Climate and population density induce long-term cohort variation in a northern ungulate

1. Density-dependent and climatic conditions experienced by individuals bef ore and after birth differ considerably between cohorts. Such early environ mental variability has the potential to create persistent fitness differenc es among cohorts. Here we test the hypothesis that conditions experienced b y individuals in their early development will have long-term effects on the ir life history traits. 2. We approached this by analysing and contrasting the effects of climate ( the North Atlantic Oscillation, NAO) and population density at year of birt h on cohort birth weight, birth date, litter size, age of maturity, surviva l and fecundity of Soay sheep, Ovies aries L., ewes in the population on th e island of Hirta, St Kilda, Scotland. 3. Significant intercohort variations were found in life history traits. Co horts born after warm, wet and windy (high NAO) winters were lighter at bir th, born earlier, less likely to have a twin and matured later than cohorts born following cold and dry (low NAO) winters. High population densities i n the winter preceding birth also had a negative effect on birth weight, bi rth date and litter size, whereas high postnatal densities delayed age of f irst reproduction. 4. High NAO winters preceding birth depressed juvenile survival but increas ed adult survival and fecundity. The negative influence of high NAO winters on juvenile survival is likely to be related to mothers' compromised physi cal condition while the cohort is in utero, whereas the positive influence on adult survival and fecundity may relate to the improved postnatal forage conditions following high NAO winters. High pre- and postnatal population densities decreased juvenile (neonatal, yearling) and adult (2-4 years) sur vivorship but had no significant effect fecundity.