Are individuals consistent? Endocrine reaction norms under different ecological challenges

Abstract

Quantifying organismal capacity for compensatory mechanisms is essential to forecast responses to environmental change. Despite accumulating evidence for individual variation in physiological plasticity, the causes and consequences of this variation remain unclear. An outstanding question is whether individual reaction norms are consistent across different environmental challenges, i.e. whether an individual that is responsive to one environmental variable will be equally responsive to a different environmental variable. Additionally, are these reaction norms themselves consistent over time, i.e. repeatable? Here, we quantified individual baseline glucocorticoid responses in house sparrows, Passer domesticus, to sequential manipulations of temperature, wind speed and food unpredictability that were repeated in discrete blocks of sampling under both control and stressor-exposed conditions. Individuals significantly decreased their baseline corticosterone levels and increased their mass during treatment exposure. This response was consistent across environmental challenge types. There was high repeatability in the intercept and slope of the baseline corticosterone reaction norm between environmental challenges but broad credible intervals in the repeatability of the reaction norm slope, suggesting that although glucocorticoid levels during baseline conditions are repeatable, among-individual variation in the shape of the glucocorticoid response may be higher than within-individual variation. Within-subject variation in baseline corticosterone levels was mainly explained by within-individual variation in body mass during stressor exposure. Despite the high lability in physiological traits, endocrine plasticity is repeatable across environmental challenges and may be able to evolve as a result of genetic accommodation, in which selection acts on genetic variation of reaction norms.

Keywords: Corticosterone; Food unpredictability; Glucocorticoids; House sparrows; Temperature; Wind.

Fig. 1.

Hypothetical illustration of corticosterone reaction norms (slope) under control and treatment conditions. A consistent responder is an individual that elevates glucocorticoid levels irrespective of environmental challenge type: here, wind (W), heat (H) and food predictability (F). A non-consistent responder is an individual whose reaction norms differ in response to environmental variable type. A consistent non-responder is an individual that does not respond to any of the environmental challenges.

Fig. 2.

Hypothetical illustration of the corticosterone reaction norms under control and treatment conditions, showing within- and among-individual plasticity. A situation of high repeatability would occur if each individual responded in the same way to different environmental challenges, such that among-individual variation is higher than within-individual variation. A situation of low repeatability would occur if each individual responded differently (in the magnitude of the response but not in the direction) to different environmental challenges, such that among-individual variation is lower than within-individual variation. Note that both scenarios would have no significant interaction between environmental challenge type and treatment (control or treatment phase), as the mean slope for each environmental challenge is the same across environments. In other words, these two scenarios apply in the situation in which individuals acted on average as consistent responders. Scenarios can occur in which there is low consistency in the response but high among-individual variation in the slope of the response, resulting in repeatability of the slope (not illustrated).

Fig. 3.

Boxplots of corticosterone levels and body mass across the three challenge types during the control and treatment phase. Box plots show median (horizontal line inside boxes), upper and lower quartiles (box) and 1.5× interquartile range (whiskers) for (A) corticosterone levels and (B) body mass.

Fig. 4.

Individual reaction norms of the baseline corticosterone levels in relation to the three environmental challenges. Each row shows the reaction norm of a single individual (e.g. B1) in the three environmental challenge exposures (food unpredictability, heat and wind exposure).

Fig. 5.

Relationship between log₁₀-transformed corticosterone levels and body mass. The dashed line represents fitted values; the shaded area is the s.e. Note that individuals are represented with multiple points. The line represents the model fit of the mixed model that accounts for the repeated nature of the data.

Fig. 6.

Within-individual relationship of log₁₀-transformed baseline corticosterone levels in relation to body mass. Each panel represents an individual (e.g. B1). Predicted lines represent the model fit, structured with common slopes but different intercepts.