American Society of Naturalists

Eve B. Cooper, Timothée Bonnet, Helen L. Osmond, Andrew Cockburn, and Loeske E. B. Kruuk (Jan 2021)

Dramatically different patterns of reproductive and survival senescence in a cooperatively breeding bird

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Abstract

Why do senescence rates of fitness-related traits often vary dramatically? By considering the full ageing 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 ageing patterns between traits by estimating standardized rates of maturation, the 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 ageing and senescence patterns between survival and reproduction, as well as between reproductive traits. In both sexes, survival started to decline from maturity onwards. 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 extra-group paternity, and while male within-group reproductive success did not change with age, extra-group reproductive success showed a dramatic increase in early ages, followed by a senescent decline, suggesting that male reproductive ageing is driven by sexual selection. We discuss how the superb fairy-wrens’ complex life history may contribute to the disparate ageing patterns across different traits.