Evidence for and possible causes of increased mortality of bull muskoxen during severe winters

Abstract

Muskoxen (Ovibos moschatus) were classified by sex and age at 12 locations on four islands in the Canadian Arctic Archipelago, Northwest Territories. We classified 147 muskoxen on Bailey Point, southwestern Melville Island, and 92 on northeastern Prince of Wales Island in August 1983. We subsequently classified 5449 muskoxen on western Victoria and Banks islands in July–August 1986, which we used as a basis for evaluating the 1983 samples. The pooled sex ratio for muskoxen ≥ 3 years old was significantly skewed toward females, and the proportion of single bulls was also lower on Bailey Point than on northeastern Prince of Wales Island in August 1983 or on Banks or Victoria islands in July–August 1986. Sexual behaviour was frequently observed at Bailey Point in August 1983, which suggests that both bulls and cows were in breeding condition. Almost no aggressive rutting behaviour was seen at Bailey Point, in contrast to observation of the complete range of sexual and aggressive behaviours on Prince of Wales Island. Above-average (1970–1985) snow depths were experienced from March to May in five of the six winters preceding winter 1982–1983. We suggest that severe winters imposed malnutrition-related mortality on spent bulls. The depleted fat reserves of these bulls apparently could not sustain them during severe and prolonged winters. This kind of winter mortality among breeding bulls likely is higher in areas of high muskox densities where more interactions between bulls during the rut lead to greater energetic costs.