Within the Anseriformes, the Anatinae (ducks) shows a wide variation in diet and feeding mechanisms, in contrast to the Anserinae (geese and swans). While grazing is common in the Anserinae, only few species within the Anatinae use terrestrial grazing as their main feeding mechanism (e.g., wigeons). This may be explained by conflicting functional demands of grazing and filter-feeding on the trophic system. In this study, the feeding performance, feeding mechanisms and oropharyngeal anatomy is compared between geese, wigeon and a general filter-feeder/pecker, the mallard (Anas platyrhynchos). The results show that the functional demands of filter-feeding and grazing are conflicting: filter-feeding requires a bald palatal surface and under-tongue transport for optimal functioning of the lingual cushion as a piston, whereas the transport mechanism of grazing requires large maxillary spines and over-tongue transport to retain grass during tongue protraction. The oropharyngeal anatomy of the wigeon shows a compromise in the small size of the maxillary spines that enable a sliding mechanism for the transport of a limited amount of grass. Filter-feeding is sometimes considered as a key adaptation that led to radiation in the anseriforms (Olson and Feduccia, 1980; Lack, 1974). We suggest, as an alternative hypothesis, that feeding on water plants may have led to the evolution of ridge-like structures in the bills, a sliding mandibular joint and the use of a water flow through the oropharynx (tongue pro- and retractions) for food transport in early anseriforms (cf. geese). A selection pressure on filter-feeding resulted in a large increase in efficiency of this system by the introduction of under-tongue transport of food and water (repatterning of bill and tongue movements) that enables the simultaneous intake and transport of a suspension of food particles (cf. Anatinae, a.o. Aythya and Anas). Terrestrial grazing later evolved by the development of maxillary spines, and in the case of the wigeon, a secondary change from the under tongue transport mechanism to over tongue transport for the grazing and pecking mechanisms only.
The feeding apparatus of the fringed turtle Chelus fimbriatus (Schneider, 1783) was studied to elucidate the feeding mechanics of an aquatic feeding specialist that has never been investigated in detail before, regarding gross morphology. The skull and hyoid apparatus as well as associated musculature were examined by computer tomography and dissection; the tongue was examined by scanning electron microscopy. The flat skull, the possibility to enormously depress the mandible combined with a cheek-like development, the large, ossified hyoid apparatus, and a well-distensible esophagus enable the turtle to produce an enormous suction force the prey is inhaled with. The jaw adductors are poorly developed in relation to other turtles and thus help keep the skull shape flat; nevertheless, they are able to generate high velocities and exhibit some new performance lines. The hyoid musculature is as well-developed as the hyoid apparatus itself, promoting the high depression velocity that is necessary for good feeding performance. The tongue is nearly reduced and lacks dorsal morphological differentiations. Taking all the morphological features into account, C. fimbriatus is an extremely well-adapted turtle making this species a very interesting object of investigation.