Computed Tomography of the Appendicular Skeleton and Arboreal Adaptations of Bradypus variegatus
DOI:
https://doi.org/10.22456/1679-9216.150901Keywords:
common sloth, tomography anatomy, suspensory locomotion, musculoskeletal adaptation, xenarthran morphologyAbstract
Background: The brown-throated sloth (Bradypus variegatus) is a highly specialized arboreal xenarthran whose unique suspensory locomotion in the Amazon canopy necessitates distinct morphological adaptations of its appendicular skeleton. While crucial for understanding its evolutionary niche, a comprehensive anatomical description of this skeletal system using modern computed tomography is lacking. This study aimed to provide a detailed computed tomography-based analysis of the B. variegatus appendicular skeleton, explicitly correlating the observed osteological features with functional demands of arboreal suspension and locomotion.
Materials, Methods & Results: This descriptive anatomical study utilized 9 frozen cadavers of B. variegatus (6 young males, 3 young females). Specimens underwent helical computed tomography scanning in dorsal recumbency with limbs extended. Anatomical analysis was performed using multiplanar reconstructions and 3-dimensional volume rendering. Results detailed significant adaptations. The thoracic limbs were markedly larger than the pelvic limbs. The scapula presented a convex dorsal margin and a well-defined spina scapula; the clavicle was absent. The humerus was a long cylindrical bone (mean length: 9.17 cm). The radius and ulna were completely separated by a distinct interosseous space. The carpus consisted of 5 diffusely arranged bones, with metacarpals and phalanges adapted to support elongated claws. In the pelvic limb, the femur had a slightly curved diaphysis (mean mediolateral diameter: 7.51 mm); a patella was not observed. The tibia and fibula also featured an interosseous space. The pelvis was characterized by a deep, rounded acetabulum, a thin ischial tuberosity, a pubis with notable lateral amplitude, and a sacrum articulating with the ilium via 6 vertebrae. These features, including the disproportionate limb size, absent clavicle, fused carpal elements, and specialized pelvic architecture, are interpreted as direct adaptations for suspensory posture and arboreal mobility.
Discussion: The morphological suite described, comprising disproportionately large thoracic limbs, a vestigial or absent clavicle, fused carpal elements, a deep acetabulum, and a modified pelvic girdle, constitutes a coherent adaptive suite for a life suspended beneath branches. These features collectively enhance skeletal stability, provide extensive surfaces for the attachment of powerful flexor musculature, and maximize mechanical advantage during climbing and stationary hanging. The absence of a clavicle increases the range of motion in the shoulder, while the pelvic morphology aids in securing the body. Primary limitations of this study include the modest sample size (n=9) and the exclusive use of juvenile specimens, which may affect the generalizability of findings related to ossification completeness (e.g., the unossified patella) and fully mature osteological proportions. The use of cadavers, while standard for anatomical studies, precludes dynamic functional assessment. In conclusions this computed tomography-based anatomical study successfully elucidates the detailed osteology of the B. variegatus appendicular skeleton, establishing a clear structure-function relationship for its arboreal and suspensory habits. The findings provide a crucial anatomical benchmark for future comparative, clinical, and evolutionary biology studies, highlighting the sloth as a remarkable model of morphological adaptation to a specialized ecological niche. Further research with adult specimens is warranted to confirm ontogenetic patterns.
Keywords: common sloth, tomography anatomy, suspensory locomotion, musculoskeletal adapta-tion, xenarthran morphology.
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Copyright (c) 2026 Michel Santos Cunha, Daniel Furtado Perdigão, José Gonçalo Monteiro Campos, Thiago da Silva Cardoso, Lucas Santos Carvalho, Luisa Pucci Bueno Borges, Sheyla Farhayldes Souza Domingues, Érika Renata Branco, Francisco Décio Oliveira Monteiro, Pedro Paulo Maia Teixeira
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