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    Sperm whale buoyancy.
    From: George Huxtable
    Date: 2007 Mar 20, 10:48 -0000

    Let me start with in apology. This has little (indeed, nothing) to do
    with navigation, but about the hydrostatics of sperm whales. If such
    question are of no interest to a reader, he should press "delete" now.
    
    I raise this matter in the hope that others who know more about the
    matter than I do can point to where I can find answers. What
    information I have comes from "Whales of the World", by Lyall Watson,
    a volume I've respected, but which hasn't satisfied my questions.
    
    My wife and I were recently introduced to two "resident" sperm whales,
    who spend their time patrolling an ocean trench which happens to come
    close inshore off Kaikoura, New Zealand.
    
    Sperm whales are the ones with an enormous squareish head, with a
    small jaw below it; the head is an immense can of pure spermaceti oil,
    containing many tons of the stuff. Their way of life is a strange one,
    even by whale standards. They earn their living by swimming down
    vertically to the bottom, at depths of a kilometre, sometimes much
    more, to graze on squid living near the bottom. These dives commonly
    last for 40 minutes or more. Then they return vertically to the
    surface to breathe air, for 10 minutes or so. A sequence that's
    regular as clockwork.
    
    It has set me wondering about the physics of the problems that face
    such a creature, alternating between regimes at such widely different
    pressure, and how it has managed to solve them. Roughly speaking, the
    pressure at a kilometre deep is 100 atmospheres, and any gas at that
    pressure is compressed to only one hundredth of its volume at the
    surface. In general, the volume of liquids and solids, and that of
    seawater, changes little with pressure. They are pretty
    incompressible.
    
    There's no way that a whale could exist on a big breath, as a
    pearl-diver does, over all that time. Instead, I understand that a
    sperm whale is adapted to rely on oxygenated blood that has been
    recharged while it was breathing at the surface, and its tissues are
    dark red or black because they hold large quantities of that
    oxygen-rich blood.
    
    My first question is this; does such a whale take a last lungful of
    air down when it dives, or not? If it does, that air provides buoyancy
    as the whale starts to dive from the surface, just when it isn't
    needed, as it would have to be overcome by downward drive from the
    tail flukes. A whale isn't built like a submarine, with a steel hull
    to resist external pressure, but is flexible, allowing outside
    pressures to be transmitted throughout its structure. So, at a few
    atmospheres of pressure, buoyancy from any inhaled air has been
    largely lost, and at a kilometre deep any air aboard is compressed to
    only 1% of its original volume, so virtually none of that buoyancy
    remains when it's at the bottom. Just where buoyancy might be most
    useful, at take-off, to return to the surface, especially in an
    emergency. It's hard to imagine that such a creature puts itself in
    such danger as to be negatively buoyant at the bottom, and completely
    reliant on tail-power to drive upwards out of any fix.
    
    It seems to me, then, that hydrostatically speaking, any air taken
    aboard prior to the dive is disadvantageous. I learn, from Watson's
    book, that the rib-cage is designed to collapse completely. Indeed,
    that's absolutely essential, to allow any air in the lungs to compress
    down to almost nothing. What I ask, is whether that collapse of the
    ribs takes place deliberately before the dive, or during it?
    
    Evidence in the other direction comes from the first spout of a sperm
    whale on surfacing, the first outward breath that gives rise to the
    old cry of "there she blows"; according to accounts I have read it is
    always more conspicuous than later blows. Does this first blow come
    from air that's been taken down to depth and brought back again, or
    does it actually follow a first, unseen, inward gasp of air? Does
    anyone know?
    
    A related question is the purpose of that immense tank of spermaceti
    oil, which takes up most of a sperm whale's enormous head. What's it
    really for? Watson points to a possible function as a focussing
    element in conjunction with the sperm whale's ultrasonic transducer,
    but also mentions the possibility of adjusting buoyancy by regulating
    its temperature, in relation the temperature variation with depth of
    the sea water. It seems to me unlikely to be possible to change the
    temperature of such a great mass of oil in a timescale that would be
    useful over a 40-minute dive. However, certain oils and waxes do
    change their volume (and therefore buoyancy) quite significantly as
    they melt and solidify; indeed, it's likely that the thermostat in the
    cooling circuit of your car engine will rely on such an effect. It's
    clear that the oil, being lighter that water, could be useful  anyway,
    in compensating for the creature's weight of bone, to get the average
    buoyancy right. Being right at the fore-end of a whale, it must tend
    to make it tail-heavy, which may be useful in keeping up the blowhole
    on the tip of its nose, out of the water, when it sleeps.
    
    Another question that arises relates to the whale's digestive system.
    No doubt, a whale's digestion continues to act while it's at the
    bottom, presumably evolving large quantities of methane and carbon
    dioxide. Under such high pressure, that compressed gas may not provide
    much volume or buoyancy at the bottom, and it seems likely that carbon
    dioxide, under pressure, will dissolve into any watery fluid, just as
    it does in a bottle of "pop" (at much lower pressures). But on return,
    as the animal approaches the surface, any gases, that it hasn't
    expelled while down below, will expand, and may also come out of
    solution, as when you take the cap off a pop bottle. Any such increase
    in volume will tend to be dramatic, as the surface is neared, doubling
    in the final 30 feet! To avoid an explosion, does a whale produce a
    gigantic fart, as it nears the surface? Whalers' accounts often refer
    to the disgusting fishy smell that comes with a whale's first blow,
    but I wonder whether the source of that smell may be not from the
    blowhole, but from the other end, bubbling up as it surfaces.
    
    How, I wonder, has a sperm whale evolved in such a way as to solve
    those problems presented by hydrostatics, Archimedes principle and the
    Gas Laws? These are no more than musings, from my position of woeful
    ignorance about whale physiology. No doubt they have been asked, and
    answered, by those with a better understanding. If anyone can provide
    those answers, or point me toward them, I will be grateful.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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