Seasonal changes in weight, condition and nutrition of free-ranging and captive muskox females

Abstract

A strong seasonal cycle of changes in weight and nutritional status occurs in wild female muskoxen; changes in fat are primarily related to reproductive status. Substantial changes in body protein also occur annually, although there was no clear association with reproduc- tion. Newly-lactating females lose more of their fat in the last 5-6 weeks of winter than during the previous 4-5 months, and lactating females compress much of their compensatory growth into late summer and fall, past the peak of fora- ge quality, as found by Thing et al. (1987). This tendency persists in captivity in Saskatoon, as it does in Fairbanks (White et al., 1989), although the seasonal variation is much reduced. This pattern may reflect priorities which change with season: initially (May-July) priority is gi- ven to calfsupport, and later to maternal tissues. Serum IGF-1 levels in wild muskoxen tended to be higher in summer, when forage quality was higher, although high IGF-1 levels persisted in wild muskoxen in the fall despite plant sene- scence. The strong seasonal pattern in captive muskoxen did not reflect their nearly constant diet; these results suggest an intrinsic predisposi- tion to high IGF-1 levels and weight gain in fall. Such a photoperiod-induced effect has been suggested in reindeer (Suttie et al., 1991). The pattern in wild females may thus reflect two in- fluences: IGF-1 levels are elevated in early sum- mer due to forage quality, and are high in late summer due to a photoperiod-induced surge. The combination of these two influences pro- duces a «plateau» of elevated IGF-1. In captive muskoxen fed a constant diet, only the second «surge» apparently exists. The low pregnancy rate among muskox cows on Victoria Island indicates that few cows are currently calving in successive years, although this can occur in wild muskoxen (Reynolds, 1989; Rowell, 1989). This may reflect the large energetic cost of lactation, particularly in the initial weeks before spring, when fat stores are depleted rapidly. Nursing females apparently do not catch up to non-lactating cows in condition during the fall weight surge, in this population. Should this poorer condition coincide with poor nutrition in fall or early winter, concep- tion or early pregnancy might be adversely affected.