Foraging ecology of female Dall's sheep in the Brooks Range, Alaska

Abstract

Most wild sheep (Ovis) are primarily diurnal. Thus, extreme cold, darkness, and limited quantities of low-quality forage during long winters above the Arctic Circle present a formidable challenge for sheep. Further, summer is particularly short at these high latitudes, providing little time for sheep to accumulate energy reserves for winter. This thesis discusses dietary and behavioral responses of wild sheep to the constraints of Arctic environments. Specifically, I determined diet composition and selection, forage quality, nutrient intake, and activity budgets of adult female Dall's sheep (ewes) (Ovis dalli dalli) near the northern extreme of the range of wild sheep for 2 years and constructed a model of the energy relationships of these animals. Ewes primarily consumed forbs and grasses during summer, and strongly selected forbs over other forages in accordance with the predictions of optimal foraging theory. Diets primarily consisted of grasses in early winter, shifted to sedges in February, and back to grasses in early spring. Shrubs were consistently the least selected class of forage. When the diet was composed of forages with varying digestibility, microhistological analyses not corrected for differential digestibility were biased toward less digestible forage. Winter forage available to Dall's sheep in the northern Brooks Range was low in both digestibility and protein content. In early summer ewes foraged during all hours of the day when sunlight was present for 24 hours. Sheep restricted their foraging almost entirely to daylight hours near the equinoxes, and foraged during all available hours of light, as well as 2.8 hours of the night in December. Daily foraging time varied from 12.9 hours in June to 7.9 hours in December, and, when measured on a daily basis, was positively correlated with average windchill and daylength. Ash-free fecal nitrogen and in vitro digestible dry matter were most highly correlated with activity level on a monthly basis. Energetics modeling indicated that ewes were in a negative energy balance for 6-8 months each winter and lost nearly 30% of their body weight. Duration of the short summer growing period was most important for weight gain, and presence of deep snow determined weight loss in winter.