Life History Consequences of Variation in Age of Primiparity in Bighorn Ewes

We investigated the life history consequences of age at primiparity in two marked populations of bighorn sheep (Ovis canadensis), studied for 19 and 12 yr. Some ewes first lambed at 2 yr of age, others at 3 or 4 yr. Lambs of 2-yr-old ewes were smaller and less viable than lambs of older ewes. Lactation at 2 yr did not affect survival to 3 yr, but in one population it was associated with reduced survival from 3 to 4 yr. Two yr olds that weaned lambs gained less mass during summer than other 2 yr olds. Overwinter mass loss from 2 to 3 yr was correlated with autumn body mass, and was lower for parous than for nonporous ewes. Reproductive success at 3 yr was independent of reproductive status at 2 yr. At 4 yr, ewes that lactated as 2 yr olds were smaller than ewes that had not lactated as 2 yr olds. In one population, negative effects of early maturation upon mass gain and subsequent reproductive success became evident at high density, and very few 2 yr olds lambed when population density was high. The lifetime reproductive success of early- maturing ewes was not lower than that of late-maturing ewes. To account for individual variation in initial reproductive potential, we controlled body mass as a yearling, a variable that affected reproductive success. Mass as a yearling, however, did not interact with age of primiparity to affect survival, growth, or reproductive success. Uncertainties about re- source availability and possible effects of genotype and body fat may explain why many ewes postponed their first reproduction despite apparently low life history costs of early maturation.