Locomotion pattern and trunk musculoskeletal architecture among Urodela

We comparatively examined the trunk musculature and prezygapophyseal angle of mid-trunk vertebra in eight urodele species with different locomotive modes (aquatic Siren intermedia, Amphiuma tridactylum, Necturus maculosus and Andrias japonicus; semi-aquatic Cynops pyrrhogaster, Cynops ensicauda; and terrestrial Hynobius nigrescens, Hynobius lichenatus and Ambystoma tigrinum). We found that the more terrestrial species were characterized by larger dorsal and abdominal muscle weight ratios compared with those of the more aquatic species, whereas muscle ratios of the lateral hypaxial musculature were larger in the more aquatic species. The lateral hypaxial muscles were thicker in the more aquatic species, whereas the M. rectus abdominis was more differentiated in the more terrestrial species. Our results suggest that larger lateral hypaxial muscles function for lateral bending during underwater locomotion in aquatic species. Larger dorsalis and abdominal muscles facilitate resistance against sagittal extension of the trunk, stabilization and support of the ventral contour line against gravity in terrestrial species. The more aquatic species possessed a more horizontal prezygapophyseal angle for more flexible lateral locomotion. In contrast, the more terrestrial species have an increasingly vertical prezygapophyseal angle to provide stronger column support against gravity. Thus, we conclude trunk structure in urodeles differs clearly according to their locomotive modes.