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Re: Feathers for S excretion (very long)
David Marjanovic reports:
"The problem assumed to have led to the
first protofeathers, whatever bristles or lengthened scales they were, is
the deconstruction of sulfur-containing amino acids, cysteine and
methionine, which leads to hydrogen sulfide (H2S). This is a poisonous gas,
as poisonous as hydrogen cyanide (blue acid, HCN). It is water soluble but I
can't imagine it can easily excreted by kidneys. (Starving people, who burn
their own proteins, smell of H2S out of the mouth.)"
This is interesting. However, have you considered Vitamin B12, a necessary
part of everyone's diet, which contains cyanide compounds? Although
hydrogen and potassium cyanide are deadly to many vertebrates in small
amounts, we consume and depend on Vitamin B12 for our health and survival,
and so far as I know, our kidneys and liver are adept at breaking B12 down.
What might be interesting to see would be the amount of sulfur-containing
amino acids produced versus the H2S or Sulfur content of hair or feathers in
various animals, and perhaps a comparison of these things with reptile
scales as well.
David reports further:
"This led me to an unusual idea: Are feathers maybe not a product of
direct adaptation, but arose out of a protein excess? [and then continues
with an evolutionary scenario wherein the ancestors of birds chased insects
through the trees for food]"
My question would be, couldn't the protein and sulfur "excretion" also have
evolved post hoc. In other words, the feathers may have become an H2S and
sulfur-containing amino acid dump AFTER they evolved, and the body just
"took advantage" of this resource once it appeared. To test your hypothesis
that sulfur excretion is tied to feather development, it might be
interesting, again, to look closely at the sulfur and sulfur-containing
amino acid content of various reptilian scales, including crocodilians. I
am aware that in certain embryological experiments, the genes for feather
development in chicks can be "toggled" such that scales, not feathers,
develop. Could such experimental animals have less, the same, or more
sulfur, etc., in their integument than control groups?
Another area to look at would be insectivorous mammals. Your hypothesis
predicts two things: 1) feathers or hair evolved as sulfur (meaning the
elements you site, sulfur-containing amino acids, etc.) dumps; and 2) an
insectivorous lifestyle could lead to flight. Is the sulfur content in the
hair of insectivorous mammals greater than that of other mammals? How many
species of flying or gliding mammals are purely insectivorous? These sorts
of questions would have to be addressed to improve the explanatory power of
your hypothesis, but it could be very interesting. You have offered a
hypothesis that can be investigated and empirically tested and repeated by
others -- we need more of this in dinosaur research.
My personal suspiscions (and note that I am not even proposing a testable
hypothesis, so take this with a very large grain of salt) are that feather
evolution was intiated by the more prosaic vagaries of evolution -- the
lengthening and/or differentiation of reptilian scales with subsequent
display advantages or perhaps just neutral survival significance were
selected for or maybe just not selected against. Perhaps all the advantages
of feathers evolved subsequent to something like this, without a "purpose"
for flight, sulfur dumping, etc. But I realize this is all speculation with
no testable hypothesis being proposed.
Interesting information and hypothesis, David.
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