Hindlimb morphology in eastern cottontail rabbits (Sylvilagus floridanus) correlation of muscle achitecture and MHC isform content with ontogeny

Abstract

Rabbits have powerful hindlimb muscles that allow them to accelerate rapidly during locomotion. Moreover, juveniles may have performance advantages relative to adults that could increase their chances of survival to reproductive maturity. To investigate the ontogeny of power capacity in rabbit hindlimb extensors, muscle architectural properties and myosin heavy chain (MHC) isoform content were quantified in 9 juvenile and 5 adult eastern cottontail rabbits (Sylvilagus floridanus). It is hypothesized that musculoskeletal features of the hindlimb will be optimized in juveniles to promote increased locomotor performance similar to that of adults. We measured architectural properties including muscle mass (MM), belly length (ML), fascicle length (l[superscript F]), pennation angle, and physiological cross-sectional area (PSCA), known to provide functional estimates of maximum isometric force, joint torque, and power. MHC isoform composition was determined by SDS-PAGE and densitometry. Overall, the growth trajectories of MM and power increase faster (i.e., positive allometry) than body size with ontogeny and suggest that adults are more developed than juveniles. However, juvenile rabbits appear to have several compensatory features that may allow for increased locomotor performance (and fitness) including higher l[superscript F]/ML ratios, faster MHC isoform composition, and greater ankle joint mechanical advantage. Therefore, the findings only partially support the hypothesis. The development of the musculoskeletal system appears to provide rabbits with some advantages to evade predation by rapid acceleration; however, adult rabbits may be over-developed affecting their performance. Further studies are required to fully understand the locomotory development of rabbits through ontogeny.