Abstract
Age-related variation in reproductive performance is central for the understanding of population dynamics and evolutionary processes. Our understanding of age trajectories in vital rates has long been limited by the lack of distinction between patterns occurring within- and among-individuals, and by the lack of comparative studies of age trajectories among traits. Thus, it is poorly understood how sets of demographic traits change within individuals according to their age. Based on 40 years of monitoring, we investigated age-related variation in five reproductive traits in female pied flycatchers (Ficedula hypoleuca) including laying date, clutch size, brood size, nest success (probability that a nest produces at least one chick) and egg success of successful nests (proportion of eggs resulting in a chick). We disentangled within- from among-individual processes and assessed the relative contribution of within-individual age-specific changes and selective appearance and disappearance. Finally, we compared the aging pattern among these five reproductive traits. We found strong evidence for age-specific performance including both early-life improvement and late-life decline in all reproductive traits but the egg success. Furthermore, the aging patterns varied substantially among reproductive traits both for the age of peak performance and for the rates of early-life improvement and late-life decline. The results show that age trajectories observed at the population level (cross-sectional analysis) may substantially differ from those occurring at the individual level and illustrate the complexity of variation in aging patterns across traits.
Highlights
The continuous change of reproductive performance with age is a fundamental characteristic of most organisms (Clutton-Brock 1988; Newton 1989)
The manner in which individual reproductive performance changes from birth to death is important for the understanding of population dynamics and life-history evolution (Stearns 1992)
We investigate age trajectories in female pied flycatchers for five reproductive traits: laying date, clutch size, brood size, nest success and egg success
Summary
The continuous change of reproductive performance with age is a fundamental characteristic of most organisms (Clutton-Brock 1988; Newton 1989). The manner in which individual reproductive performance changes from birth to death is important for the understanding of population dynamics and life-history evolution (Stearns 1992). Reproductive performance typically increases in early life, reaches a plateau at intermediate ages and declines at old ages Senescence is defined as the decline in fitness components with increasing age (Rose 1991) and has been extensively studied. While current evolutionary theories explain the presence of senescence, they provide little basis for the understanding of the variability in aging patterns across different fitness components. It is expected that the strength of natural selection against the late-life decline in a given phenotypic trait depends on how
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