as we shall see, is determined by size, as well as the body temperature and evolutionary history of the animal concerned.

Nutrients must also be transported into the organism and waste products removed. Again, for a single-celled organism simple diffusion is enough, but larger animals need to develop absorption and excretion systems. The small intestine of mammals is designed for rapid absorption of nutrients. Its inner surface has many ridges, which give a three-fold increase to the surface area. These ridges are themselves lined with tiny, finger-like projections, called villi, which protrude 1 mm into the intestine and increase its effective surface area a further 10-fold. The surface of the cells lining these villi have a brush-like border of microvilli, each 1 micron long and 0.1 microns in diameter, giving a further 20-fold increase in surface area. Taking all this into account, the total surface area available for absorbing food in an adult human is an enormous 250 m².

The rate at which chemicals and gases can diffuse into the cytoplasm limits the maximum size of a spherically shaped cell to an average of around 50 microns across. The largest cells in the human body, some nerve cells, may be more than half a metre in length, but they are very thin and thus have a relatively large surface area. There are some giant single-celled organisms, mostly in the plant kingdom, such as the marine Acetabularia, which can grow up to 1 cm in length, though they also have a structure with a relatively large surface area. For an organism to be any bigger it has to be multicellular 鈥� this can allow certain groups of cells to specialise in particular tasks, and to become 鈥渙rgans鈥�.

The evolution of multicellular life and the specialised organs associated with it probably occurred because many of the biochemical reactions necessary for life are most efficient when carried out under specialised conditions. For instance, digestion of food is most effective in many animals in an acid environment, but an acid environment would play havoc with the nervous system or the bones. The single-celled animal must do its best to separate the processes of digestion, locomotion and reproduction within one cell. But the larger animal can begin to keep these biochemical reactions apart, and improve the overall efficiency of its use of energy and nutrients.

However, once these various functions are separated from each other in larger animals, it is essential for the organs to communicate. The two principal methods of internal communication used by animals are hormones (the endocrine system) and the nervous system. Hormones circulate in the blood and usually pervade the whole body. Nerve impulses in mammals can travel at up to 100 metres a second.

The chemical reactions which keep animals alive also generate heat. At the same time, the rates at which biochemical reactions occur are affected by temperature, a 1 掳C rise in temperature causing the rate of reaction to rise by 14 per cent. Conversely, a fall in temperature slows reactions down.

Much of the carbon chemistry on which life depends is most efficient at about 38 掳C. At lower temperatures, although the reactions occur more slowly, they are generally less efficient 鈥� more of what little energy is released tends to be wasted as heat. At higher temperatures, many enzymes start to disintegrate.

By allowing their biochemical reactions to occur at prevailing temperature, some animals have adapted to unpredictable or unsuitable temperature environments. But as a consequence, they are not the most efficient users of energy. Examples of animals in this category are the krill of the Antarctic seas that have enzyme systems adapted to icy temperatures, or bacteria which live in the boiling water that gushes from hot springs and can only use enzymes capable of withstanding such heat.

Those animals that have struggled to keep their internal temperatures near to 38 掳C have tended to adopt a two-fold approach during evolution. On the one hand they have elaborated novel types of physiology and biochemistry to improve the efficiency of their energy use, allowing them to speed up their metabolism and generate heat, while on the other, changing body shape or behaviour to improve temperature control.

The mobility of larger animals can be highly significant in regulating temperature. The tiny amoeba must take its ambient temperature as it finds it. But the lizard can find a place in the sun in the early morning, a shady spot at noon and a crevice in the warm sun-baked rocks at sunset.

Some scientists think that the dinosaurs ingeniously exploited this technique with sails. The evidence suggests that the sails of dinosaurs, such as Dimetrodon, were rich in blood supplies, and were used to heat the animal鈥檚 blood at dawn and dusk, when the Sun鈥檚 rays are closest to the horizontal. At midday, when the Sun was overhead, the sail could transfer excess heat to the passing breeze.

Size turns out to have a very special role in the speed at which energy is used by animals, for reasons which are still not entirely clear, despite many years of study.

