Living Suspended Beneath Branches: The Skeletal Anatomy of Sloths
Sloths spend much of their day in trees, often moving with their bodies suspended beneath branches while alternately hooking their limbs onto supports ahead of them. In most quadrupedal animals, the limbs primarily bear the forces generated as body weight presses downward against the ground. Sloths frequently experience a very different mechanical situation: their limbs must support the body's weight while it hangs below the point of attachment for prolonged periods. What features has this unusual lifestyle left on their skeleton? One study used computed tomography (CT) to examine the appendicular skeleton of the brown-throated sloth (Bradypus variegatus), from the scapula and digits to the pelvis and toes, and explored how these skeletal structures are associated with suspensory locomotion.
The study examined nine frozen cadavers of B. variegatus, including six young males and three young females. The animals originated from wild populations near Castanhal in the Brazilian state of Pará. After thawing, the cadavers underwent helical CT scanning, and the resulting images were used to generate three-dimensional reconstructions of the skeleton.
The thoracic limbs of B. variegatus were markedly larger than the pelvic limbs. The humerus measured approximately 9.17 cm in length, the ulna 8.19 cm, and the radius 7.07 cm, whereas the femur measured about 5.87 cm and the tibia 5.48 cm. For an animal that frequently suspends itself beneath branches, long and robust forelimbs can increase reach and provide much of the support needed as the body moves from one attachment point to the next. The scapula is relatively flat and bears a well-defined scapular spine, which provides surfaces for muscle attachment. No clavicle was visible in the CT images, nor was an acromioclavicular joint observed. A reduced or absent clavicle may allow greater mobility at the shoulder, which would be advantageous when the forelimbs must repeatedly change orientation during suspensory locomotion. Previous anatomical studies of B. variegatus, however, have produced inconsistent observations: some individuals possess a rudimentary clavicle, and its presence may vary with age or among individuals. The absence of a clavicle in these nine young specimens therefore does not establish that the species has completely lost this bone.
Farther down the forelimb, the radius and ulna are completely separated by a distinct interosseous space. At the wrist, the arrangement of the bones becomes more compact. Five carpal bones were identified, with some carpal elements connected to the bases of the metacarpals and some medial and lateral elements showing reduction or fusion. The authors interpreted this configuration as providing a more stable wrist for suspension, although at the expense of some degree of fine mobility.
The structures that actually secure a sloth to a branch lie at the ends of the digits. Nine phalanges were identified in the forelimb, forming three functional digits with complete sets of phalanges, each terminating in a long, strongly curved claw. These claws can hook around branches, allowing body weight to be transmitted through the digits and wrist to the forearm and shoulder. Previous studies of sloth musculature have shown that the forearm and digital flexors are well suited to maintaining prolonged gripping forces, and the CT images in this study provide the corresponding skeletal framework for that function.
The pelvic limbs are shorter, but they still provide additional attachment points during climbing and suspension, helping stabilize the body beneath branches. As in the forelimb, the tibia and fibula are separated by a distinct interosseous space. The foot contains five tarsal bones, including fused cuneiform elements. The first and fifth metatarsals are proportionally smaller than the others, while the three principal sets of toe phalanges also terminate in elongated claws, allowing the hind feet to function as hooks around supporting branches.
No patella was observed at the knee joint. All of the specimens, however, were young animals that had not yet reached full skeletal maturity, and sloths are known to have an unusual pattern of ossification. The authors therefore considered it possible that the apparent absence of the patella simply reflected incomplete ossification at the age of these individuals.
The pelvis also contains several features associated with suspensory stability. The acetabulum is deep and rounded, providing a secure socket for the femoral head. The ischial spine projects cranially, the ischial tuberosity is relatively thin, and the pubis extends broadly in the lateral direction. The sacrum is composed of six vertebrae, several of which articulate with the ilium through the sacroiliac joint. The deep acetabulum and distinctive pelvic architecture may help stabilize the body during suspension while also providing the bony framework required for muscle attachment and support by the pelvic limbs.
From the shoulder and forearm to the wrist, pelvis, and toes, the skeleton of the brown-throated sloth reflects a body specialized for supporting its weight beneath branches. Its seemingly slow and effortless upside-down posture depends on a highly modified appendicular skeleton shaped around the mechanical demands of suspensory life.
Author: Shui-Ye You
Reference:
Cunha MS et al. (2026). Computed Tomography of the Appendicular Skeleton and Arboreal Adaptations of Bradypus variegatus. Acta Scientiae Veterinariae.
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