Age-Sex Differences in the Diets of Wintering Rocky Mountain Bighorn Sheep

Abstract

The winter diets of Rocky Mountain bighorn sheep in Banff National Park were studied to determine whether the age—sex classes have evolved differences in food exploitation patterns under selective pressure to reduce intrapopulation competition. Rams remained primarily on the north end of the study area while the "non—ram" superclass (ewes, lambs, yearlings) occupied primarily the south end. Vegetation analysis indicated that the southern part of the range was characterized by relatively more Bromus, Carex, Koeleria, Poa, Oxytropis, and Juniperus and less Festuca, Fragaria, Hedysarum, Arctostaphylos, and Rosa than the northern portion. In addition, the southern part of the range was typified by less soil, more xeric conditions, lower plant biomass and apparently overgrazed conditions relative to the more northern parts. Ewes, lambs, and yearlings were found to have diets more similar to each other than to that of adult rams. Class effects in multivariate statistical analysis were significant; the major univariate differences were that non—rams ate more Carex and less Festuca than did rams. This class effect was found to be wholly attributable to the fact that rams and non—rams segregate onto separate portions of the range which contain different availabilities of forage plants. It was considered to be unlikely that the spatial separation arose from diet preferences but rather from some unrelated factor. The observed differences should not be considered as an adaption for competition reduction.