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Feathers for S excretion (very long)
There seem to be some misconceptions on this hypothesis onlist. First,
nobody says feathers evolved for sulfATE excretion -- this is water soluble
and totally unproblematic AFAIK. 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.) In a small insectivorous
endotherm H2S can surely become a problem. Also, it has been asked why
mammals have not evolved such a mechanism for excreting sulfur without H2S
production -- let me ask why mammals have hairs (all over their bodies,
instead of only whiskers on the snout)???
The misconceptions are probably caused by the fact that both (mind the
number) papers which I know on this topic are in German and have appeared in
relatively obscure magazines. In particular:
Josef H. Reichholf: Die Vogelfeder -- ein Eiweiß-Endlager? [ = The bird
feather -- a final disposal site for protein?], kosmos 4/97, 68 -- 71
same author: Evolution der Vogelfeder [ = Evolution of the bird feather],
Praxis der Naturwissenschaften Biologie, 5/47 (Urvögel), 20 -- 24 (15 July
The former says [my translation -- phew]: "Both theories [arboreal,
cursorial] have one thing in common: They start with a direct effect by the
environment and try -- in a typically darwinistic way -- to understand the
evolution of the bird feather as an adaptation to [...] [the environment].
This, of course, means that the very first beginnings of the origin of
feathers must either have brought very big advantages in comparatively short
time, or their little advantages must have persisted unchanged or growing
for a very long time; despite all vicissitudes of life. Otherwise natural
selection would not have favored the feather-bearers.
But is this realistic enough? How many reptiles that climbed around
in the trees would have had to fall down and break their necks so that the
survivors would have gained, thanks to their small scale enlargements, the
massive advantages for surviving that were necessary for the origin and
further evolution of the feather? Why do numerous, in their climbing
abilities very different, but still living reptiles manage their lives in
trees without braking scale enlargements? The cursorial theory also has its
difficulties to make the advantages of its beginning stages. When the scales
are large enough, the animal can take off pretty suddenly, as the chicks of
grouse [Galliformes: Phasianidae: Tetraoninae] show. These can fly several
meters some days after hatching with their highly incompletely developed
wings, but not before; the growing feathers must have become big enough! The
same problem poses itself to feather evolution. Both usual theories can't
therefore answer this important point at the beginning.
But perhaps the first feathers weren't evolved for conquering the
air? [...] [Presents the hypothesis that feathers evolved for insulation.]
Both the arboreal and the cursorial theory are, according to this
hypothesis, merely ideas for explaining flight and don't primarily deal with
the origin of feathers.
But can these ideas really explain the beginnings of the evolution?
Many reptile species live without feathers -- and obviously not badly. So
where is the all-deciding advantage supposed to be? Furthermore this model
requires that the [...] [ancestors of birds] were already warm-blooded. Why
should they have been? [Because they were dinosaurs, IMHO.] And how
[important question!] had they arrived at this warm-bloodedness?
I'd like to suggest a different approach. It isn't based on the
environment and its (selective) pressures, but on the "inside world", the
physiology of the animal.
The feather is a product of protein metabolism. It is composed of
keratin [...]. A typical feature [of keratin] is its delicately fibrous
structure which results mostly from the sulfur content of keratin. Therefore
sulfur-containing amino acids are necessary for keratin production.
Feather production consumes relatively much protein. The more
surprising is the phenomenon of moulting. During moulting birds don't
discard only worn feathers, but for the most part fully functioning ones,
although they are always able to replace single feathers in case of massive
damage. But protein is a rather rare building substance. So why do birds
throw away well-preserved feathers? Wouldn't it be more economical to retain
them as long as possible? A forcing outer necessity for moulting is, at
least in most cases, not obvious. But feather production and protein budget
seem to be connected. Anyhow the sulfur content of bird feathers, up to 3 %,
is surprisingly high.
This led me to an unusual idea: Are feathers maybe not a product of
direct adaptation, but arose out of a protein excess? This would explain
many things. In particular, in the early Mesozoic the insects had radiated
explosively. This offered the chance for some reptile forms [basal
ornithodirans and their ancestors, IMHO] to use this new alimentary
resource. This had advantages, because insects contain lots of easily
digestable protein and energy-providing fat -- in any case more and better
usable one than plants or other reptiles [...]. But they are very mobile.
Using this new resource meant leaving the leisurely reptile life towards
higher running speeds. This of course required more energy [and leaky cell
membranes, but this seems to be just one mutation], but this was provided by
[...] [insect fat]. [...] [And there was now enough fuel for endothermy.]
Pretty soon a problem must have surfaced. Insects contain, aside
from fat, very much protein high in sulfur. Apparently more [sulfur] than
the reptile organism could use or excrete through the cloaca -- which isn't
easy, particularly for sulfur compounds and an intensive metabolism. Thus
some animals may have deposited this excess [...] elegantly in the form of
enlarged scales that they may already have dropped from time to time. As an
aside effect these enlarged scales could have provided insulation -- the
animals equipped with this didn't need as much fat as their rivals and thus
had an advantage. [Once feathers are here good old Darwinian selection
strikes back. =8-) ] [...] Such a process doesn't need selectionary
advantages and can go on for a very long time [that probably wasn't even
needed] without environmental pressure, because the topic is the removal of
substances from the body.
Recent birds, with their high basic metabolic rates, also need much
energy. If they use fat from insects they have excess protein and must get
rid of the sulfur-containing amino acids. Maybe this is why many species
moult when they [...] [eat much] and when they build up fat reserves for
winter or migration -- instead of changing their feathers slowly and
continuosly over the year. It is also conspicuous that males of many bird
species produce larger feathers for display. The females invest part of
their excess protein into their young and the very large, protein-rich eggs,
while the males produce conspicuous, over-dimensioned display feathers [...]
or moult several times in one season, like the eider ducks with their
protein-rich clam diet. On the other hand birds with protein-poor diet, such
as parrots, moult not much and often not regularly.
This new model of feather evolution has the advantage that all
conclusions and assumptions can be tested by metabolism physiology."
The other paper repeats all this in more detail and a more scientific style
(with refs and stuff), along with illustrations, and mentions that the
sulfur content of feathers is over 3 % in songbird feathers. "Hydrogen
sulfide is quite poisonous, in particular at the body temperature of birds
which exceed 40 °C." Heavy metals are also deposited in feathers (in complex
compounds with sulfur?). Once more it is stressed that this model is
experimentally testable to a large degree.
I hope this helps more than it stuffs mailboxes!!!