Locomotor variation and bending regimes of capuchin limb bones

Abstract

Primates are very versatile in their modes of progression, yet laboratory studies typically capture only a small segment of this variation. In vivo bone strain studies in particular have been commonly constrained to linear locomotion on flat substrates, conveying the potentially biased impression of stereotypic long bone loading patterns. We here present substrate reaction forces (SRF) and limb postures for capuchin monkeys moving on a flat substrate ("terrestrial"), on an elevated pole ("arboreal"), and performing turns. The angle between the SRF vector and longitudinal axes of the forearm or leg is taken as a proxy for the bending moment experienced by these limb segments. In both frontal and sagittal planes, SRF vectors and distal limb segments are not aligned, but form discrepant angles; that is, forces act on lever arms and exert bending moments. The positions of the SRF vectors suggest bending around oblique axes of these limb segments. Overall, the leg is exposed to greater moments than the forearm. Simulated arboreal locomotion and turns introduce variation in the discrepancy angles, thus confirming that expanding the range of locomotor behaviors studied will reveal variation in long bone loading patterns that is likely characteristic of natural locomotor repertoires. "Arboreal" locomotion, even on a linear noncompliant branch, is characterized by greater variability of force directions and discrepancy angles than "terrestrial" locomotion (significant for the forearm only), partially confirming the notion that life in trees is associated with greater variation in long bone loading. Directional changes broaden the range of external bending moments even further.

Arboreal locomotion in rats – the challenge of maintaining stability

“…Impulse angles in the transverse plane during ground and pole locomotion were determined at midstance (Lee et al, 2004;Demes and Carlson, 2009) and corrected to the animal's anatomical plane. Discrepancy angles were estimated following the method of Demes and Carlson (Demes and Carlson, 2009).…”

“…Most of the studies focusing on the locomotion of arboreal mammals to date have been restricted to those animals that possess prehensile extremities (e.g. Schmitt, 1994;Schmitt and Lemelin, 2002;Schmitt, 2003c;Schmitt and Hanna, 2004;Carlson et al, 2005;Franz et al, 2005;Demes and Carlson, 2009). Prehensile extremities enable animals to generate torques around the substrate to avoid falling down by counteracting and reducing mediolateral (m-l) forces (Preuschoft, 2002).…”

“…To increase stability, the latter display the following strategies during locomotion on small branches: avoidance of vertical oscillations and side-to-side fluctuations, reduction of velocity and stride frequency, and decrease in the height of proximal pivots to lower the center of mass relative to the substrate surface (Schmitt, 1994;Schmitt and Lemelin, 2002;Schmitt, 2003c;Schmitt and Hanna, 2004;Carlson et al, 2005;Franz et al, 2005;Demes and Carlson, 2009).…”

Phenotypic plasticity and constraint along the upper and lower limb diaphyses of Homo sapiens

“…[ Figure 5 here] Just as muscle contractions constitute a mechanical load on bone (Özkaya et al, 2012), ground reaction force vectors are also capable of influencing skeletal morphology (Whalen, 1988;Kohrt et al, 1997;Crossley et al, 1999;Demes et al, 2001;Demes and Carlson, 2009;Wilks et al, 2009). Traditional cursorial mammals including red deer (Skedros et al, 2003), horses (Nicholson and Firth, 2010) and sheep (Lieberman, 2003) have been reported to exhibit more frequent episodes of haversian remodeling in the distal limb compared to more proximal segments.…”

“…Traditional cursorial mammals including red deer (Skedros et al, 2003), horses (Nicholson and Firth, 2010) and sheep (Lieberman, 2003) have been reported to exhibit more frequent episodes of haversian remodeling in the distal limb compared to more proximal segments. This phenomenon is likely due in part to the distal elements' exposure to shock, effectively buffering impact forces for the rest of the limb (Biewener, 1989;Skedros et al, 2003;Demes and Carlson, 2009;Wilks et al, 2009). The greater surface area of proximal and distal long bone sections may therefore be necessary to accommodate cancellous bone and cartilage for shock absorption without adding extraneous mass from cortical tissue.…”