Young females pay higher costs of reproduction in a short-lived bird

Abstract

Theory predicts a trade-off between current reproduction and future reproduction or survival. Nevertheless, costs of reproduction are often not found owing to heterogeneity in environment or individuals, or to studies not evaluating multiple costs or reproductive metrics that could influence costs. Detecting costs of reproduction is further complicated by the fact that they may increase with age if physiological condition declines in older individuals, or decrease with age if younger individuals are less efficient at acquiring resources than older individuals. We used a 37-year study to evaluate costs of reproduction in song sparrows (Melospiza melodia). Our results support theoretical expectations for short-lived species by demonstrating costs of reproduction on future survival, but not on future reproduction. We examined two metrics of reproductive allocation—reproductive effort and termination of breeding—and found that only higher reproductive effort increased costs. Thus, testing of multiple allocation metrics may be necessary because results may not be coincident between metrics. Lastly, we observed that younger females paid higher costs of reproduction than older females. Although older female sparrows senesced, they had lower costs of reproduction than younger females who may be less able to acquire food or high-quality mates. By taking into account variation among individuals and examining multiple metrics, our study provides strong support for costs of reproduction and a decrease in costs with age. One premise of life history theory is that reproduction is costly, but evidence of such costs remains mixed. Mixed results may arise because resources are not always limiting for all individuals or in all environments, leading to temporal, spatial, and individual heterogeneity in trade-offs between current reproduction and future reproduction or survival that can be hard to detect. Costs may also vary with age, depending on how resource acquisition and allocation vary with age. We used a 37-year study of female song sparrows (Melospiza melodia) to control statistically for individual and environmental heterogeneity and test multiple cost metrics. We demonstrate marked costs of reproduction on future survival, particularly in young females.