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feathers as N2 excretors
The recent suggestion that feathers are actually a pathway for the excretion of Nitrogen products has recently surfaced on this list. This is not a new idea and has been most recently advocated by Josef Reichholf (Archaeopteryx 14: 27-38 1996). He begins with the premise that feathers are 'expensive' to make and then are molted every year, in spite of the face that they are not worn completely. To account for this economic loss, he points out that feathers are rice in to chemical elements, S and N, that are potentially toxic. N of course is converted metabolically to simple compounds such as urea and uric acid and excreted directly. In addition to the amino acids of the feather proteins, N also comes from the metabolism of nucleic acids. S appears in feathers in the amino acids cystine (there is next to no methionine in feathers). Reichholf argues that rather than convert this S in feather amino acids to hydrogen sulfide (H2S) which is highly toxic, the S-containing AA's represent a pathway to excrete the material where it is insoluble and therefore of no danger to the body. He rationalizes the annual molt as a detoxification mechanism.
He makes some other arguments regarding dietary energy that are difficult to accept. He uses these to establish 'excess" proteins as related to the formation of keratins. He then relates this to the proposed excretory function of feathers. In the article (in German with English summary) he also postulates that the origin of feathers proceeded the evolution of flight. Nothing new here except that he tries to make the origin of feathers a response to 'excess' protein intake. It's convoluted, as it is difficult to image just what 'excess' protein is (he believes it comes from dietary shifts) and that feathers contain no essential amino acids. Further, S metabolism essentially is based on water soluable compounds and is rarely or never reduced to H2S. It occurs most commonly combined with O, H as soluble ions.
My personal opinion is that he is slightly off the mark on this, but that's an opinion. There are certainly other reasons for molting feathers.
Alan H. Brush
92 High St.
Mystic, CT 06355