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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@huxtable.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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