Forces of Nature. Andrew Cohen

Чтение книги онлайн.

СКАЧАТЬ animals float, which means the effects of gravity are not relevant. With the necessity for strong bones to support their weight removed, their forms are freed from this particular constraint. Here is how Simplicio, Sagredo and Salviati put it, from their metaphorical park bench. I can’t help but hear them as a sort of three-way Renaissance version of Pete and Dud…

      Simplicio: This may be so; but I am led to doubt it on account of the enormous size reached by certain fish, such as the whale which, I understand, is ten times as large as an elephant; yet they all support themselves.

      You read that in a Dagenham accent, didn’t you?

      Simplicio: A very shrewd objection! And now, in reply, tell me whether you have ever seen fish stand motionless at will under water, neither descending to the bottom nor rising to the top, without the exertion of force by swimming?

      Simplicio: In aquatic animals therefore circumstances are just reversed from what they are with land animals inasmuch as, in the latter, the bones sustain not only their own weight but also that of the flesh, while in the former it is the flesh which supports not only its own weight but also that of the bones. We must therefore cease to wonder why these enormously large animals inhabit the water rather than the land, that is to say, the air.

      Sagredo: I am convinced and I only wish to add that what we call land animals ought really to be called air animals, seeing that they live in the air, are surrounded by air, and breathe air.

      Salviati: I have enjoyed Simplicio’s discussion, including both the question raised and its answer. Moreover I can easily understand that one of these giant fish, if pulled ashore, would not perhaps sustain itself for any great length of time, but would be crushed under its own mass as soon as the connections between the bones gave way.

      Freed from the tyranny of gravity, aquatic animals can be larger than their land-based cousins, but they don’t have complete freedom from the laws of physics.

      Every winter the warm waters of Florida are home to one of Nature’s apparently less elegant shapes. The caveat is important, because the clumsy-looking manatee is as well adapted to its environment as the most aesthetically refined butterfly. The West Indian manatee is the largest living example in the Sirenia order of wholly aquatic, herbivorous mammals. A less-than-taxonomically accurate but nonetheless accurate image can be conjured by imagining a 4m-long aquatic cow with no legs, unhurriedly grazing on the sea grasses that grow in the slow-moving waterways along the Floridian coast.

      During the summer months the manatee roam as far north as Massachusetts, but as the seasonal temperatures fall they must return to warmer seas. They are unable to survive in waters below 20 degrees Celsius for long. The need for warm winter waters drives the manatees to congregate in large groups around the warm springs that dot the Florida coast, where temperatures remain above 22 degrees Celsius all year round. They also take advantage of human activity, gathering in the outflows of power plants near Apollo Beach and Fort Myers. The manatee is a strange animal indeed; it is more closely related to an elephant than to the other marine mammals – they share a common ancestor around 60 million years ago, not long after the dinosaurs became extinct. The ancestor may have looked like the modern-day hyrax, which at around 50cm in length looks nothing like an elephant or a manatee; 60 million years is plenty of time for the un-directed tinkering sieve of natural selection to sculpt an animal to take advantage of an environmental niche.

      The elephant’s niche is to be the biggest land animal, which undoubtedly gives it an advantage against predators, but it also displays the anatomical evidence of a tussle with gravity. As dictated by the square-cube law, the elephant has evolved with exceptionally thick legs to support its substantial weight. There is also the matter of cooling; heat escapes from an organism through its surface. As the volume of the animal increases, so does the amount of heat it generates, but its surface area decreases in proportion, according to the square-cube law. This presents a problem for a land-dwelling animal, and the elephant has solved it by developing an ingenious cooling system – its big ears.

      The manatee filled a different niche. The transition from a coastal land-dweller to an aquatic mammal saw their front limbs evolve into flippers, although they still possess their ancient finger-bone structure and fingernails. The rear limbs have become a giant paddle-shaped caudal fin, a gradual evolutionary change wonderfully documented in the fossil record. The limbs of the ancient ancestor that grew thick to resist gravity in the elephant have become streamlined to allow the manatee to swim at up to 12 km/hour. The manatee can dive deep, for up to twenty minutes at a time, but being an air-breathing mammal it must surface for air eventually. Its time under water is maximised by slowing down its heartbeat and metabolic rate, reducing the need for oxygen; but this is where biology comes into conflict with physics. A low metabolic rate means limited heat production, and water is an extremely good conductor of heat away from the body, so there is a danger of becoming too cold. The compromise solutions discovered, naturally, by natural selection, are to get bigger, which reduces the surface-area-to-volume ratio and therefore decreases the rate of heat loss per unit volume, and to get spherical (see illustration, here).

      This graph shows how the surface area decreases for rounder shapes and the surface-area-to-volume ratio decreases as the volume increases.

      This is a beautiful example of a naturally occurring shape reflecting a deeper mathematical reality. The sphere is the three-dimensional shape with the lowest surface-area-to-volume ratio. If you want to generate lots of heat by having a large volume, but lose as little through your surface as possible, you’ll be spherical – and the manatee is the most spherical mammal on Earth. What a wonderful thing to be – unless you are an astronomer. The astronomer Fritz Zwicky is credited with calling a group of his colleagues Spherical Bastards, because they are bastards, whichever way you look at them. Which brings us nicely back to the subject of symmetry. If a physicist designed a manatee it would be spherically symmetric. Symmetrical shapes such as planets tend to be the result of the action of symmetrical laws of Nature, unless there are reasons for the symmetry to be broken. There are no perfectly symmetric large organisms in biology. Why?

Symmetry and symmetry breaking in biology

      Leonardo da Vinci’s ‘Vitruvian Man’ is perhaps the most famous drawing of the human form in history. It depicts a man in two superimposed positions within a circle and a square. The proportions are carefully calculated in an attempt to represent the underlying perfection of Man and to link him directly to the Universe. Da Vinci was inspired by one of the great classical works, De architectura, written by the Roman architect Vitruvius. The relationship of the human form to a circle and square reflects ancient ideas – dating back to Plato, Pythagoras and earlier mystic traditions – which attempted to forge a link between Nature and geometry. Kepler’s early work on the motion of the planets was firmly rooted in this tradition, and he only jettisoned the idea that the motion of the planets could be described in terms of the perfect ‘Platonic’ solids when the data forced him to conclude that planets actually move in elliptical orbits rather than circular ones. It is interesting to reflect on the fact that the explanation for the motion of the planets is more elegant and beautiful than Kepler’s hoped-for geometrical perfection. As we’ve seen, the motion of all the planets and moons in the Solar System, and indeed every solar system in the Universe, can be described by the application of Newton’s laws of motion and Universal Gravitation; a profound simplification that would surely have appealed to Kepler, and to Plato before him, because Newton’s laws do embody a ‘perfect’ spherical symmetry, which is hidden but still evident in the structures СКАЧАТЬ