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Skeletal development in salamanders is greatly influenced by their complex life history. Although the relationship between skull development and life history has been investigated in a variety of salamander species, few studies have compared skull development among lineages. Here, we describe the cranial skeletogenesis of the Japanese giant salamander, Andrias japonicus (family Cryptobranchidae), for the first time. To conduct interspecific comparisons, we also describe the cranial skeletogenesis of Hynobius nebulosus (Hynobiidae), Pleurodeles waltl (Salamandridae) and Ambystoma mexicanum (Ambystomatidae). Our comparative analyses reveal interspecific differences in cranial skeletogenesis, particularly in the timing of the onset of ossification, the direction of ossification (e.g. from anterior to posterior) and the ossification pattern of the hyobranchial skeleton. The early onset of jaw ossification, posterior extension of the maxilla and posterolateral inclination of the squamosal in An. japonicus might be adaptations that allow An. japonicus to gulp feed, primarily using the suction force generated by opening the mouth wide, while also supplementing that force by depressing the hyobranchial skeleton. Multivariate regression of skull shape on log10-transformed centroid size revealed that the unique skull shape of adult and subadult An. japonicus forms through hypermorphosis.

We investigated ontogenetic changes in the trunk muscles of the Japanese black salamander (Hynobius nigrescens) before, during and after metamorphosis. Given that amphibians change their locomotive patterns with metamorphosis, we hypothesized that they may also change the structure of their trunk muscles. The trunk muscles were macroscopically observed, and the weight ratios of each trunk muscle group were quantified at six different developmental stages. Immediately after hatching, we found that the lateral hypaxial muscle was composed of one thick M. ventralis, from ventral edge of which M. transversus abdominis arose later, followed by M. obliquus externus and M. rectus abdominis. The weight ratios of the dorsal and abdominal muscles to the trunk muscles increased with growth. We suggest that a single thick and large lateral hypaxial muscle facilitates swimming during early developmental stages. The increase in the weight ratios of the dorsal and abdominal muscles with growth possibly assists with gravity resistance necessary for terrestrial life.

Clarification of the trunk structure in Urodela is important in understanding the locomotive evolution of basal tetrapods. The components of the muscular trunk wall among Urodela using different modes of locomotion were compared. Since the whole trunk may be used for swimming and the effect of limbs may be small in the more aquatic species, they showed smaller differences in the trunk muscles among anterior, middle and posterior sections of the trunk. By contrast, in the more terrestrial species, the dorsal and abdominal muscles are larger in the middle section than those in the anterior and posterior sections. High compressive stresses occur in the supporting limbs and their insertion at the trunk on the ventral side, and spread from the forelimbs along the back to the supporting hindlimbs on the dorsal side. Tensile stresses occur in the middle ventral part. The components of the trunk muscles among the three sections may reflect differences in stresses occurring in the trunk of the more terrestrial species. The findings also suggest that in the middle section, larger dorsal muscles for stiffening the back to maintain posture and larger abdominal muscles are responsible for balancing the body weight while it is supported by the limbs in the more terrestrial species.

Trunk musculature in Urodela species varies by habitat. In this study, trunk musculature was examined in five species of adult salamanders representing three different habitats: aquatic species, Amphiuma tridactylum and Necturus maculosus; semi-aquatic species, Cynops pyrrhogaster; terrestrial species, Hynobius nigrescens and Ambystoma tigrinum. More terrestrial species have heavier dorsal and ventral trunk muscles than more aquatic forms. By contrast, the lateral hypaxial musculature was stronger in more aquatic species. The number of layers of lateral hypaxial musculature varied among Urodela species and did not clearly correlate with their habitats. The M. rectus abdominis was separated from the lateral hypaxial musculature in both terrestrial and semi-aquatic species. In aquatic species, M. rectus abdominis was not separated from lateral hypaxial musculature. Lateral hypaxial musculature differed in thickness among species and was relatively thinner in terrestrial species. In more terrestrial species, dorsal muscles may be used for stabilization and ventral flexing against gravity. Ventral muscle may be used in preventing dorsally concave curvature of the trunk by dorsal muscles and by weight. The lengthy trunk supported by limbs needs muscular forces along the ventral contour line in more terrestrial species. And, the locomotion on well-developed limbs seems to lead to a decrease of the lateral hypaxial musculature.

