Distinctive morphofunctional features of the bill and hyoid apparatus of turacos (Aves, Musophagidae): their relation to frugivory.

The present study points to morpho-anatomical features that allow vegetaria n but mainly frugivorous turacos to fill at best their specific feeding req uirements. Mechanisms are analysed which the bird can use to detach a fruit and avoid it rolls out of the bill. It also examined how vegetable items c an be processed particularly when they have to be cut, and what can facilit ate their ingestion particularly when they are fruits that to be swallowed entirely and are large or have dry or fibrous skin or flesh. The skeletal a nd muscular anatomy of the bill and hyoid apparatus is described in details and illustrated. The particularly short and stout bill coupled with a rela tively simple system of aponevroses of the adductors reflects a trade-off b etween two important jaw functional requirements: huge efforts for clamping , and a wide opening for plucking, processing and swallowing relatively lar ge fruit. The clearly oblique orientation of the modified outer adductor se ems an adaptation to the detachment of fruits. The os uncinatum, particular ly developed in this bird family, is interpreted as maintaining transversal stability, particularly when jaws act as a pair of scissors. Most food ite ms being processed near the base of the bill, mechanism have been developed which contribute to overcome the risk of breakage at the level of the prok inetic hinge because of a vertically oriented force there.