Seasonal patterns in body composition and reproduction of female muskoxen (Ovibos moschatus)

To analyse the effects of season and reproductive status on seasonal changes in body composition of female muskoxen (Ovibos moschatus), we examined 196 muskox cows from Victoria Island, Northwest Territories, Canada, during 16 collections carried out by Inuit hunters in April, May, July, August, September, and November from 1989 to 1993. During this time, the density of muskoxen on the island increased slowly and they numbered about 45,000 in 1993. Hunting and predation by wolves (Canis lupus) were limited, and there was little evidence of muskox mortality due to malnutrition. Adult muskox cows (greater than or equal to 4 years old) were leanest in May, when the ingesta-free body weight (IFBW) and estimated fat weight of newly-lactating cows were 116 +/- 2 kg (adjusted mean +/- S.E.) and 13.7 +/- 0.8 kg, respectively, and those of non-lactating cows were 112 +/- 3 and 12.2 +/- 1.6 kg. These weights peaked in September at 166 +/- 4 kg and 37.8 +/- 1.8 kg in lactating cows and 176 +/- 7 and 45.1 +/- 3.4 kg in non-lactating cows. The most rapid losses of fat (242 g/day) and protein (55 g/day) occurred in lactating cows in late winter. The weight of ingesta, primarily rumen contents, increased during the year from April (20.9% of body weight) to November (27.8%) and was exceptionally high for ruminants. Seasonal patterns in 2- and 3-year-old females were generally similar to those in older cows. although the younger cows were leaner and weighed less. Pregnancy rates increased from 0 in females 18-23 months old to 25% in cows 30-35 months old and 63% in females greater than or equal to 42 months old. Mammary gland weight in lactating females peaked in July and August, suggesting that peak milk production did not occur until at least two months after calving, and lactation was sometimes extended to a year or more. Lactation in November did not preclude pregnancy, but females which were pregnant at this time were fatter and heavier than non-pregnant cows. These results indicate a high-amplitude annual cycle of body reserves in female muskoxen, and suggest a considerable sensitivity of pregnancy rates to nutritional influences.