Morphological reflections of evolutionary adaptations in the tongue of the white-headed duck.

During an organism's evolution, functional adaptations help species to become better suited to their ecological niches. From the morphological aspect, these adaptations are reflected in the anatomical specializations of different organs. Specializations of the lingual organ is a critical adaptation of birds, such as the white-headed duck (Oxyura leucocephala), that enables their nutritional requirements to be met. For optimal use of the available food resources, the white-headed duck utilizes three methods of food collection, namely pecking, grazing and filter-feeding. Since this species is classified as endangered, we conducted the present study on two carcasses of the white-headed duck (death due to natural causes) employing routine histological methods, light microscopy and scanning electron microscopy. Our results show that the tongue of this bird shares some similarities and some differences with the tongue of other members of the family Anatidae. The results confirm that it is better adapted to the filter-feeding method rather than to other types of food intake. This adaptation is reflected by anatomical specializations of its lingual structures, including the stair-like outline shape, bi-sectional lingual body, a deep median sulcus, lateral conical papillae, mucus secreting glands, lack of serous secreting glands, cartilaginous skeleton and the triangular fibromuscular structure of the lingual body. The so-called triangular structure and cartilaginous skeleton are the major structures involved in the lingual motions during the filter-feeding method. The presence of the triangular structure and its connection with the cartilaginous skeleton and lingual mucosa have not previously been reported in any species of birds.