Meeting Abstract
Sloths are canopy-dwelling inhabitants of American neotropical rainforests that exhibit suspensory locomotion and posture. These abilities involve hanging below branch from hook-like feet, and although they require great strength and endurance, the skeletal muscle mass of sloths is reduced, thus requiring modifications to muscle architecture for large joint torque. We hypothesized that intrinsic muscle properties also are modified for fatigue resistance by homogeneous expression of slow myosin heavy chain (MHC) fibers that rely on aerobic metabolism for sustained contraction. This hypothesis was tested by determining MHC fiber type distribution and energy metabolism in the forelimb muscles of two-toed (C. hoffmanni, N=4) and three-toed (B. variegatus, N=5) sloths using protein gel electrophoresis, immunohistochemistry, and enzyme activity assays. A primary expression of the slow MHC-1 isoform as well as moderate expression of fast MHC-2A fibers and few hybrid MHC-1/2A fibers are found in both species. MHC-1 fibers are larger in cross-sectional area (CSA) than MHC-2A fibers and comprise the greatest %CSA in each muscle sampled. The enzyme assays show elevated activity for anaerobic enzymes compared to low activity for aerobic markers. Notably, enzyme activity and MHC fiber type are uncorrelated. The lack of support for the predicted fiber type properties suggests that sloth limb muscles have appreciable tolerance for lactate accumulation, but may rely more heavily on rapid ATP re-synthesis pathways. Still, the intrinsic properties observed match well with their locomotor requirements, and these modifications may have further evolved in unison with low metabolism and body temperature, and slow movement patterns as means to conserve energy.