In Anolis lizards, sexual dimorphism has been reported in morphological and ecological traits. Males show larger body size and longer limbs related to territorial combat and courtship display with the dewlap. Although functional-anatomical traits are closely related to locomotor behaviors, differences between sexes in musculoskeletal traits on limbs remain unclear. We explored the relationships among sexual dimorphisms in musculoskeletal morphology, habitat, and locomotor traits in Anolis lizards. Specifically, we examined appendicular musculoskeletal morphology in three species of Cuban Anolis by measuring muscle mass and lengths of moment arms. Through comparisons of crossing locomotion, we found that the runner species possessed larger extensors in hindlimbs, which are advantageous for running, whereas the masses of the humeral and femoral retractors were larger in climber species, allowing these lizards to hold up their bodies and occupy tree substrates. Comparisons between the sexes showed different trends among the three species. Males of A. porcatus, which inhabit narrow branches or leaves, had stronger elbow extensors that maintain the display posture. In contrast, males of A. sagrei, which occupy broad surfaces, did not show sexual differences that affected social display. Moreover, A. bartschi indicated sexual differences despite the absence of dewlapping behavior. Our findings suggest that both sexes show fundamentally similar relationships between muscular morphology and locomotor habits to adapt arboreal or terrestrial substrates, and yet sexual dimorphism in forelimb muscles may additionally affected by male specific display with the dewlap.

We investigated ontogenetic changes in the trunk muscles of the Japanese black salamander (Hynobius nigrescens) before, during and after metamorphosis. Given that amphibians change their locomotive patterns with metamorphosis, we hypothesized that they may also change the structure of their trunk muscles. The trunk muscles were macroscopically observed, and the weight ratios of each trunk muscle group were quantified at six different developmental stages. Immediately after hatching, we found that the lateral hypaxial muscle was composed of one thick M. ventralis, from ventral edge of which M. transversus abdominis arose later, followed by M. obliquus externus and M. rectus abdominis. The weight ratios of the dorsal and abdominal muscles to the trunk muscles increased with growth. We suggest that a single thick and large lateral hypaxial muscle facilitates swimming during early developmental stages. The increase in the weight ratios of the dorsal and abdominal muscles with growth possibly assists with gravity resistance necessary for terrestrial life.

We examined the diversity of the musculoskeletal morphology in the limbs of Anolis lizards with different habitats and identified variations in functional and morphological adaptations to different ecologies or behaviors. Dissection and isolation of 40 muscles from the fore- and hindlimbs of five species of Anolis were performed, and the muscle mass and length of the moment arm were compared after body size effects were removed. Ecologically and behaviorally characteristic morphological differences were observed in several muscles. Well-developed hindlimb extensors were observed in ground-dwelling species, A. sagrei and A. bremeri, and were considered advantageous for running, whereas adept climber species possessed expanded femoral retractors for weight-bearing during climbing. Moreover, morphological variations were observed among arboreal species. Wider excursions of the forelimb joint characterized A. porcatus, presumably enabling branch-to-branch locomotion, while A. equestris and A. angusticeps possessed highly developed adductor muscles for grasping thick branches or twigs. These findings suggest divergent evolution of musculoskeletal characteristic in the limbs within the genus Anolis, with correlations observed among morphological traits, locomotor performance, and habitat uses.

We propose a temporal multiplexing method of two holograms based on off-axis digital holography using acousto-optic modulators. The proposed method can overcome the limitation of the frame rate of an image sensor for digital holography.