Overwinter gains in body fat challenge assumptions about winter nutrition in northern ungulates

Abstract Animals in northern environments contend with seasonally variable resources and have evolved life histories to cope with the flush of nutrients in summer and scarcity in winter. Ungulates cope with seasonal variation in resources by accreting fat stores when resources are plentiful during the growing season and then mobilizing these stores to meet nutritional requirements when resources are more limited during winter. While this generalization makes sense, previous work has suggested that caribou ( Rangifer tarandus spp.) may break this rule; however, previous works have suffered various limitations that leave this question unanswered. As part of a larger study to understand the role of nutrition as a potential factor limiting the Mulchatna Caribou Herd (MCH) in southwestern Alaska, we initiated a longitudinal study in which we captured the same individual caribou twice annually in autumn and late winter. We aimed to test the hypothesis that winter nutrition limited the MCH and predicted that caribou would lose fat between late autumn and late winter. During captures, we determined body fat percentage of lactating, adult females using validated ultrasonography and manual palpation techniques, and determined pregnancy status in late winter using transabdominal ultrasonography. Repeated measures indicated lactating females gained body fat over winter, averaging ~2.8 percentage points from 6.1% ± 0.1% (mean ± SE) in autumn to 8.9% ± 0.4% in late winter ( z = 5.425, p < 0.001), with maximum individual gains of up to 8.5 percentage points. Our findings provide empirical support for overwinter gains in fat among caribou and indicate that winter nutrition is not limiting the MCH. Our findings illustrate the value of longitudinal studies in ecology, have implications for research designs for studies of nutrition in northern ungulates, and highlight important considerations for caribou relative to unique nutritional aspects of their life history.