Foot of two‐toed sloths: Its anatomy and potential uses relative to size of support

Abstract

Feet of two-toed sloths (Choloepus) are long, narrow, hook-like appendages with only three functional digits, numbers II, III, and IV; Rays I and V are represented by metatarsals. Proximal phalanges of complete digits are little more than proximal and distal articulating surfaces. All interphalangeal joints are restricted, by interlocking surfaces, to flexion and extension. Ankle and transverse tarsal joints, however, allow extreme flexion and inversion of foot. Powerful digital flexion is augmented by several muscles from extensor compartment of leg. Intrinsic foot musculature is reduced to flexors and extensors but these, with the exception of lumbricals, are large and well developed. Choloepus uses its feet much like hooks with distal phalanges and covering claws forming the "hook" element. These hook-like appendages are seemingly best suited for supports less than 50 mm in diameter suggesting that two-toed sloths may prefer supports of this size in their natural habitat.

The Convergent Evolution of Suspensory Posture and Locomotion in Tree Sloths

“…This also had kinematic consequences, because the tarsometatarsus no longer contributes to step length. The foot always needs to be oriented perpendicular to the support (Mendel 1981b) and therefore actually reduces step length (Nyakatura et al 2010). This negative effect on step length is partly negated by movements of the thoracolumbar spine that lead to distinct displacements of the pelvis and hip in Choloepus (Nyakatura and Fischer 2010b).…”

“…Moreover, the wrist of Choloepus is characterized by shallow fossae of the distal ulna and radius as well as decreased contact between carpal bones (pisiform and triquetum) and the radius and ulna, respectively (Mendel 1979(Mendel , 1981a. The tarsus of two-toed sloths is characterized by a highly mobile joint between the talus and the navicular that allows a pivoting motion (Mendel 1981b), but the ankle joint also has remarkable mobility that is little if at all restricted by ligaments (Mendel 1981b). Again, Bradypus has been studied in less detail, but Ursing (1932) described the mobility of the ankle to allow adduction/abduction, flexion/extension as well as emphasized long-axis rotation of the foot of up to 90°.…”

Developmental bias in the evolution of phalanges

“…In arboreal and digging mammals, e.g., in Grant's Golden Mole (31) and in certain extinct taxa of sloths (32), this evolutionary challenge has been circumvented differently, by condensing phalangeal elements in normal proportions but then fusing phalanges with one another or with the metapodial to produce a long segment from shorter phalanges. Other arboreal mammals, such as the Two-Toed Sloth (33,34), have paralleled the digging/perching/predatory birds in extending their distal phalanges to a greater extent than the proximal ones. An insight into at least a potential mechanism by which this extension might have occurred comes from a study (15) in which Fgf signaling was maintained experimentally at the distal tip of a growing digit ray.…”

Gait kinetics of above- and below-branch quadrupedal locomotion in lemurid primates

“…The second theory attributes a shift to below-branch movement to constraints of balancing on branches; as the body size to support diameter ratio increases, the ability to remain balanced above the support becomes more difficult (Cartmill, 1985;Napier, 1967). One solution is for arboreal animals to move below branches and adopt suspensory positional behaviors (Cartmill, 1985;Mendel, 1981;Napier, 1967). While these strategies may solve problems of balance and food acquisition, individuals moving from above-to below-branch quadrupedal locomotion experience a novel biomechanical environment and may need to adjust locomotor behavior in order to effectively move below the support (Ishida et al, 1990;Jouffroy and Stern, 1990;Stern, 1975;Swartz et al, 1989).…”

“…sloths, bats, kinkajous), few studies (Fujiwara et al, 2011;Ishida et al, 1990;Jouffroy and Petter, 1990;Jouffroy and Stern, 1990;Mendel, 1981;Nyakatura, 2012;Parsons and Taylor, 1977;Turnquist, 1975) have specifically focused on the mechanics of below-branch quadrupedal locomotion in mammals, and of those only one has measured substrate reaction forces (Ishida et al, 1990). Ishida et al (1990) collected multiaxial force data from freely moving slow lorises (Nycticebus coucang) on an upright and inverted instrumented runway.…”