Tears and 'salt excretion'. Was Re: tears

Bill Burnett (bbur@wpo.nerc.ac.uk)
Wed, 18 Oct 1995 15:15:12

In article <462e56$ei0@scotsman.ed.ac.uk> jamesb@hgu.mrc.ac.uk writes:
>hubey@pegasus.montclair.edu (H. M. Hubey) wrote:

>>>One problem with this: the salinity of human tears matches the salinity
>>>of the body. They are not used as a salt excretion mechanism.
>>First sentence doesn't justify the second. If you can get rid
>>of salt from your body, it's a salt excretion mechanism.

>Quite right, even though the tears are only as salty as body fluids, the
>weeper still loses salt.

>James Borrett.

Yes, well, so they do. On the other hand, if you want to REDUCE internal salt
levels then you need to INCREASE the concentration of your excretion product.
Hence rather a lot of rather nifty adaptations (e.g. the loop of Henle) for
increasing urine concentration in that other (better documented) excretory
organ the kidney. If tears don't concentrate salt (or something else) then
all they do is reduce total body fluid volume. Tears in the marine crying
examples I mentioned before (and the beavers) are highly concentrated.

Actually, come to think of it, that's a pretty good argument against our tears
evolving in an aquatic environment... at least in a salty one. You can't
afford to waste water in a salty environment or you get dehydrated very

Something else you have to explain is, of course, that whether or not they
actually cry when upset, all those mammals which don't have dry corneas have
tear ducts of some description, or am I wrong? I'm pretty sure that's all of
them but I'm open to correction. What this means is that for those mammals
which didn't evolve in the sea (:-)) there's some reason other than the
increased salt stress of living in a marine environment to explain tears and
the AAT 'becomes' unnecessary. Doubtless a return to the all mammals evolved
in water at 98.1F argument will be forthcoming from certain quarters...

Incidentally, do I have to produce a definite rebuttal of the AAT tear
producing mechanism to convince you? Or would a sufficient alternate
hypothesis be enough to sow the seeds of doubt?