Forelimb and hind limb loading patterns during below branch quadrupedal locomotion in the two‐toed sloth

Abstract

The living sloths are the most suspensory of all extant mammals, and therefore represent ideal models for investigating the effects that suspensory behaviours have on bone and joint morphology. While the anatomy and kinematics of sloths are well known, no research has reported kinetic patterns of sloth locomotion. This study examines peak force patterns generated by the two‐toed sloth Choloepus didactylus during below branch quadrupedal walking to infer how kinetic patterns of sloths compare to data reported on non‐human primates. Values for vertical, fore‐aft, and mediolateral peak forces were collected for the forelimb and hind limb, and analyses between the magnitude and timing of these peaks were compared between forelimbs and hind limbs. Patterns and timing of fore‐aft peak forces were similar between sloths and non‐human primates, and were characterized by first a propulsive force as the limb first made contact with the support followed by braking force prior to lift‐off. Similarly, both sloths and primates demonstrate a medially directed force bias onto the substrate during below branch quadrupedal locomotion, although the magnitude observed in sloths exceeds values reported in primates. Peak vertical forces applied by the forelimbs and hind limbs of sloths were not statistically different in magnitude from each other. Data from this study indicate the forelimbs and hind limbs of sloths are functioning similarly to each other during below branch quadrupedal locomotion, and that forelimb‐biased weight support, the pattern typical of primates, is not a mechanical requirement of suspensory locomotion across all mammals. These findings provide important information about the mechanical necessities of below branch movement, and data from this study should be used when considering mechanical convergence among suspensory taxa and the interpretation of suspensory limb adaptations in the fossil record.

Forelimb and hindlimb loading patterns during quadrupedal locomotion in the large flying fox (Pteropus vampyrus) and common vampire bat (Desmodus rotundus)

“…However, in both species the forelimb still supports a relatively large proportion of body weight (~40% in large flying foxes and ~60% in common vampire bats). Similar values have been reported in primates and sloths during suspensory quadrupedal locomotion (Ishida et al ., ; Granatosky et al ., ; Granatosky & Schmitt, ). Therefore, to suggest that bats have a less‐equipped forelimb for weight support during suspensory quadrupedal locomotion due to the presence of the wing would be incorrect.…”

“…While no kinetic data has been collected during suspensory locomotion in bats, such data have been reported for primates and sloths (Ishida, Jouffroy & Nakano, ; Nyakatura & Andrada, ; Granatosky, Tripp & Schmitt, ; Granatosky & Schmitt, ). In general, suspensory quadrupedal locomotion in primates is characterized by: (1) Vpk forces that are relatively higher in the forelimb compared to the hindlimb; (2) the limb (forelimb or hindlimb) applies a propulsive force to the substrate during the first portion of support phase, followed by a braking force throughout the remainder of support phase and (3) mediolateral forces are low in both the forelimbs and hindlimbs, and animals more frequently apply a medially directed force to the substrate (Ishida et al ., ; Granatosky et al ., ).…”

“…These patterns are opposite compared to data collected during terrestrial locomotion in primates (Demes et al ., ; Schmitt, ; Granatosky et al ., ). Kinetic patterns in sloths are largely similar, with the exception that mediolateral forces tend to be relatively higher, and the increased forelimb weight‐bearing role observed in primates during suspensory locomotion is not present (Granatosky & Schmitt, ).…”

A suspensory way of life: Integrating locomotion, postures, limb movements, and forces in two‐toed sloths Choloepus didactylus (Megalonychidae, Folivora, Pilosa)

“…Only walking strides (i.e., duty factor over 50%) in which the animal was traveling in a straight path and not accelerating or decelerating (i.e., steady‐state locomotion) were selected for analysis. Steady‐state locomotion was determined by calculating the instantaneous velocity between subsequent video frames throughout the entire stride, and then using regression analysis to determine whether velocity changed throughout the stride (Granatosky, ; Granatosky & Schmitt, ; Granatosky et al, ). Only strides with no change in speed (i.e., slope not significantly different from zero) were analyzed.…”

“…To collect kinetic data, a small subsection of the runway was instrumented with two Kistler force plates (model 9317B; Kistler, Amherst, NY) that have been used in previous studies (Granatosky & Schmitt, ; Granatosky et al, ). Force plate output was sampled at 12,000 Hz, imported, summed and processed using BioWare™ v.5.1 software, and then filtered (second‐order Low Pass Butterworth, 30 Hz) in MATLAB.…”