Ontogenetic changes in mineralization and bone geometry in the femur of muskoxen (Ovibos moschatus)

Bone mineralization and bone geometry at femoral midshaft were analysed for 33 muskoxen Ovibos moschatus ranging in age from 3 weeks to 4.5 years and in body mass from 9 to 215 kg, to assess changing mechanical properties ass ociated with postnatal ontogeny. Analysis of the mineral density of bone (B MD) indicates little or no change in femoral mineralization until after 4 m onths of age. Between 4 and 6 months of age BMD increases markedly, and com plete mineralization occurs by about 18 months of age. Femoral length (L) a nd bone strength (as measured by Z, the section modulus) scale negatively a llometric with body mass (L infinity M-0.26 and Z infinity M-0.88) indicati ng that neonate muskoxen have longer and stronger femora for their body siz e than do older animals. When the neonate data were excluded, however, bone length and bone strength regressed against body mass did not differ signif icantly from isometry (L infinity M-0.32 and Z infinity M-1.03), implying t hat different growth trajectories characterize postnatal ontogeny before an d after 4 months of age. As a consequence of the timing of ontogenetic chan ges in bone mineralization and cross-sectional geometry, safety factors cal culated for the femur (derived from estimates of peak bending stress and fr acture strength) are largest and smallest in neonates and 4-month-old calve s, respectively. It is hypothesized that increased bone mineralization and accompanying diaphyseal stiffness after 4 months of age are necessary to ma intain structural integrity of the femur.