Abstract
ABSTRACTThe hormone corticosterone (CORT) has been hypothesized to be linked with fitness, but the directionality of the relationship is unclear. The ‘CORT-fitness hypothesis’ proposes that high levels of CORT arise from challenging environmental conditions, resulting in lower reproductive success (a negative relationship). In contrast, the CORT-adaptation hypothesis suggests that, during energetically demanding periods, CORT will mediate physiological or behavioral changes that result in increased reproductive investment and success (a positive relationship). During two breeding seasons, we experimentally manipulated circulating CORT levels in female tree swallows (Tachycineta bicolor) prior to egg laying, and measured subsequent reproductive effort, breeding success, and maternal survival. When females were recaptured during egg incubation and again during the nestling stage, the CORT levels were similar among individuals in each treatment group, and maternal treatment had no effect on indices of fitness. By considering variation among females, we found support for the CORT-adaptation hypothesis; there was a significant positive relationship between CORT levels during incubation and hatching and fledging success. During the nestling stage CORT levels were unrelated to any measure of investment or success. Within the environmental context of our study, relationships between maternal glucocorticoid levels and indices of fitness vary across reproductive stages.
Highlights
Within and among species individuals vary in the strategies used to maximize fitness, by adjusting the relative effort put into current versus future reproductive events (Williams, 2005; Hansen et al, 2016)
There was no significant difference between the Sham and CORTnest Treatment (CORT) treatments in the percentage of females that retained their implants and subsequently laid eggs [Sham: 45% (20 of 43), CORT: 31% (14 of 44); χ2=0.865, d.f.=1, P=0.352; Table 1; a single CORT and Sham female of unknown age were omitted from the analysis]
We found no relation between CORT levels during incubation (CORTinc) and clutch mass; female tree swallows with higher CORTinc levels had greater hatching success and higher fledging success
Summary
Within and among species individuals vary in the strategies used to maximize fitness, by adjusting the relative effort put into current versus future reproductive events (Williams, 2005; Hansen et al, 2016). There is ample evidence that increased energy expenditure and effort during one breeding bout results in decreased reproductive success, probability of re-nesting, or survival in subsequent bouts (Nager, 2006; Crossin et al, 2013; 2016; Harms et al, 2014; Bleu, Gamelon and Sæther, 2016; Henderson et al, 2017). Glucocorticoids (GCs) have been hypothesized to be a mediator of the trade-off between current and future reproduction In response to an environmental stressor, GC levels increase rapidly, resulting in increased availability of metabolic substrates, and adjustment of behaviors toward immediate survival (Wingfield and Sapolsky, 2003; Romero, 2004) while inhibiting reproductive behavior and physiology (Sapolsky et al, 2000; Dantzer et al, 2014), i.e. the CORT-trade-off hypothesis (Patterson et al, 2014)
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