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.

State-dependent foraging by caribou with different nutritional requirements

Foraging by animals is hypothesized to be state-dependent, that is, varying with physiological condition of individuals. State often is defined by energy reserves, but state also can reflect differences in nutritional requirements (e.g., for reproduction, lactation, growth, etc.). Testing hypotheses about state-dependent foraging in ungulates is difficult because fine-scale data needed to evaluate these hypotheses generally are lacking. To evaluate whether foraging by caribou (Rangifer tarandus) was state-dependent, we compared bite and intake rates, travel rates, dietary quality, forage selection, daily foraging time, and foraging strategies of caribou with three levels of nutritional requirements (lactating adults, nonlactating adults, subadults 1–2 years old). Only daily foraging times and daily nutrient intakes differed among nutritional classes of caribou. Lactating caribou foraged longer per day than nonlactating caribou—a difference that was greatest at the highest rates of intake, but which persisted even when intake was below requirements. Further, at sites where caribou achieved high rates of intake, caribou in each nutritional class continued foraging even after satisfying daily nutritional requirements, which was consistent with a foraging strategy to maximize energy intake. Foraging time by caribou was partially state-dependent, highlighting the importance of accounting for physiological state in studies of animal behavior. Fine-scale foraging behaviors may influence larger-scale behavioral strategies, with potential implications for conservation and management.

Straight from the caribou’s (Rangifer tarandus) mouth: detailed observations of tame caribou reveal new insights into summer–autumn diets

High-quality habitats for caribou (Rangifer tarandus (L., 1758)) are associated primarily with lichens, but lichens alone fail to satisfy summer nutritional requirements. To evaluate the summer forage value of plant communities across northeastern British Columbia (BC), where populations of northern and boreal ecotypes of caribou are declining, we observed foraging by tame, female caribou. We compared diet composition with forage abundance to determine forage selection and to quantify forage availability. Deciduous shrubs, not lichens, largely dominated summer diets. Caribou were highly selective foragers, with 28 species comprising 78% of diets. Caribou avoided ≥50% of understory vegetation in all communities, especially conifers, evergreen shrubs, mosses, and two genera of terrestrial lichens. Availability of accepted forage (species not avoided) was strongly heterogeneous across landscapes. Alpine shrub areas and mid-elevation spruce–fir stands in the mountains, as well as treed rich fens and white spruce communities in the boreal forests, provided the greatest quantities of accepted forage for caribou. Dry alpine sites and unproductive black spruce communities provided the least accepted forage. Our work has direct implications to caribou conservation by contributing to a greater understanding of the forage value of summer habitats, with implications to habitat selection, seasonal movements, and distribution ecology.

Nutritional implications of increased shrub cover for caribou (Rangifer tarandus) in the Arctic

Shrubs are increasing in the annual range of arctic caribou (Rangifer tarandus (L., 1758)), but it is unknown how much summer browse caribou could consume. We measured instantaneous intakes of resin birch (Betula glandulosa Michx.) and feltleaf willow (Salix alaxensis (Andersson) Coville) by caribou during summer. Daily intake of a formulated diet without toxins was measured during the same period to monitor appetite. Caribou appetite increased from 64.1 to 86.7 g DM·kg–0.75·day−1 as animals gained body mass from 96.8 to 113.5 kg. We estimated that caribou required 645 kJ·kg–0.75·day−1 of digestible energy to maintain body mass and 1113 kJ·kg–0.75·day−1 to gain body mass for autumn reproduction. Caribou had the same bite mass (9.7 mg·bite−1·kg–0.75) and instantaneous intake rate (0.17 g DM·min−1·kg–0.75) on both forages; however, birch contained more phenols (3.3% vs. 1.5%) and less available protein (6.2% vs. 10.2%) than willow. A 100 kg female caribou would need to consume 2.4–8.7 kg of fresh browse, requiring 3.1–8.5 h·day−1 of eating time to meet daily energy requirements. Birch is unlikely to provide enough nitrogen for maintenance of body protein. Therefore, caribou may depend on abundance and diversity of plants to offset toxin loads and low protein intake from shrubs during summer.

Responses of caribou and reindeer (Rangifer tarandus) to acute food shortages in spring

Migratory caribou and sedentary reindeer (Rangifer tarandus (L., 1758)) can encounter acute food shortages during spring. We examined the response to short-term food restrictions by measuring individual food intake, body mass, and activity of 2-year-old unbred female caribou and reindeer from 25 April to 29 May 2011. Caribou lost 2%–3% of body mass on days when mean dry matter (DM) intakes (60 ± 6 g DM·kg−0.75·d−1) were restricted up to 75%. Caribou regained body mass as intake increased to 98 ± 8 g DM·kg−0.75·d−1 following restriction without a change in digestibility (82%–83%). In reindeer, digestibility increased (78%–83%) as intakes decreased (67–45 g DM·kg−0.75·d−1). Food restriction did not affect activity for either subspecies. We suggest that, at high digestive efficiency, Rangifer have “spare capacity”, to increase DM intake to compensate for lost foraging opportunity or to use patches of emerging high-quality forage. Furthermore, caribou with large fat reserves lost proportionally more body mass, consumed less food, and were less active than leaner caribou. Our data indicate that Rangifer use flexible responses of food intake, digestion, and body condition to maximize survival and reproduction in both migratory and sedentary ecotypes at the end of winter.