Habituation and individual variation in the endocrine stress response in the Trinidadian guppy (Poecilia reticulata)

T.M Houslay,R.L Earley,A.J Young,A.J Wilson

doi:10.1016/j.ygcen.2018.10.013

T.M Houslay, R.L Earley + Show 2 more

Open Access

https://doi.org/10.1016/j.ygcen.2018.10.013

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Abstract

The vertebrate stress response enables individuals to react to and cope with environmental challenges. A crucial aspect of the stress response is the elevation of circulating glucocorticoids. However, continued activation of the stress response under repeated exposure to stressors can be damaging to fitness. Under certain circumstances it may therefore be adaptive to habituate to repeated exposures to a particular stressor by reducing the magnitude of any associated release of glucocorticoids. Here, we investigate whether Trinidadian guppies (Poecilia reticulata) habituate to repeated exposure to a mild stressor, using a waterborne hormone sampling approach that has previously been shown to elicit a stress response in small fish. We also test for individual variation in the extent of habituation to this stressor. Concentrating on freely circulating cortisol, we found that the first exposure to the assay induced high cortisol release rates but that guppies tended to habituate quickly to subsequent exposures. There were consistent differences among individuals in their average cortisol release rate (after accounting for effects of variables such as body size) over repeated exposures. Our analyses did not find evidence of individual differences in habituation rate, although limitations in statistical power could account for this finding. We repeated the analysis for free 11-ketotestosterone, which can also respond to stressors, but found no obvious habituation pattern and no among-individual variation. We also present data on conjugated forms of both hormones, which were repeatable but did not show the expected time-lagged habituation effect. We discuss consistent individual differences around the general pattern of habituation in the flexible stress response, and highlight the potential for individual variation in habituation to facilitate selection against the deleterious effects of chronic stress.

Highlights

Stress responses involve a complex suite of behavioural, neuroendocrine, and physiological processes that act to maintain organismal health and homeostasis in the face of unpredictable environmental challenges (Selye 1973; Korte et al 2005; Romero, Dickens & Cyr 2009)
Our results show a striking pattern of change in the mean stress-induced free cortisol 406 concentrations expressed by guppies across repeated stress exposures
While we find some limited support for our final prediction that individuals would differ in their rates of habituation with regard to stress-induced free cortisol release, statistical evidence is equivocal

Summary

Introduction

Stress responses involve a complex suite of behavioural, neuroendocrine, and physiological processes that act to maintain organismal health and homeostasis in the face of unpredictable environmental challenges (Selye 1973; Korte et al 2005; Romero, Dickens & Cyr 2009). While disagreements over terminology persist (McEwen and Wingfield, 2010; Romero et al, 2009), stress response mechanisms are broadly seen as underlying a process of “achieving stability through change” (McEwen and Wingfield, 2003) and are vital for dealing with acute environmental challenges. These same mechanisms can be damaging to fitness when organisms are exposed to stressors repeatedly or for prolonged periods (Boonstra, 2013; Dallman et al, 1992; Huether, 1996).

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