Winter extremes of temperature and food shortage limit the distribution of arctic animals. North American porcupines (Erethizon dorsatum) are one of the most widely distributed mammals in North America and range from deserts to arctic tundra. In Alaska porcupines remain active at low winter temperatures (i.e., −39°C) while consuming woody plants that are low in nitrogen (N) and high in both fiber and plant secondary metabolites. Porcupines conserved lean body mass in winter by using fat stores. Fat mass declined from 50% ± 3% to 27% ± 7% of body mass over winter. Animals with small fat stores might be more reliant on food intake during winter, because proportional fat loss was correlated positively with total fat mass at the start of winter. Fat losses were minimized by lowering rates of energy expenditure. Field metabolic rate was 440 ± 18 kJ kg−0.77 day−1. Water turnovers were slow at 26.62 ml kg−0.75 day−1 in wild porcupines. Body temperatures were not reduced to save energy; core temperatures were maintained at 37.3°C ± 0.1°C despite variation in ambient air temperature from 7°C to −38°C in captivity. Persistence of porcupines at the northern limits of their range is due to plasticity of food intake and tolerance of low-quality diets and low ambient temperatures. Minimal expenditures of energy and N in winter are combined with the conservation of lean mass. Porcupines rely on abundant summer forages to replenish their stores of fat and protein for reproduction and survival in the subsequent winter.
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Vol. 92 • No. 3