Morphology and mechanics of tongue movement in the African pig-nosed frog Hemisus marmoratum: A muscular hydrostatic model

The goal of this study was to investigate morphological adaptations associa ted with hydrostatic elongation of the tongue during feeding in the African pig-nosed frog Hemisus marmoratum. Whereas previous studies had suggested that the tongue of H, marmoratum elongates hydraulically, the anatomical ob servations reported here favour a muscular hydrostatic mechanism of tongue elongation. H. marmoratum possesses a previously undescribed compartment of the m, genioglossus (m, genioglossus dorsoventralis), which is intrinsic t o the tongue and whose muscle fibres are oriented perpendicular to the long axis of the tongue. On the basis of the arrangement and orientation of mus cle fibres in the m, genioglossus and m, hyoglossus, we propose a muscular hydrostatic model of tongue movement in which contraction of the m, geniogl ossus dorsoventralis, together with unfolding of the intrinsic musculature of the tongue, results in a doubling in tongue length. Electron micrographs of sarcomeres from resting and elongated tongues show that no special adap tations of the sarcomeres are necessary to accommodate the observed doublin g in tongue length during feeding. Rather, the sarcomeres of the m, geniogl ossus longitudinalis are strikingly similar to those of anuran limb muscles . The ability to elongate the tongue hydrostatically, conferred by the pres ence of the m, genioglossus dorsoventralis, is associated with the appearan ce of several novel aspects of feeding behaviour in H. marmoratum. These in clude the ability to protract the tongue slowly, thereby increasing capture success, and the ability to aim the tongue in azimuth and elevation relati ve to the head. Compared with other frogs, the muscular hydrostatic system of H, marmoratum allows more precise, localized and diverse tongue movement s. This may explain why the m, genioglossus of H. marmoratum is composed of a larger number of motor units than that of other frogs.