Conservation implications of a lack of relationship between baseline glucocorticoids and fitness in a wild passerine

Abstract

The application of physiological measures to conservation monitoring has been gaining momentum and, while a suite of physiological traits are available to ascertain disturbance and condition in wildlife populations, glucocorticoids (i.e., GCs; cortisol and corticosterone) are the most heavily employed. The interpretation of GC levels as sensitive indicators of population change necessitates that GCs and metrics of population persistence are linked. However, the relationship between GCs and fitness may be highly context-dependent, changing direction, or significance, depending on the GC measure, fitness metric, life history stage, or other intrinsic and extrinsic contexts considered. We examined the relationship between baseline plasma corticosterone (CORT) levels measured at two periods of the breeding season and three metrics of fitness (offspring quality, reproductive output, and adult survival) in female Tree Swallows (Tachycineta bicolor). Specifically, we investigated whether (1) a relationship between baseline CORT metrics and fitness exists in our population, (2) whether the inclusion of energetic contexts, such as food availability, reproductive investment, or body mass, could alter or improve the strength of the relationship between CORT and fitness, and (3) whether energetic contexts could better predict fitness compared to CORT metrics. Importantly, we investigated these relationships in both natural conditions and under an experimental manipulation of foraging profitability (feather clipping) to determine the influence of an environmental constraint on GC-fitness relationships. We found a lack of relationship between baseline CORT and both short- and long-term metrics of fitness in control and clipped birds. In contrast, loss in body mass over reproduction positively predicted reproductive output (number of chicks leaving the nest) in control birds; however, the relationship was characterized by a low R (5%), limiting the predictive capacity, and therefore the application potential, of such a measure in a conservation setting. Our results stress the importance of ground-truthing GC-fitness relationships and indicate that baseline GCs will likely not be easily employed as conservation biomarkers across some species and life history stages. Given the accumulating evidence of temporally dynamic, inconsistent, and context-dependent GC-fitness relationships, placing effort towards directly measuring fitness traits, rather than plasma GC levels, will likely be more worthwhile for many conservation endeavours.

Glucocorticoid–environment relationships align with responses to environmental change in two co‐occurring congeners

“…Glucocorticoid hormones (GCs) are particularly appropriate indicators of short-term physiological responses to variable environmental conditions due to their critical role in allostasis (Sapolsky et al 2000, Wikelski and Cooke 2006, Busch and Hayward 2009. Chronically elevated GCs are usually considered maladaptive (Bonier et al 2009), although support for this hypothesis is mixed (e.g., Thierry et al 2013, Burtka et al 2016, Hansen et al 2016, Madliger and Love 2016. In general, baseline GCs are thought to reflect the social, environmental, and energetic challenges to homeostasis faced by individuals (Sapolsky et al 2000).…”

Effects of parental and nest-site characteristics on nestling quality in the Tree Swallow (Tachycineta bicolor)

“…In our study population, nestlings with lower body mass have lower first-year survival (Harriman 2014). Although the relationship between CORT and individual quality is no doubt context dependent and need not always be negative (Bonier et al 2009;Rivers et al 2012;Madliger and Love 2016), our predictions are based on previous work with the same study population showing that higher levels of CORT from feathers were associated with poorer nestling quality (reduced nestling growth and fledging success; Fairhurst et al 2013), as well as being hatched and raised in plywood boxes (Fairhurst et al 2012a). Here, we also measured levels of CORT in nestling feathers (CORT f ), which provides an integrated measure of CORT during feather growth (Bortolotti et al 2008;Fairhurst et al 2013;Hõrak et al 2013;Jenni-Eiermann et al 2015; for a review see Romero and Fairhurst 2016) that reflects energy management during development (Johns et al 2018) and, thus, is a convenient way of assessing nestling quality during early life in Tree Swallows.…”

Maternal glucocorticoid levels during incubation predict breeding success, but not reproductive investment, in a free-ranging bird

“…In fact, there may exist no consistent relationship between CORT and fitness, due to a variety of factors masking directionality ( Madliger and Love, 2016a ). For example, a lack of relationship could be due to different functions of CORT; when resources are plentiful, elevated CORT could stimulate energy mobilization and parental provisioning; however, CORT could also be elevated in parents experiencing stressors ( Vitousek et al, 2014 ).…”