Seasonal patterns in nutritional condition of caribou (Rangifer tarandus) in the southern Northwest Territories and northeastern British Columbia, Canada

Evaluating nutritional condition provides insights of nutritional influences on wildlife populations. We sampled three measures of condition - body fat, body mass, and loin thickness- of adult female caribou (Rangifer tarandus (Linnaeus, 1758)) in boreal settings in the Northwest Territories (NT), Canada, in December and March, 2016-2018, and in mountain and boreal settings in British Columbia (BC), Canada, in December and February, 2014-2015. We evaluated the effect of calf-rearing on condition in December, compared influences of summer-autumn versus winter on condition over winter, and developed an annual profile of nutritional condition with estimates from caribou dying in summer. Mean December body fat was 8.4% in females with calves and 11.4% in females without calves, demonstrating the influence of lactation on condition. Over winter, nutritional condition did not decline in northeastern BC and it declined slightly in NT: body fat by 0.55 percentage points, mass by 2.8 kg, and loin thickness did not change. Body fat peaked in December, changed little over winter, but declined to a minimum by early summer, temporally coinciding with elevated rates of adult female mortality. Consistent with those of other ungulate studies worldwide, our findings suggest a need to focus on nutritional limitations operating in late spring through early autumn.