EARLY DEVELOPMENT AND POPULATION FLUCTUATIONS IN SOAY SHEEP

1. In the Soay sheep population of Hirta (St. Kilda), high winter mortality occurs every 3-4 years following summers when population density exceeds 2.2 sheep ha-1. During these die-offs, more than 50% of adults, 70% of yearlings and 90% of lambs die and population density falls by around 65%. 2. This paper investigates the extent to which density-related changes in early growth might be responsible for fluctuations in over-winter survival and population size. Individual differences in birth weight and subsequent growth rates exert a strong influence on the survival of lambs through the neonatal period and through the first winter of life when the effects of population density on survival have been taken into account. 3. High population density in summer is associated with reductions in the weight of lambs entering the winter and low birth weights the following spring. Density-related changes in birth weight are closely correlated with changes in neonatal mortality. However, annual variation in neonatal mortality contributes little to fluctuations in population size. 4. Despite the relationship between individual variation in birth weight and survival through the first winter, density-related changes in the weight of lambs at birth and at 4 months contributed little to density-related changes in survival through the first winter. This was primarily because birth weight and early growth were most heavily depressed in springs that followed die-offs and were followed by winters when mortality (and density) were low. These results indicate that density-related changes in lamb growth are unlikely to be an important cause of die-offs in this population.