Ontogeny of myosin isoform expression and prehensile function in the tail of the gray short-tailed opossum (Monodelphis domestica)

Muscle architecture and muscle fibre type composition in the forelimb of two African mole-rat species, Bathyergus suillus and Heterocephalus glaber

The scratch-digging Cape dune mole-rat (Bathyergus suillus), and the chisel-toothed digging naked mole-rat (Heterocephalus glaber) are African mole-rats that differ in their digging strategy. The aim of this study was to determine if these behavioural differences are reflected in the muscle architecture and fibre-type composition of the forelimb muscles. Muscle architecture parameters of 39 forelimb muscles in both species were compared. Furthermore, muscle fibre type composition of 21 forelimb muscles were analysed using multiple staining protocols. In B. suillus, muscles involved with the power stroke of digging (limb retractors and scapula elevators), showed higher muscle mass percentage, force output and shortening capacity compared to those in H. glaber. Additionally, significantly higher percentages of glycolytic fibres were observed in the scapular elevators and digital flexors of B. suillus compared to H. glaber, suggesting that the forelimb muscles involved in digging in B. suillus provide fast, powerful motions for effective burrowing. In contrast, the m. sternohyoideus a head and neck flexor, had significantly more oxidative fibres in H. glaber compared to B. suillus. In addition, significantly greater physiological cross-sectional area and fascicle length values were seen in the neck flexor, m. sternocleidomastoideus, in H. glaber compared to B. suillus, which indicates a possible adaptation for chisel-tooth digging. While functional demands may play a significant role in muscle morphology, the phylogenetic differences between the two species may play an additional role which needs further study.

Hind foot drumming: Myosin heavy chain muscle fiber distribution in the hind limb muscles of three African mole-rat species (Bathyergidae)

Hind foot drumming as a form of seismic signaling plays a pivotal role in the communication of various mammalian species including Bathyergidae (African mole-rats). The aim of the present study was to histologically determine if the action of hind foot drumming would influence the number of type II fibers present in the hind limb muscles of two drumming (Georychus capensis and Bathyergus suillus) and one non-drumming (Cryptomys hottentotus natalensis) bathyergid species. Twenty-one frozen muscles of each species were selected for the purpose of mid-belly cryostat sections. These sections were immunohistochemically labeled for myosin heavy chain slow muscle fibers (MHCs). In addition, oxidative capacity was determined by means of histochemical staining. A high percentage of fast type II muscle fibers was found in all the functional muscle groups, although there were no statistical differences between the drumming and non-drumming species. Bathyergus suillus had significantly fewer type II fibers in mm. semitendinosus, gluteofemoralis, tibialis cranialis, plantaris, and the medial head of m. gastrocnemius compared to the other two species. In all three species, the majority of the muscle fibers in all functional muscle groups demonstrated low oxidative capacity which correlated with the expression of type II muscle fibers. It therefore seems likely that the number of type II muscle fibers in the hind limb muscles of the Bathyergidae species studied here is more influenced by either body size or digging strategy rather than being an adaptation for hind foot drumming.

Coming to grips with life upside down: how myosin fiber type and metabolic properties of sloth hindlimb muscles contribute to suspensory function

Sloths exhibit almost obligatory suspensory locomotion and posture. These behaviors require both strength and fatigue resistance, although we previously found muscle fiber type characteristics in the forelimbs of sloths that belied these initial expectations. Based on locomotor roles of the forelimbs versus hindlimbs in propulsion and braking, respectively, sloth hindlimb musculature should be adapted for force production and energy savings by a near homogeneous expression of slow myosin heavy chain (MHC) fibers. This hypothesis was tested by determining MHC fiber type (%) distribution and energy metabolism in the hindlimbs of three-toed (B. variegatus, N = 5) and two-toed (C. hoffmanni, N = 3) sloths. A primary expression of the slow MHC-1 isoform was found in the hindlimbs of both species. Slow MHC fiber type (%) was significantly greater in the flexors of B. variegatus, whereas expression of fast MHC-2A fibers was significantly greater in the extensors of C. hoffmannni. MHC-1 fibers were largest in cross-sectional area (CSA) and comprised the greatest %CSA in each muscle sampled from both species. Enzyme assays showed elevated activity for anaerobic enzymes (CK and LDH) compared with low-to-moderate activity for aerobic enzymes (3-HAD and CS), and only CK activity was related to body size. These findings emphasize a joint stabilization role by the hindlimbs during suspension, especially in smaller three-toed sloths, and suggest that larger two-toed sloths could have muscles further modified for greater power output and/or prolonged arboreal maneuvering. Moreover, modifications to muscle metabolism rather than MHC expression may be more reflective of functional adaptation in sloth limbs.

Cheap labor: myosin fiber type expression and enzyme activity in the forelimb musculature of sloths (Pilosa: Xenarthra)

Sloths are canopy-dwelling inhabitants of American neotropical rainforests that exhibit suspensory behaviors. These abilities require both strength and muscular endurance to hang for extended periods of time; however, the skeletal muscle mass of sloths is reduced, thus requiring modifications to muscle architecture and leverage for large joint torque. We hypothesize that intrinsic muscle properties are also modified for fatigue resistance and predict a heterogeneous expression of slow/fast myosin heavy chain (MHC) fibers that utilize oxidative metabolic pathways for economic force production. MHC fiber type distribution and energy metabolism in the forelimb muscles of three-toed ( Bradypus variegatus, n = 5) and two-toed ( Choloepus hoffmanni, n = 4) sloths were evaluated using SDS-PAGE, immunohistochemistry, and enzyme activity assays. The results partially support our hypothesis by a primary expression of the slow MHC-1 isoform as well as moderate expression of fast MHC-2A fibers, whereas few hybrid MHC-1/2A fibers were found in both species. MHC-1 fibers were larger in cross-sectional area (CSA) than MHC-2A fibers and comprised the greatest percentage of CSA in each muscle sampled. Enzyme assays showed elevated activity for the anaerobic enzymes creatine kinase and lactate dehydrogenase compared with low activity for aerobic markers citrate synthase and 3-hydroxyacetyl CoA dehydrogenase. These findings suggest that sloth forelimb muscles may rely heavily on rapid ATP resynthesis pathways, and lactate accumulation may be beneficial. The intrinsic properties observed match well with suspensory requirements, and these modifications may have further evolved in unison with low metabolism and slow movement patterns as means to systemically conserve energy. NEW & NOTEWORTHY Myosin heavy chain (MHC) fiber type and fiber metabolic properties were evaluated to understand the ability of sloths to remain suspended for extended periods without muscle fatigue. Broad distributions of large, slow MHC-1 fibers as well as small, fast MHC-2A fibers are expressed in sloth forelimbs, but muscle metabolism is generally not correlated with myosin fiber type or body size. Sloth muscles rely on rapid, anaerobic pathways to resist fatigue and sustain force production.