Modes of thermal protection in newborn muskoxen (Ovibos moschatus)

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Abstract
The muskoxen (Ovibos moschatus), a native of Greenland and the Canadian North West Territories, give birth in late April, and the newborn calves are known to tolerate an ambient temperature (Ta) of ‐35°C.At birth the calves weigh about 8 kg, increasing in weight with 0.6 kg. day‐1 for the first 30 days. With a deep body temperature (DBT) of 39.5°C (range 37.7–41.3°C) the newborn calves are consequently able to maintain a thermogradient of at least 70°C between body core and the environment. The calves use primarily two modes of thermal protection: High metabolic heat production and prime fur insulation. Metabolic rate was about 3.5 W.kg‐1 at thermoneutrality in calves aged from 8 h to 7 days. Lower critical temperature at this age was about ‐7°C and a drop in Ta to ‐30°C increased metabolism to about 5.3 W.kg‐1.Upper critical temperature at age 4–7 days is as low as 20°C, while it in calves aged only 18–24 h appears to be even lower. The calves possess great amounts of brown adipose tissue (BAT) at birth. Mitochondria from the BAT deposits were isolated and found to be in an extremely loose‐coupled state with a great capacity for thermogenesis. Skeletal muscle contained very few mitochondria and is hardly employed in aerobic non‐shivering thermogenesis. Calves shiver visibly while drying just after birth, but are normally not seen shivering thereafter. The conductance value for the dry pelt of newborn calves averaged 3.2 W.m‐2.°C‐1 (n=4).Wetting of the pelt with ice‐water at a Ta of 3°C increased conductance to 8.8 W.m‐2.°C‐1.The conductance of the pelt was also influenced by wind, being 10 W.m‐2.C‐1 at a windspeed of 10 m. sec‐1.The legs of the newborn calves are heavily furred and countercurrent circulation is not present, subcutaneous temperature just above the hooves being +29.8°C at Ta of ‐24°C as compared to 37.5°C on the back. The newborn calves could cope with a Ta of ‐30°C without apparent problems under experimental conditions, but they suffered hypothermia when exposed to a Ta of ‐33°C in combination with wind of 10 m. sec‐1.At birth these calves are, however, able to walk, albeit with difficulty, with a DBT of 31°C.Wetting of the intact newborn calf with ice‐water at a Ta of +6°C was tolerated, but the calves will most certainly succumb to the unlikely event of wetting of the pelt combined with strong winds.