Do Hormones, Telomere Lengths, and Oxidative Stress form an Integrated Phenotype? A Case Study in Free-Living Tree Swallows

Abstract

Synopsis All organisms must anticipate and balance energetic demands and available resources in order to maximize fitness. As hormones coordinate many interactions between an organism's internal condition and the external environment, they may be key in mediating the allocation of resources to meet these demands. However, given that individuals differ considerably in how they react to changes in energetic demand, we asked whether variations in endocrine traits also correspond with life history variation. We tested whether natural variation in glucocorticoid hormone levels, oxidative stress measurements, and condition related to reproductive effort in a free-living songbird, the tree swallow, Tachycineta bicolor We then tested whether any of these traits predicted the probability of a particular individual's return to the local population in the following two years, an indicator of survival in this philopatric species. We found that males and females with longer telomeres had lighter nestlings. Moreover, individuals with lower plasma antioxidant capacity and higher reactive oxygen metabolites (i.e., greater oxidative stress) were less likely to return to the population. However, none of these traits were related to glucocorticoid levels. Our findings suggest a trade-off between reproduction and survival, with individuals with shorter telomeres having heavier nestlings but potentially paying a cost in terms of higher oxidative stress and lower survival. Interestingly, the evidence of this trade-off was unrelated to natural variation in glucocorticoids.

Fig. 1

Adult tree swallows with longer telomeres (estimated as a principle component of average and the distribution of telomere lengths at each 10% percentile) had lighter offspring (residuals controlling for age). Females are represented by solid circles and solid regression line. Males are represented by open circles and dashed regression line.

Fig. 2

Whether adult tree swallows returned to breed in the population is predicted by the magnitude of oxidative stress they experience. Oxidative stress (OxS) was expressed as the first axis of a principal component analysis of antioxidant status and oxidative damage. There is a typo here. It should read: Higher principle component values indicate low antioxidant capacity and higher ROMs. Raw data are represented with the predicted mean of the logistic model presented in the text. Values are jittered along the y-axis for better visibility.