25 to 35 %
||Gross, J. (1963). In Comparative Biochemistry, vol. 5, ed. Florkin, M. & Mason, H. S., p. 308 4th paragraph. New
York: Academic Press.
||A. Neuberger, in "Fibrous Proteins and Their Biological Significance", Symposia Soc. Exptl. Biol. No. 9, 72-84 (1955).
||"Collagen, the major fibrous element of the extracellular connective
tissues, is probably the most ubiquitous and plentiful protein in the
animal kingdom. Neuberger (primary source) has compiled a list of collagen content
of a variety of mammalian tissues. He states: "There is no quantitative
information available about the total collagen content of any mammal
but it can be estimated that between 25 and 35% of the total protein of
the body is collagen." In certain invertebrate forms, such as the Keratosa
among the sponges, and some species of Echinodermata, such as the sea
cucumber, it accounts for an even larger proportion of the total composition
of the organism."