Sloths are unique mammals known for their slow, arboreal lifestyle. This often leads to questions about their physical characteristics, particularly their “fingers” or digits. Their limb anatomy reveals specialized structures perfectly adapted to their environment.
Sloth Digits and Claws
Sloths do not possess individual, articulating fingers like humans. Instead, their forelimbs and hindlimbs feature fused digits, forming a hook-like structure from which prominent, long, and curved claws emerge. These claws are not merely extensions of nails; they are formed by elongated, curved distal phalange bones, covered by a keratin sheath. This sheath, the same material found in human fingernails and hair, contributes to their strength and sharpness.
A sloth’s “hand” structure is highly specialized. For instance, in two-toed sloths, only two functional digits (II and III) bear prominent claws, with rays I and IV (equivalent to a thumb and ring finger) primarily represented by metacarpals. The proximal phalanges of these functional digits are shortened with restricted joint movement, making the entire structure behave as a cohesive unit. This anatomical arrangement means sloths lack dexterity for fine manipulation but possess an exceptionally strong gripping mechanism.
Specialized Adaptations for Arboreal Life
The design of sloth digits and claws is a direct adaptation to their arboreal lifestyle. These hook-like structures allow sloths to grip branches firmly, enabling them to hang suspended for extended periods with minimal energy expenditure. Their grip is so strong that sloths can remain securely attached to branches even after death. This is partly due to specialized tendons in their hands and feet that can lock into place, reducing the need for continuous muscle contraction.
Sloths spend up to 90% of their lives hanging upside down, and their anatomy supports this inverted posture. Their muscles are specialized for pulling and gripping motions, more developed than their pushing muscles. This muscle specialization, combined with their curved claws, makes them efficient for slow movement through the canopy, reaching for leaves, and maintaining a secure hold. This adaptation contributes to their low metabolic rate, helping them conserve energy from their leaf-based diet, which offers limited nutrients.
Distinctions Between Sloth Species
The terms “two-toed” and “three-toed” sloths refer to the number of prominent claws on their forelimbs. While both types generally have three claws on each hindlimb, their common names distinguish them by their front limbs. For example, a two-toed sloth (e.g., Hoffmann’s) has two claws on each forelimb, while a three-toed sloth (e.g., brown-throated) has three.
Beyond forelimb claw count, two-toed and three-toed sloths belong to different, distantly related families, having evolved similar arboreal adaptations independently. Two-toed sloths tend to be slightly larger and have a wider diet than the more specialized leaf diet of three-toed sloths. Three-toed sloths also possess extra cervical vertebrae, allowing them to rotate their heads up to 270 degrees—a feature not found in two-toed sloths.