Glycosaminoglycans in the tongue of birds
We examined the literature to verify whether adaptations and modifications in the structure and glandular secretions of birds' tongues are related to habitat and can be ascribed to evolutionary processes. The data are discussed in relation to species taxonomy, following the Sibley and Ahlquist classification [C. G. Sibley and J. E. Alquist, Philogeny and Classification of Birds. A Study in Molecular Evolution (Yale University Press, New Haven, 1990)]. The following conclusions are drawn: gustatory papillae and taste buds are present in varying numbers in most species. The composition of gland secretions is also found to be variable. Proteic secretion is documented in Larus modestus, Sula variegata, Fulica atra only. Acid proteoglycans both with sulfomucins and carboxymucins, and also glycoproteins, are consistently found. Sialic and hyaluronic acids are found in many species. Our overview indicates that the presence or absence of gustatory papillae is related to adaptation processes that these structures undergo in response to environmental factors, and that the absence of front tongue glands can be ascribed to habitat and feeding habits. Referring to the Sibley and Ahlquist classification, proteins are present in the glandular secretion of less evolved species, whereas more evolved species exhibit a gradual decrease in proteins with the exception of hyaluronic acid, which is absent, and a progressive increase in glycoproteins and acid proteoglycans.