Abstract
It is widely expected that physiological and behavioral stress responses will be integrated within divergent stress-coping styles (SCS) and that these may represent opposite ends of a continuously varying reactive–proactive axis. If such a model is valid, then stress response traits should be repeatable and physiological and behavioral responses should also change in an integrated manner along a major axis of among-individual variation. While there is some evidence of association between endocrine and behavioral stress response traits, few studies incorporate repeated observations of both. To test this model, we use a multivariate, repeated measures approach in a captive-bred population of Xiphophorus birchmanni. We quantify among-individual variation in behavioral stress response to an open field trial (OFT) with simulated predator attack (SPA) and measure waterborne steroid hormone levels (cortisol, 11-ketotestosterone) before and after exposure. Under the mild stress stimulus (OFT), (multivariate) behavioral variation among individuals was consistent with a strong axis of personality (shy–bold) or coping style (reactive–proactive) variation. However, behavioral responses to a moderate stressor (SPA) were less repeatable, and robust statistical support for repeatable endocrine state over the full sampling period was limited to 11-ketotestosterone. Although post hoc analysis suggested cortisol expression was repeatable over short time periods, qualitative relationships between behavior and glucocorticoid levels were counter to our a priori expectations. Thus, while our results clearly show among-individual differences in behavioral and endocrine traits associated with stress response, the correlation structure between these is not consistent with a simple proactive–reactive axis of integrated stress-coping style. Additionally, the low repeatability of cortisol suggests caution is warranted if single observations (or indeed repeat measures over short sampling periods) of glucocorticoid traits are used in ecological or evolutionary studies focussed at the individual level.
Highlights
Ecology and Evolution published by John Wiley & Sons Ltd
We have previously demonstrated a strong axis of among-individual variation in boldness that is stable over long periods, that is, representative of expected life span (Boulton et al 2014)
Univariate analyses, assuming the test statistic to be asymptotically distributed as a mix of 50:50 v20andv21, were statistically significant at P < 0.05 for all individual traits except time in middle
Summary
When challenged by adverse and uncontrollable environmental stimuli, animals use behavioral and physiological components of the stress response to maintain homeostasis (Selye 1973; Johnson et al 1992; Chrousos 1998) and minimize loss of fitness (Levine and Ursine 1991; Blas et al 2007; Breuner et al 2008; Koolhaas et al 2011).Stress response may vary among individuals within a population (Huntingford 1976; Verbeek et al 1996; Devries 2002), a phenomenon that has led researchers to postulate the existence of “stress-coping styles” (SCS) (Benus et al 1991; Koolhaas et al 1997, 1999; Korte et al 2005). Under the SCS model, it is widely expected that behavior and physiology will be integrated within divergent coping styles typically characterized as being either proactive or a 2015 The Authors. Testing the Stress Coping Style Model reactive (Koolhaas et al 1997). Proactive individuals actively challenge stressors and present behavioral profiles consistent with bold personalities (e.g., Brown et al 2007; Thomson et al 2011), rapidly develop rigid routines and usually have low hypothalamic–pituitary–adrenal (HPA) (or in fishes hypothalamic–pituitary–interrenal (HPI) activity). Often presented as dichotomous, proactive and reactive coping styles may represent opposite ends of a continuously varying axis of SCS (Barreto and Volpato 2011). Using a freshwater fish population, we investigate among-individual (co)variation in behavioral and endocrine stress response traits to test these predictions and evaluate the SCS
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