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Conceptual Revision and Synthesis of Proximate Factors Associated with Age-Related Improvement in Reproduction


  • (Invited Review)


Improvement in reproductive performance with age, up to the point of senescence, is a predominant pattern among vertebrates. Predictions from life-history theory provide a powerful framework for understanding the evolutionary basis of these patterns. However, based on the growing number of publications on this topic, there is increased interest in understanding the proximate causes of age-related improvements in reproductive performance (ARIRP). A formal conceptual framework through which factors related to ARIRP can be examined is lacking. Here, we establish hypotheses with testable predictions for social and ecological factors, including resource quality, mate fidelity, site fidelity, prior breeding experience, and changes in ability to attract mates. We use this conceptual framework to review 55 empirical studies published (between 1900 through 2013) on avian species as birds have the greatest representation in empirical studies of ARIRP. Our synthesis revealed that tests of the breeding experience hypothesis are most prevalent in the literature, whereas tests of the site fidelity hypothesis are least prevalent. Overall, the role of increased mate attraction with age seems to be an important predictor of ARIRP, whereas changes in resource quality with age show the least support among published studies. Because many studies suffered from small sample sizes and did not control for confounding variables, we suggest experimental methodologies for teasing apart hypotheses in empirical investigations and offer statistical approaches for longitudinal datasets. From an ultimate perspective, we also highlight the role of life-history variation, in shaping within-individual improvements. Future work should employ a standardized framework to study patterns of ARIRP, as set forward here, to allow for more quantitative comparison of results across studies.