Winter bioenergetics of Rocky Mountain bighorn sheep

Changes in voluntary dry matter intake, body weight, and resting metabolic rates at 10 and −10 °C were measured in four adult Rocky Mountain bighorn sheep from October until May. Voluntary intake in mid-February decreased to 0.55 of that in mid-October. Body weights increased until January after which stasis or slight declines occurred. Resting metabolic rate at 10 °C fluctuated without a consistent pattern over winter. Resting metabolic rates at −10 °C were lowest in February and highest in May. The lower critical temperatures of fed animals in winter pelage were below −20 °C. As temperatures were lowered from −20 to −30 °C, metabolic rates increased 1.37- to 1.39-fold. Wind speeds from 4 to 8 m/s increased metabolic rates only at temperatures below −20 °C.