Re: There's something fishy about human brain evolution

On 23 Feb 2006 14:05:41 -0800, "nickname" <alas_my_loves@xxxxxxxxx>
wrote:

r norman wrote:
There is one species of frog, Rana cancrivora, that can tolerate
seawater. It uses urea as part of the mechanism to maintain
osmoregulation is such circumstances even though its skin is
permeable. That is the trick used by sharks, and the coelocanth, but
nowhere else among tetrapods as far as I know.

Iodine is widely found in soil and is available in plants grown on
such soils. Iodide ions in seawater (and soil) gets oxidized to
iodine which is volatile and is found in air (about 0.7 micrograms per
cubic meter). It then rains down to be absorbed by soil. There are
areas where the soil is deficient in iodine/iodide and in those
regions, animals can suffer from thyroxine deficiency. Severe
maternal iodide deficiency does cause poor fetal growth and brain
development.

I didn't know of the marine frog. Would you elaborate a bit on the
function of urea in it?
Reason for request: IMO rather than primarily "waste products, heat
regulators, or friction enhancers" as I've seen written, I think the
primary function at least "originally" of eccrene sweat (slightly
saline water), sebum (acidic oil), and urea (slightly alkaline?) was as
anti-pathogenic skin covering, especially in the presence of sunlight
(UV/IR), thus protecting against a diverse array of microbes, some of
which were tolerant of acid but intolerant of salt (fungal), others
tolerant of salt but intolerant of acid (algae), etc. No doubt this is
known, but I haven't seen anything written that includes the
combination of these protectants working together with sunlight to
shield the human body, largely effective in seaside and freshwater
environments, unless skin is penetrated or wounded, or the pathogen
enters via an alternate route (orifices).
This shield of overlaying grids on human skin includes:
1.dead proteinacious squamae (continuously shed skin flakes physically
removes pathogen)
2.acidic sebaceous oils
3.saline eccrene sweat
4.alkaline urea?
5.sunlight
6. possible ochre/fat (iron:anti-pathogen?) inland
7.clothing (terrestrial thorns & grasses blades) not needed on tropical
beach?

But why would frog need urea on skin? Dave Deden

Sweat has nothing at all to do with the frog story. Frogs don't
"sweat" in the sense of secreting fluid onto the skin to produce
evaporative cooling and hence thermoregulate when the ambient
temperature is higher than the body temperature.

The problem frogs face is that their skin is permeable to water.
Hence if their extracellular fluid has a different osmotic
concentration than the fluid environment they inhabit, then water
moves by osmosis across their skin. A frog living in fresh water
constantly gains water by osmosis through the skin. As a result, it
never drinks and has kidneys and a bladder that is specialized to
produce a very dilute urine to excrete that excess water.

However a frog living in salt water has the opposite problem: it
loses water by osmosis through its skin. If there is no source of
fresh water, it has no ability to retain enough water in its body
unless it drinks the salt water. But then it has the problem of
getting rid of the excess salt. Its kidneys can't handle that
problem: since it derived from ancestors living in fresh water, its
kidneys cannot produce urine more concentrated than the body fluids.
As a result, there are virtually no amphibians that live in salt
water. Other tetrapods (reptiles, and most birds) that do live in
salt water have specialized extra-renal glands that excrete very
concentrated salts solutions but amphibians never evolved that
ability. Mammals evolved kidneys with the Loop of Henle to produce
concentrated urine to solve that problem.

So some animals use a trick -- they allow urea to accumulate in their
extracellular fluids. Ordinarily, urea is nitrogenous "waste": a way
to excrete the amine groups produced by metabolism of amino acids in a
way that avoid the very toxic production of ammonia. Urea is
relatively non-toxic and many animals tolerate a rather high
concentration. So that particular fish allows urea to accumulate in
its blood and extracellular fluids. This increases the osmotic
concentration of these fluids so it doesn't have the osmotic problem
of losing water when living in sea water but still has the lower salt
concentration characteristic of vertebrates. The particular marine
frog, Rana cancrivora, uses this trick. Sharks and other
elasmobranchs use the same trick, as does the coelocanth. It is
purely related to osmotic regulation and has nothing at all to do with
sweat.

I believe that fact that human sweat contains urea is simply a
reflection of the fact that blood and other extracellular fluids of
humans contains urea. Sweat is not particularly an effective method
of excreting material from the body -- that is the major function of
the urine. However material does leave the body through the sweat so
that, in extreme kidney malfunction, sweat excretion might serve some
useful function.


.

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