The Ancestor\'s Tale(c.1) by Richard Dawkins

Author:Richard Dawkins
Language: eng
Format: mobi
Tags: prose_contemporary
ISBN: 9780618005833
Published: 2011-09-29T22:00:00+00:00

The Pike's Tale

In the sad province of Ulster, where ‘the Mountains of Mourne sweep down to the sea’, I know a beautiful lake. A party of children were swimming naked there one day, when somebody shouted that they had seen a large pike. Instantly all the boys — but not the girls — fled to dry land. The northern pike, Esox lucius, is a formidable predator of small fish. It is beautifully camouflaged, not against predators but to help it steal up on its prey. A stealth predator, and not particularly fast over a distance, it hangs almost motionless in the water, creeping imperceptibly forward until within striking distance. During the deadly creep, it propels itself with imperceptible movements of the rear-mounted dorsal fin.

This whole hunting technique depends upon the ability to hang in the water at the desired level, like a drifting dirigible, without any effort, in perfect hydrostatic equilibrium. All locomotor work is concentrated on the clandestine business of creeping forwards. If a pike needed to swim in order to maintain its level, as many sharks do, its ambush technique would not work. Effortless maintenance, and adjustment, of hydrostatic equilibrium is what teleost fish are supremely good at, and it may be the single most significant key to their success. How do they do it? By means of the swim bladder: a modified lung filled with gas, which provides sensitive dynamic control of the animal's buoyancy. Except for some bottom-dwellers who have secondarily lost the swim bladder, all teleosts have it — not just pike and not just their prey.

The swim bladder is often explained as working like a Cartesian Diver, but I think that is not quite correct. A Cartesian Diver is a miniature diving bell containing a bubble of air, which hangs at hydrostatic equilibrium in a bottle of water. When the pressure is increased (usually by squeezing down the cork in the neck of the bottle), the bubble is compressed and less water is displaced by the diver as a whole. Therefore, by Archimedes’ Principle, the diver sinks. If the cork is eased slightly upwards so that the pressure in the bottle decreases, the bubble in the diver expands, more water is displaced, and the diver floats a little higher. So, with your thumb on the cork, you can exert fine control over the level at which the diver finds its equilibrium.

The key point about a Cartesian Diver is that the number of air molecules in the bubble remains fixed, while the volume and the pressure are changed (in inverse proportions, following Boyle's Law). If fish worked like Cartesian Divers, they would use muscle power to squeeze, or relax, the swim bladder, thereby changing the pressure and volume but leaving the number of molecules the same. That would work in theory, but it isn't what happens. Instead of keeping the number of molecules fixed and adjusting the pressure, the fish adjust the number of molecules. To sink, the fish absorbs some molecules of gas from its swim bladder into the blood, thereby reducing the volume.

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