Early work on metabolism focused on mammals and it was found that in resting mammals, the amount of oxygen consumed or heat generated was related to the animal鈥檚 mass. But though larger animals used more energy, it was not a simple multiple of the body mass. For instance, an average man weighs about the same as 5000 mice. But the combined oxygen consumption of 5000 resting mice is about 17 times that of a resting man.

It turned out that basal metabolic rate (BMR), the energy turnover of the resting animal, was proportional to the 戮 power of body mass 鈥� for every quadrupling of mass there is only a trebling of oxygen consumption. At first, when only a few animals had been tested, biologists assumed that BMR must be determined by the rate of heat loss across the surface area of the body, which, according to the example of the wooden cube, should vary as the 鈪� power of mass 鈥� close enough to 戮 to be within experimental error. But as more data were added, the exponent remained stubbornly on 戮. It was also shown that breathing rates and heart rates were also tied to the same relationship.

More difficult still to explain from the principles of heat loss, was the discovery of a similar exponent among the 鈥減oikilothermic鈥� multicellular animals such as fish, amphibians and reptiles. (Poikilothermic is the correct term for animals popularly described as cold-blooded meaning living at 鈥渧arious temperatures鈥�, while homoiothermic animals, the birds and mammals, preserve a relatively fixed temperature.) Among the single-celled animals there is a similar relationship between BMR and mass. However, when the influence of size within each of these three 鈥渕etabolic groups鈥� is taken out, the single-celled animals have the lowest BMR, followed by the poikilothermic animals, with birds and mammals having the highest. Within each of these three groups, a knowledge of an organism鈥檚 size allows a surprisingly accurate prediction of BMR, both between species and between young and old of the same species. Each gram of a young animal鈥檚 body has a higher rate of metabolism than that of an older animal from the same species (see Diagram).

By measuring the oxygen uptake and heat output of resting humans, we know that the BMR of a human weighing 75 kilograms is about 340 kilojoules per hour (kJ/h). This gives a rate per kilogram of body mass of 4.5 kJ/h. Compare this with a baby weighing 3 kg which will have a metabolic rate of 29 kJ/h. It has a metabolic rate per kilogram of body mass of 9.6 kJ/h

On a day-to day basis, metabolism can vary quite a bit. The few studies that have been done suggest that the metabolic rate of active animals averages about two or three times the BMR. During brief periods of maximum exertion, up to 15 times BMR has been recorded in humans lifting heavy weights.

Whatever the underlying factors which relate metabolic rate to the 戮 power of body size, it is significant that the figure does not diverge too far from the 鈪� surface relationship. If the metabolic power function was higher than 戮, animals larger than the optimum mass would certainly overheat, while small mammals would be even more susceptible to the cold than they are already. If the metabolic power function were lower than 鈪�, the opposite would be true.

Developing embryo

Changing times

THE NEED for ever-increasing levels of sophistication as an organism increases in size is demonstrated elegantly by observing the growth of a human egg (ovum) once it is fertilised.

Starting with only one cell, all the nutrients and gases needed for life and growth can be exchanged directly with the surrounding fluid in the uterine tube. However, by the third day after fertilisation, the embryo consists of a solid ball of 16 cells, and its surface area is becoming too small to cope with its volume. It can no longer rely on simple diffusion, and must organise a better supply of raw materials if it is to survive.

Now the embryo buries itself in the uterine wall and develops a more substantial nutrient supply and a distribution system. By the end of the second week of development, the embryo builds a primitive placenta 鈥� a special fetal outgrowth which invades maternal tissue to improve exchange. One week later, a miniature heart starts to circulate blood around a rudimentary set of vessels. In doing this the embryo is developing a method that effectively increases its surface area. The effective surface area is not simply restricted to the outside of the layer of cells, but is also the surface of every blood vessel (see Diagram).

The embryo grows in the safety of the amniotic sac, which is a fluid-filled bag containing the fetus and which limits the effects of gravity upon its delicate bones and organs. Over the next eight months the placenta continues to develop, exposing a large surface area of fetal blood vessels to maternal blood. As the fetal organs and tissues grow, they are invaded by capillaries of the fetal blood circulation so that it is very rare for a cell to be more than 50 microns away from a capillary. By adulthood, a human will have 10 billion capillaries, with a total surface area of about 600 m². This means that each human cell manages to have somewhere near the same effective surface area as the free-living, single-celled amoeba.

