Just a comment on why diving birds don't get Decompression Sickness,
Nitrogen Narcosis or Oxygen Toxicity and a question, "why don't they get
Pulmonary Baratrauma of Descent from free diving to such depths?" (not to be
mistaken for Pulmonary Barotrauma of Ascent from compressed air diving).
My understanding is that Decompression Sickness can only be contracted by
diving with compressed air at depth and ascending too fast for the stored
nitrogen to escape from the body without forming large bubbles that
interfere with body function. The nitrogen concentration increases in a
diver's body with depth and time as s/he continues to take in compressed air
delivered by a regulator at the atmospheric pressure the diver is at. At
less than 10m (2 ATA = 2 Atmospheres Absolute = total ambient pressure at
that depth) a diver cannot contract decompression sickness (unless maybe
they shoot to the surface). Unless penguins have a tank of compressed air
we don't know about, the thought of such a hazard probably never entered
their minds.
I am not so confident about Nitrogen Narcosis. The symptoms manifest
themselves on the nervous system after a particular pressure is reached,
thus increasing the partial pressure of nitrogen to 80% of the ATA reached.
The symptoms get worse with depth, but not with continued exposure after the
initial onset of symptoms. All divers are 'narked' to a degree when they
reach 30m (4 ATA). However as the onset of symptoms is not immediate the
small delay of a few minutes suggests to me that they may need to be
breathing compressed air (ie have access to more than just a single breath
of air) to contract symptoms. Notably humans can build up a resistance to
nitrogen narcosis with continued dives and the affect of narcosis varies
enormously with species (in test chambers, mostly on mammals).
One can also decrease the effect of narcosis on performance by carefully
focussing on the task to be done to the exclusion of nearly every thing else
(you don't ignore your air supply gauge or dive timer of course). It would
be intersting to know exactly what the penguins do while underwater. Do
they only descend to the really deep depths to catch a fish from a school
they may detect from shallower water (ie lower partial pressure)? Do they
"muck around" down there or are they focussed highly on a task? As their
metabolism and body temperature appear to drop tremendously (from what I
have read from the chat on this subject) maybe they don't waste energy or
effort, but remain totally focussed. Certainly there would be an
opportunity for the old "natural selection" to zap any birdie who got it wrong!
Similarly Oxygen Toxicity may not occur without an artificial air supply. It
also operates on reaching a particular partial pressure and gets worse with
depth. It can quickly kill a human being breathing air at 90m (10 ATA).
All people diving below 20m (3 ATA) (lots of recreational SCUBA divers do)
get mild Oxygen Toxicity but do not show symptoms because they need to dive
for many hours to become symptomatic. I have my doubts that one breathful
of oxygen (20% of air) would amount to much particulary when it is being
metabolised. I have not ever heard of free divers getting Nitrogen Narcosis
or Oxygen Toxicity.
Now the real question to my way of thinking is "how do these birds avoid
Pulmonary Baratrauma of Descent"? As a diver descends on a breath hold
(free)dive, fluids gather in the lungs equalising the pressure inside the
lung with the ambient pressure. My understanding is that when the diver
reaches 40m (5 ATA)(yes people do free dive this deep and deeper) his/her
lung tissues break down with the overload of fluid. How do penguins over
come this???
I hope this puts a few things into perspective.
Cheers from
Julie Raines
(Honourable bird and fish watcher!!)
......and, no I haven't seen any penguins on SCUBA lately.
Julie Raines
Australasian Ecological Services
E-mail
Snail PO Box 312, Wanneroo, Western Australia 6065
Tel/fax + 61 9 306 1642 Mobile 014 081 702
|
