Stride frequency in relation to allometric growth in ghost crabs

Body size has significant impacts on fundamental processes of locomotion, including the operational frequency of skeletal muscle contraction, which declines systematically with increasing size. Although this shift in operation frequency and contractile kinetics is well documented in the literature, the mechanisms responsible for these changes are still incompletely understood. One important factor is that the mechanical properties of the musculoskeletal system possess resonant properties that favor higher frequencies in small animals. Another significant element is the physiological properties of the skeletal muscles, which may be tuned for faster contractions in smaller animals. These two components are interrelated, but precisely how muscle physiology and musculoskeletal mechanics interact to shape patterns of locomotion is complex. Ghost crabs O cypode quadrata present an interesting model to study these processes because they are proficient runners that exhibit systematic changes in stride frequency as they grow. In the current study, we focused on anatomical changes that might occur with allometric growth in ghost crabs to test the hypothesis that changes in mechanical parameters contribute to the slowing of stride frequency. We paired basic anatomical measurements with kinematic analyses of crabs running at top speeds on a treadmill and experimentally weighted crabs to determine if the relative mass of larger crabs affects running frequency. We found that biologically relevant mechanics of the leg joints do not change with growth, as mechanical advantage and muscle fiber length relative to joint moment arm were unaffected by body size. Loading crabs had similar effects on stride frequency in both large and small animals alike. In contrast, muscle shortening velocity, estimated directly from angular velocity of the leg joints, decreased significantly with increasing body size. These data suggest that fundamental changes to the contractile properties of skeletal muscles during growth are primarily responsible for the changes in stride frequency observed in ghost crabs.