HABITAT SELECTION BY DALL SHEEP (OVIS DALLI) - MATERNAL TRADE-OFFS

Habitat selection by female Dall's sheep (Ovis dalli dalli) during lam bing was studied in interior Alaska, U.S.A., in 1988 and 1989. Selecti on of habitat changed with the chronology of lambing. During the pre-l ambing period, maternal females selected sites with forage and avoided snow-covered areas. During peak lambing, food and steep slopes contin ued to be selected. Distance to escape terrain was a critical componen t of habitat selection by females throughout lambing. Maternal bands t hat ventured farther from escape terrain were in larger groups. Abunda nce of forage also increased in areas that were farther from this terr ain. Habitat selection by females also differed between years. In 1989 , a late spring storm resulted in deep snow and delayed phenology of p lants; forage was of lower quality, and the growing season was much sh orter in 1989 than in 1988. Births of lambs also occurred later and le ss synchronously in the second year. Females selected terrain characte ristics that were related to avoiding predators in 1988 when forage wa s more plentiful, but selected sites with forage in 1989 when food was less abundant. Group size of maternal bands was larger in 1988 than i n 1989, and females foraged most efficiently during 1988. Large groups may not have occurred in 1989 because lambs were born asynchronously, preventing cohesive movements of maternal bands with lambs of differi ng ages. Both the chronology of lambing and a variable environment aff ected habitat selection by females. Maternal females made trade-offs b etween the requirements for forage to meet the high energetic costs of lactation and the risk of predation.