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Conflicting demands on the trophic system of Anseriformes and their evolutionary implications

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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.


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