Robust Responses of Female Caribou to Changes in Food Supply

AbstractUngulates can respond to changes in food supply by altering foraging behavior, digestive function, and metabolism. A multifaceted response to an environmental change is considered robust. Short seasons of plant growth make herbivores sensitive to changes in food supply because maintenance and production must be accomplished in less time with fewer options in a more fragile response. Caribou live at high latitudes where short summers constrain their response to changes in food supply. We measured the ability of female caribou to resist and tolerate changes in the quality and quantity of their food supply during winter and summer. Caribou resisted changes in food abundance and quality by changing food intake and physical activity with changes in daily temperature within each season. Peak food intake rose by 134% from winter pregnancy to summer lactation (98 vs. 229 g kg^-0.75 d^-1), as digestible requirements to maintain the body increased by 85% for energy (1,164 vs. 2,155 kJ kg^-0.75 d^-1) and by 266% for N (0.79 vs. 2.89 g N kg^-0.75 d^-1). Caribou required a diet with a digestible content of 12 kJ g^-1 and 0.8% N in pregnancy, 18 kJ g^-1 and 1.9% N in early lactation, and 11 kJ g^-1 and 1.2% N in late lactation, which corresponds with the phenology of the wild diet. Female caribou tolerated restriction of ad lib. food intake to 58% of their energy requirement (680 vs. 1,164 kJ kg^-0.75 d^-1) during winter pregnancy and to 84% of their energy requirement (1,814 vs. 2,155 kJ kg^-0.75 d^-1) during summer lactation without a change in stress level, as indicated by fecal corticosterone concentration. Conversely, caribou can respond to increased availability of food with a spare capacity to process digestible energy and N at 123% (2,642 vs. 2,155 kJ kg^-0.75 d^-1) and 145% (4.20 vs. 2.89 g N kg^-0.75 d^-1) of those respective requirements during lactation. Robust responses to changes in food supply allow caribou to sustain reproduction, which would buffer demographic response. However, herds may decline when thresholds of behavioral resistance and physiological tolerance are frequently exceeded. Therefore, the challenge for managing declining populations of caribou and other robust species is to identify declines in robustness before their response becomes fragile.