EFFECTS OF BODY-SIZE, POPULATION-DENSITY, AND MATERNAL CHARACTERISTICS ON AGE AT FIRST REPRODUCTION IN BIGHORN EWES

The factors affecting variation in age at first reproduction of bighor n ewes (Ovis canadensis) were investigated in two marked populations i n Alberta. One population was studied for 20 years, the other for 11 y ears. As yearlings, females that lactated at 2 years of age were on av erage heavier and larger, and had longer horns than females that did n ot lactate at 2 years. However, there was wide overlap in body mass be tween early and late producers, and increases in body mass over the th reshold for reproduction had little effect on the probability of early lambing. The body mass of females at 4 months of age explained less t han half of the variance in female body mass at 1 year or at 15 months . In one population, the proportion of 2-year-old ewes lactating was n ot correlated with density and declined after a pneumonia epizootic. I n the other population, the proportion of 2-year-old ewes lactating wa s higher during an experimental reduction of density, and dropped to n ear zero as density increased. There was a significant interaction eff ect of body mass and population density upon the probability that a ew e would lactate at 2 years of age. Independently of body mass, yearlin gs were less likely to lactate at 2 years of age at high population de nsity than at low density. The number and age distribution of rams did not affect the proportion of I-year-old ewes lactating. The mothers o f lactating 2-year-olds were not older or heavier than the mothers of ewes that did not lactate at 2 years. Although some of the variation i n age at first reproduction was due to differences in mass and populat ion density, much of it remained unexplained and could be due to genet ic factors.