Abstract
Senescence is a decrease in functional capacity, increasing mortality rate with age. Sexual signals indicate functional capacity, because costs of ornamentation ensure signal honesty, and are therefore expected to senesce, tracking physiological deterioration and mortality. For sexual traits, mixed associations with age and positive associations with life expectancy have been reported. However, whether these associations are caused by selective disappearance and/or within‐individual senescence of sexual signals, respectively, is not known. We previously reported that zebra finches with redder bills had greater life expectancy, based on a single bill colour measurement per individual. We here extend this analysis using longitudinal data and show that this finding is attributable to terminal declines in bill redness in the year before death, with no detectable change in presenescent redness. Additionally, there was a quadratic relationship between presenescent bill colouration and survival: individuals with intermediate bill redness have maximum survival prospects. This may reflect that redder individuals overinvest in colouration and/or associated physiological changes, while below‐average bill redness probably reflects poorer phenotypic quality. Together, this pattern suggests that bill colouration is defended against physiological deterioration, because of mate attraction benefits, or that physiological deterioration is not a gradual process, but accelerates sharply prior to death. We discuss these possibilities in the context of the reliability theory of ageing and sexual selection.
Highlights
One of the most intriguing things about life is that it will inevitably end
To contrast the results of a cross-sectional analysis with the within-individual analyses that follow, we first tested for the six separate years of our study whether the individuals that died in the subsequent year following our measurement had lower bill hues (Fig. 1)
We find that for both males (z = À2.40, P = 0.016) and females (z = À1.73, P = 0.08) lower bill hues are associated with lower survival in the subsequent year (Fig. 1)
Summary
The correlation between age-specific declines in reproductive performance – a measure of condition – and mortality rate varies widely between species, suggesting that physiological markers of performance need not always track mortality rate (Burger & Promislow, 2006; Bouwhuis et al, 2012). A positive relationship between trait expression and survival can come about via terminal declines of sexual signals, variation between individuals in senescence or associations with the level of presenescent sexual signal expression (Reed et al, 2008). We dissect these intricate relationships between mortality and sexual signal senescence in zebra finches (Taeniopygia guttata) using longitudinal data, allowing us to separate between- and within-individual variation. Carotenoid supplementation can affect later reproduction in the same captive environment we use in this study (Simons et al, 2014b) These considerations have led us to interpret bill colouration as an indicator of physiological state (Perez-Rodrıguez, 2009), in our captive environment. We analysed patterns of ageing and investigated the contribution of terminal effects in bill redness and its association with mortality in both male and female zebra finches
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