Aging and Senescence across Reproductive Traits and Survival in Superb Fairy-Wrens (Malurus cyaneus)

Why do senescence rates of fitness-related traits often vary dramatically? By considering the full aging trajectories of multiple traits, we can better understand how a species' life history shapes the evolution of senescence within a population. Here, we examined age-related changes in sex-specific survival, reproduction, and several components of reproduction using a long-term study of a cooperatively breeding songbird, the superb fairy-wren (Malurus cyaneus). We compared aging patterns between traits by estimating standardized rates of maturation, age of onset of senescence, and rates of senescence while controlling for confounding factors reflecting individual variability in life history. We found striking differences in aging and senescence patterns between survival and reproduction as well as between reproductive traits. In both sexes, survival started to decline from maturity onward. In contrast, all reproductive traits showed improvements into early adulthood, and many showed little or no evidence of senescence. In females, despite senescence in clutch size, number of offspring surviving to independence did not decline in late life, possibly due to improvements in maternal care with age. Superb fairy-wrens have exceptionally high levels of extragroup paternity, and while male within-group reproductive success did not change with age, extragroup reproductive success showed a dramatic increase in early ages, followed by a senescent decline, suggesting that male reproductive aging is driven by sexual selection. We discuss how the superb fairy-wrens' complex life history may contribute to the disparate aging patterns across different traits.