Grip it good: in vivo grip force across substrate diameters in the brown-throated three-toed sloth (Bradypus variegatus)

Sloths are among the most specialized arboreal species and have evolved a suite of anatomical features that support suspensory behaviors. While sloths generally show a reduction in skeletal muscle mass, the digital flexor musculature (i.e., the muscles associated with grip strength) remains relatively large. It may therefore be inferred that grip strength represents an important selective criterion for sloths. However, no in vivo data are available to assess the validity of this hypothesis. In this study, we explore how grip force varies as a function of substrate diameter, sidedness, and between the forelimb and hindlimb in brown-throated three-toed sloths (Bradypus variegatus; N = 5). Grip force was measured using a custom-built support, comprising two semi-circular prisms, the lower half of which was instrumented with a medium-load force plate. Maximum recorded grip forces exceeded 100% bodyweight. While no differences were observed between the fore- and hindlimb pairs, both substrate diameter and sidedness significantly influenced grip strength. Specifically, B. variegatus exhibited the greatest grip force potential on the left side of their body and on large-diameter supports. Equal gripping performance between the fore- and hindlimbs may be attributable to previous observations in sloths of a near-equal distribution of bodyweight support between the limbs during suspensory locomotion. Moreover, a tendency to produce the greatest grip forces on large-diameter substrates agrees with their ecological substrate preferences. The consistent tendency for left sidedness across the individuals studied was unexpected, and future work should explore the potential ecological and anatomical correlates of such a finding.

Duke Scholars

Author Melody Young