Macroparasites potentially play a significant but often ignored role in the ecology and dynamics of wild ruminant populations. In the Arctic, parasites may impact on host populations by exacerbating the effects of seasonal and limited forage availability on the condition, fecundity and survival of individuals. We studied the effects of abomasal nematode parasites and warble flies, Hypoderma tarandi, on condition and pregnancy of caribou Rangifer tarandus in the Dolphin-Union herd, Nunavut, Canada. By the end of winter, female caribou over 2 years old showed a significant decrease in body weight with increasing nematode burden, and a decrease in back fat depth with increasing warble abundance. These effects were exaggerated in the non-pregnant fraction of the population. High warble larvae burdens were also associated with significantly reduced probability of being pregnant. Our research demonstrates a negative relationship between parasites and caribou condition that may have consequences for their fitness. Additionally, we discuss the possibility that muskox Ovibos moschatus share some parasite species with the caribou and could lead to elevated burdens in the sympatric host. Parasites may have been a contributory factor in a previous winter range-shift of the caribou herd and this may reflect a form of apparent competition between the two ungulate species.

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 associated with postnatal ontogeny. Analysis of the mineral density of bone (BMD) indicates little or no change in femoral mineralization until after 4 months of age. Between 4 and 6 months of age BMD increases markedly, and complete mineralization occurs by about 18 months of age. Femoral length (L) and bone strength (as measured by Z, the section modulus) scale negatively allometric with body mass (L ∞ M0.26 and Z ∞ M0.88) indicating that neonate muskoxen have longer and stronger femora for their body size than do older animals. When the neonate data were excluded, however, bone length and bone strength regressed against body mass did not differ significantly from isometry (L ∞ M0.32 and Z ∞ M1.03), implying that different growth trajectories characterize postnatal ontogeny before and after 4 months of age. As a consequence of the timing of ontogenetic changes in bone mineralization and cross-sectional geometry, safety factors calculated for the femur (derived from estimates of peak bending stress and fracture strength) are largest and smallest in neonates and 4-month-old calves, respectively. It is hypothesized that increased bone mineralization and accompanying diaphyseal stiffness after 4 months of age are necessary to maintain structural integrity of the femur.