Morgab tears /salt

Elaine Morgan (
Thu, 09 Nov 1995 15:14:27 GMT

Question: Does our weeping excrete salt?

The recent argument over this was one of definition. Our tears are
hypotonic i.e less salty than blood. So although some salt is excreted
in the literal snse of ejected out of the body, this would not help
the condition of having too much salt in the tissues. Because
relatively more H2o than NaCl is taken out of the blood, it ends up
slightly saltier than before. Hence we can't cure thirst by drinking

This does not contradict anything I've ever said. I merely speculated
that at one time our tears and sweat might have been hypertonic, like
salt glands - a proposition neither provable nor disprovable, but it
seemed to me a possible way of accommodating some facts otherwise hard
to explain. The line of thinking was:

It has been scientifically established that non-reflex tearing has
evolved in birds and reptiles, that it has evolved always and only in
marine species, and that it has evolved in such species more than once.
"The location of the (salt) gland in the turtle indicates that it has a
different evolutionaryt origin." (Schmidt-nielson, Sci. Amer. Vol 200,
No 1, p 116) Similarly wings have evolved more than once - in insects,
in birds, in bats - but always for the same purpose. When we find a
flightless bird, we do not reason :"these attenuated wings are no good
for flight, so they must have evolved for some other reason". We assume
that they must have had ancestors that flew. It did not seem to me an
untenable proposition that the excretory powers of human tears could
have attenuated in the same way, to save energy, when their usefulness
was gone.

However, the position with mammals was never quite so unambiguous, and
now thanks to the bracing effect of defending the issue on the Net, I
am having a rethink. The chief stumbling-block is that Schmidt-Nielsen
(op cit) seems to imply that seals' tears are hypotonic like ours. He
does not actually say this, only that injecting seawater into their
stomachs does not cause the tears to flow. But I suspect that it might
follow .

N. B. Please note that this (to me) uncomfortable fact was not
contributed by the redoubtable JDM but by the allegedly rabidly biassed
EM, who could very easily have kept her mouth shut about it. I look
forward to the day when JDM will contribute a quote which will be a
gift to the opposition. But I am not holding my breath.

It is uncomfortable because the seals are still in the sea, and if
their tears were ever salt-excretors I would have expected them to
remain so. The other, smaller, snag is the reference to weeping
beavers. I'm not dead sure about them. They are not mentioned in the
beaver books I have read. But if they do weep, it is uncomfortable
because they are freshwater mammals and probably always have been.

So perhaps the salt theory was wrong. For SMH'ers that is the end of
it. They can rest on their laurels, say "Elaine admits she may have
been wrong", and as with many other anomalies say simply "We do not
know the reason for our tears. There may no reason and if there is we
may never find it, so let's talk about something else and thank God for

It is not the end for me. I don't go along with the "may be no reason"
cop out. If I had the wrong reason there must be a right one
somewhere. Perhaps the tears of aquatic mammals have nothing to do
with salt - merely something to do with water. Diving under water -
ecpecially if you have to keep your eyes open to look for fish, or
shellfish, or to find the right place to jam a stick into your nice
new dam - may present its own dangers to the eyes. The naturally
protective coating of tears common to all mammals may get washed away
too quickly by the water (this is surely inevitable) and an
occasional more prolific supply would be desirable

If the danger was bacterial or viral, there would be no immediate
physical irritation to the eyeball of the kind necessary to trigger
off reflex weeping.

Mammalian tears contain a variety of protective elements. One of the
most effective is lysozyme; some of this is found in the tears of the
higher primates. (A. Fleming, Proc. Roy. Soc. (London) B93, 306 (1922))
It has been shown to inactivate the viruses of many infections such
as herpes simplex (if you get a cold sore on your lip get a friend to
weep onto it) and herpes zoster and warts and vaccinia, etc. (R.
Ferrari et al, Nature, 183, 548 , 1959)

Prediction time. This new hypothesis would predict that among the
differences between chimps and humans would be a higher concentration
of protective substances in the tears of humans, especially in the
latest-evolved type, namely psychic tears. Okay?

"In human tear fluid, relatively high concentrations of proteins like
immunoglobin A, lactoferrin, tear-specific pre-albumin, and lysozyme
are found, besides high activity of the tear-specific enzymes
peroxidase and amylase." ("Species differences in tears; comparative
investigation in the chimpanzee" by VMW Bodelier et al. Primates,34(1)
pp77-84, Jan 1993)

"The species comparisons performed until now indicate that human tears
remain unique in this high content of lactoferrin" (Ibid)

And, as previously noted, psychic tears contain more of
these proteins than the old-fashioned reflex ones.

JDM will now explain to the seminar why a savannah/mosaic
anthropoid needed more immunising components in its eyes than its
relatives in the forest.