Re: Tears and 'salt excre

J. Moore (
Sun, 12 Nov 95 21:43:00 -0500

: Sea mammals, along with marine birds, reptiles, and bony fishes,
: do *not* have a physiological compostion "almost the same as
: seawater"; such a composition restricted to some invertebrates,
: although elasmobranch fishes (non-bony fishes such as hagfish,
: rays, and sharks) have plasma which is isotonic to seawater
: (that of sharks and rays are actually somewhat hypertonic to
: seawater due to large amounts of urea in the plasma).

SS> OOooops! did I say mammals? I meant animals.
SS> ....and please replace almost the same with "more similar to".

Well, *some* sea animals, but of course as these animals are
either non-bony fish or invertebrates this has less and less to
do with hominids, as when talking about hominids we're definitely
dealing with mammals who had been mammals for a not inconsiderable
time. ;-)

: Sea mammals therefore do have to be able to eliminate sodium in
: the same manner as we do, which is why they have large, complex
: kidneys compared to us.
SS> I'm not arguing that they don't have kidneys. I'm saying
SS> that there are differences in body ion concentrations which
SS> elevate some of the work performed by the kidneys of
SS> aquitic ANIMALS.
SS> Sean Stinson

For non-mammalian animals, the kidneys are not the most important
route (in most cases) for ion-water balance. In those animals which
actually do have a sodium-water balance which is isotonic or
nearly so ("more similar to" seawater), the kidneys are *definitely*
not the most important route (since these animals are either fish
or invertebrates) -- gills are, for fish, and as for invertebrates,
I haven't looked that one up.

For all mammals, however (and mammals are the base subject matter
when it comes to paleoanthropology) the renal system is by far the
most important system in regulating this necessary sodium-water
balance, since it's the only one which can actually produce
the required hypertonic fluid.

Jim Moore (

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