Sloths are known for their slow, deliberate movements through the rainforest canopies of Central and South America. These arboreal mammals, comprising the two-toed and three-toed species, possess an anatomy strikingly different from most other mammals. Their entire existence is adapted to hanging upside down, a lifestyle that has molded their skeletal structure in surprising ways. This specialization means the sloth’s skeleton often deviates from standard biological rules.
Determining the Total Bone Count
Determining the total number of bones a sloth has is complicated because the count is not fixed across all species, unlike in many mammals where the bone count is highly conserved. The total count varies significantly between the two distinct genera: the three-toed sloths (Bradypus) and the two-toed sloths (Choloepus). This skeletal variability primarily stems from the axial skeleton, specifically the number of vertebrae in the neck and thorax.
Most mammals possess a total bone count in the low 200s, but sloths often deviate from this standard due to the number of individual elements in their spine and rib cage. Three-toed sloths generally have a higher number of bones overall because they possess extra vertebrae in their neck. Two-toed sloths also contribute to the variability, as they can have up to 21 pairs of ribs, a number far greater than any other mammal.
The Unique Cervical Vertebrae
The number of cervical (neck) vertebrae is the most unique aspect of the sloth skeleton, as it breaks a rule observed in nearly all other mammalian species. Almost every mammal, from a mouse to a giraffe, has exactly seven cervical vertebrae, but sloths are a notable exception. Three-toed sloths (Bradypus) typically possess eight or nine cervical vertebrae, an increase that provides them with an exceptional range of motion.
This extra flexibility allows the three-toed sloth to rotate its head through an arc of almost 270 degrees. This ability enables the animal to scan a wide area for predators and food without having to move its entire body. Conversely, the two-toed sloth (Choloepus) usually has between five and seven cervical vertebrae, a number that can be less than or equal to the typical mammalian count.
This vertebral variation is thought to result from a phenomenon called homeotic transformation, where one type of vertebra developmentally takes on the identity of another. In three-toed sloths, the shift causes vertebrae that would normally be thoracic (rib-bearing) to become cervical (ribless), thus increasing the neck length. This transformation contrasts with two-toed sloths, which show a more complex pattern of variation, including a highly variable number of thoracic vertebrae that result in their numerous ribs. This high degree of skeletal variability in the spine is rare in mammals.
Skeletal Features for Hanging
Beyond the vertebral column, the rest of the sloth’s skeleton is specialized to support a life spent permanently inverted. The limbs are not built for standing or walking but for grasping and hanging. Three-toed sloths have forelimbs significantly longer than their hindlimbs, which creates a natural leverage advantage when suspended from a branch.
Their famous claws are formed by elongated and curved distal phalange bones at the tips of the digits. These bones are covered in a sheath of keratin, functioning as strong, natural hooks that require minimal muscular effort to maintain a grip. A powerful tendon-locking mechanism allows the sloth to hang for extended periods without expending much energy.
To manage the constant downward pull of gravity on their organs, sloths have evolved a robust rib cage and unique internal support. Fibrous sheets of connective tissue anchor the stomach, liver, and kidneys to the lower ribs and pelvis. This internal “taping” prevents the weight of a full stomach—which can account for up to a third of the sloth’s body mass—from compressing the lungs, conserving the energy that would otherwise be spent on breathing against the internal pressure.