Mortality of Northern Montana Pronghorns in a Severe Winter

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Abstract
Winter mortality of pronghorn antelopes (Antilocapra americana) was studied near Glasgow, Montana, in April, 1965. A minimum loss of 500 pronghorns was associated with severe weather and occurred primarily on foothill grassland along the Milk River. Bone marrow examination of 59 carcasses indicated that malnutrition was the principal cause of death. Sex and age composition of 327 carcasses was 28, 33, and 39 percent fawns, adult males, and adult females, respectively. Comparison of carcass age ratios with ratios observed in summer herds suggested a differentially high mortality for fawns but not for yearlings. A low mortality rate was indicated for 2 1/2-year-olds; the rate apparently increased for animals older than 2 1/2, Sex ratio comparisons were inconclusive for fawns but indicated that 1 1/2-3 1/2-year-old females had a higher mortality rate than males of the same age. The mortality rate of males 4 1/2-7 1/2 years and older appeared greater than females of the same age group. Food habits were studied from analysis of 14 carcass rumens and 6113 instances of plant use recorded at pronghorn feeding sites. Rumen contents were characterized by fringed sagewort (Artemisia frigida), creeping juniper (Juniperus horizontalis), and a variety of forbs. Comparison of rumen samples with feeding site examinations suggested that starvation occurred while animals were restricted to the grassland vegetative type. Among pronghorns in the Glasgow herd fawn production was 39-55 fawns per 100 does as compared to a normal of 90-110. On the Malta winter range where sagebrush was abundant, available, and heavily utilized, losses were minor and fawn production normal.

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