A major complication faced by a developing mammalian fetus is that at birth the surface used to obtain oxygen and nutrients and dispose of waste products, the placenta, is lost. During life in the uterus, a fetus needs to prepare a second set of external surfaces. Lungs develop in preparation for performing gas exchange, stomach and intestines develop ready to absorb food, and kidneys prepare to clean waste out of the body. At birth, the blood circulation also has to be subtly redirected to send a much greater flow to the lungs and the gut than occurred when it was a fetus.

Again, increased size does not just affect diffusion; systems of control and communication are also required. With the blood supply running so close to every cell in the body, it is an ideal transport system for distributing messages. These messages come in the form of hormones. For example, insulin is released from cells in the pancreas when blood glucose rises, and causes its reduction. In fetuses, insulin appears to be one of the key hormones that regulates growth, and the pancreas is established by the end of the seventh week of development.

However, hormones have limitations. They cannot travel faster than the blood stream and cannot be targeted very accurately to individual organs because they are pumped all over the body. This is where the nervous system comes in. Its signals can be transmitted with speed and accuracy. Development of this system starts within a few weeks of conception.

Life history and ecology

Energy and evolution

ANIMALS have diverged from plants in the uses to which they put the energy that passes through them. The more complex plants have become increasingly less mobile in all stages of their life history and more productive of organic matter. By contrast, the more complex animals put more of their energy into work and heat, and less into the production of organic matter, that is to say growth and reproduction. The divergence has made plants and animals mutually dependent.

The more complex plants, such as the flowering plants, rely increasingly on animals to break down organic matter and to transport reproductive bodies and fruit, while all animals rely ultimately on plants for their food. For instance, bees and many other insects transport pollen between flowering plants while consuming nectar. Birds eat many fruits, transporting seeds away from the parent plant and depositing them elsewhere in the faeces, conveniently enclosed in fertiliser.

The mammals and the birds, which are relatively recent in evolutionary terms, have taken the divergence away from the plant kingdom to the greatest extreme. As a proportion of their total energy consumption, mammals and birds tend to devote the smallest proportion of their metabolism to physical growth, and the greatest to work and heat. However, because of their greater overall metabolism compared with poikilothermic animals (associated with their higher average body temperatures), many mammals and birds of a given size still grow as fast or faster than many poikilothermic animals. Thus, mice will grow faster than goldfish, but for every gram of increased weight the growing mouse requires much more food.

The primates, including humans, are, for any given size, among the slowest growing of all the mammals despite having a typical mammalian metabolic rate. The primates devote a very low proportion of their energy to physical growth, and a great deal to work and heat.

In addition, the larger species in each of the three major metabolic groups tend to take longer to reach maturity. This makes it more difficult for larger animals to recover their numbers after any catastrophe, such as a volcanic eruption, flooding, or fire. For example, the larger primates, which are doubly slow-growing as a consequence of their size and their ancestry, are vulnerable to sudden disruptions. In general, it is supposed that natural selection 鈥� that is to say the better reproductive success of animals better adapted to their environment 鈥� favours small size in environments that are subject to regular floods, droughts or other ecological upsets, while a fairly predictable environment is necessary fbr the survival of larger-bodied species.

But given a fairly stable environment, does large size offer any other clear advantages? Large size may reduce the risk of being eaten by another animal and improve temperature regulation in cold climates, but another factor may be at work here, especially among the mammals. Larger animals tend to have both longer lives and larger brains 鈥� though neither are directly proportional to size. Nor is there any intimate connection between brain size and intelligence. Nevertheless, throughout the millions of years of evolution of the fishes, reptiles, birds and mammals, brain size has increased. It seems likely that some advantage must be conferred by a larger brain. So perhaps larger animals in any group of related species may have an advantage in learning and remembering complex behaviours, such as the changing pattern of fruit-bearing trees within a home range, and, especially for females in the animal world, parental care.

Humans cannot escape many of the biological constraints imposed by their physical size. However, by making tools and using techniques passed on by each generation, humans learnt to gather, process and eventually grow food, create external warmth and build shelters. This allows us to overcome some of the ecological difficulties faced by our nearest relatives, the apes, which have a relatively low population density, and are dependent on tropical forest for a predictable, warm environment in which food is abundant.