Fitness consequences of delayed maturity in female wood ducks

Age at maturity has been suggested to be an important life-history trait with substantial potential for influencing the evolution of life histories. Using known reproductive histories of 90 female wood ducks (Aix sponsa) from a long-term study (1982–96), we investigated the fitness consequences of changes in age at maturity. Female wood ducks that attained sexual maturity as yearlings had a significantly shorter lifespan than those that delayed reproduction until 2 years of age or later. However, females that delayed reproduction neither produced a larger number of ducklings during their first reproductive event, nor did they have a greater number of successful lifetime breeding attempts than those maturing as yearlings. Females that attained sexual maturity as yearlings had the greatest individual fitness; the results indicated a directional selection for early maturity. These results are consistent with life-history theory predictions and suggest that reduced lifespan in female wood ducks is the primary cost associated with early maturity; however, costs of early maturity are outweighed by the associated benefits in fitness currency. Regression of individual fitness on age at maturity indicated strong directional selection for early maturity. These results suggest that attaining sexual maturity as yearlings may be an evolutionarily stable strategy in our study population, and that females delaying reproduction do so because they are genetically or environmentally constrained, not because of the potential fitness benefits of delayed maturity.