Reproductive aging and age-dependent parental effects on offspring in a long-lived seabird.

Reproductive aging is a widespread process in wild populations, affecting both females and males across many species. It also plays a key role in shaping parental effects as older parents are expected to be unable to invest optimally in reproduction late in life, or such investments may be compromised by the detrimental consequences of aging. In most species, reproductive performance increases over the first breeding attempts, reaches a plateau, and then declines at older ages. Many long-lived species, however, deviate from this pattern, with older individuals maintaining high breeding performance or even improving it, often resulting in offspring of higher quality. Studies examining sex-specific patterns of reproductive aging in long-lived species from natural populations and their consequences for offspring fitness remain scarce compared to the extensive human literature. We used a long-term longitudinal dataset of reproductive data collected from a population of a long-lived seabird, the Scopoli's shearwater, to analyze (i) how a set of parental traits (i.e., reproductive success, body mass and egg size) change with maternal and paternal age, and (ii) potential age-dependent parental effects on the body mass and skeletal traits of the offspring. We found that age strongly affected reproductive success. Early-life improvement was steeper in females than in males, whereas both sexes showed the typical late-life decline expected with advancing age. Adult body mass followed a similar, though partially reversed, pattern: males gained mass more rapidly than females early in life before reaching a plateau. Age also affected female egg volume and width, with older and heavier females laying larger and wider eggs. Parental effects on offspring body mass were mainly driven by pair experience, whereas skeletal traits depended only on chick age and sex. Our study shows that reproductive aging varies across traits in long-lived species, providing support for the asynchronous theory of aging, and reveals unexpected sex-specific patterns of age-related reproductive changes in monogamous species. Moreover, it suggests that advanced parental age is not necessarily associated with negative effects on offspring fitness.