Population fluctuations, reproductive costs and life-history tactics in female Soay sheep

1. Though models of life-history decisions are traditionally based on age-related changes in the costs and benefits of reproduction, in nature both costs and benefits vary with individual differences in phenotype as well as with environmental changes. 2. Using long-term records of individual reproduction and survival in the Soay sheep of St Kilda, we show that the costs and benefits of breeding to animals of different weight categories vary with population density. 3. Subsequently, we use stochastic dynamic programming to predict the optimal fecundity of animals belonging to each category at high and low population density. Optimal strategies of fecundity vary with population density as well as between different weight categories of sheep. However, there is no evidence that the sheep track density-related changes in optimal fecundity. Instead, their behaviour approximates to an average, weight-related optimum that is well adapted to the range of conditions that they encounter.