RESEARCH ARTICLE Does selection for behavioral and physiological performance traits alter glucocorticoid responsiveness in bank voles? Malgorzata M. Lipowska 1, *, Edyta T. Sadowska 1 , Ulf Bauchinger 1,2 , Wolfgang Goymann 3 , Barbara Bober-Sowa 1 and Pawel Koteja 1 ABSTRACT One of the key elements of an animal’s Darwinian fitness is its ability to adequately respond to and cope with challenging situations. Glucocorticoid hormones, such as corticosterone, affect an organism’s ability to overcome such challenges. We hypothesized that changes in the glucocorticoid response curve contribute to the evolution of increased performance during challenging conditions, and tested it on bank voles (Myodes glareolus) from a multidirectional artificial selection experiment, which involves lines selected for high aerobic exercise metabolism achieved during swimming (A – Aerobic), predatory behavior towards a cricket (P – Predatory) and ability to maintain body mass on a low-quality herbivorous diet (H – Herbivorous), as well as unselected control lines (C – Control). We elicited a glucocorticoid response either by restraining the animal or by maximum pharmacological stimulation, and measured plasma corticosterone levels at baseline, during the response and during the recovery phase. Response-level corticosterone was higher in females, and recovery from maximal level was faster than that of males. Selection did not affect baseline or stress-induced corticosterone levels, but it decreased the maximum corticosterone level in Aerobic and Predatory lines, reducing the difference between stress-induced and maximum levels. Recovery from restraint-induced corticosterone level tended to be slower in the Herbivorous than in the other lines, an effect that was stronger in females than in males. In conclusion, successful selection for increased performance in challenging conditions was not associated with changes in absolute values of the glucocorticoid response to stress, but can affect other characteristics of the glucocorticoid response curve. KEY WORDS: Evolution, HPA axis, Corticosterone, Artificial selection, Myodes glareolus INTRODUCTION In nature, animals are frequently exposed to challenges such as encounters with predators or competitors, harsh weather conditions or food deficiency. Successfully coping with such challenges involves a spectrum of psychological and physiological processes activated or modulated as a part of a stress response to the situation (Wingfield and Ramenofsky, 1999). An adequate stress response can help an animal to overcome the challenge, and, consequently, determine its chances of survival and reproduction. Therefore, evolving a better ability to cope with a particular challenge may involve not only physiological or biomechanical performance traits but also modification of the stress response mechanisms. Here, we tested this hypothesis within the framework of a unique experimental evolution model system: lines of a common rodent, the bank vole (Myodes glareolus), from a multidirectional selection experiment (Chrzą  scik et al., 2014; Sadowska et al., 2015, 2008). In vertebrates, the hypothalamic–pituitary–adrenal (HPA) axis is one of the main mediators of the stress response and is an important regulator of homeostasis (Koolhaas et al., 2011; McEwen and Wingfield, 2003). By releasing glucocorticoids, the HPA axis affects a wide spectrum of an organism’s functions, such as metabolism, energy mobilization, immune system, behavior and gene expression (Bonier et al., 2011; Coppens et al., 2010; Phuc Le et al., 2005; Schmid et al., 2013; Wingfield and Ramenofsky, 1999), and hence helps the organism to cope with the stressor (Bonier et al., 2009; Koolhaas et al., 1997; Patterson et al., 2014; Sapolsky et al., 2000; Wingfield et al., 1998). Under metabolically demanding conditions, glucocorticoids stimulate energy substrate mobilization (Jimeno et al., 2017; McEwen and Wingfield, 2003). This is reflected, for instance, by increased glucocorticoid levels observed during breeding (Kenagy and Place, 2000; Romero, 2002), at low ambient temperatures (de Bruijn and Romero, 2018) or following intense physical activity (Coleman et al., 1998; Duclos and Tabarin, 2016; Hill et al., 2008). However, a strong glucocorticoid response to metabolically demanding situations may hinder rather than promote an animal’s performance (Breuner et al., 1998; Lipowska et al., 2019; Munck and Náray-Fejes-Tóth, 1992; Wingfield and Ramenofsky, 1999). Moreover, prolonged elevation of glucocorticoids can compromise an organism’s health (Cohen et al., 2007; Sapolsky et al., 2000). Thus, the glucocorticoid response should be finely adjusted to the duration and intensity of the challenge. Under natural conditions, a multitude of factors affects HPA axis activity, making it nearly impossible to single out changes occurring in response to a particular selection factor. However, experimental evolution offers a powerful tool to study responses to selection for well-defined traits controlling for random processes (Henderson, 1997; Swallow et al., 2009). The activity of the HPA axis is known to be heritable (Almasi et al., 2010; Béziers et al., 2019; Odeh et al., 2003a,b). Divergent selection in Japanese quail produced lines of quail with high and low corticosterone responses to brief restraint (Cockrem et al., 2010; Satterlee and Johnson, 1988). Quails from the two selection lines also differed in fearfulness (Jones et al., 1992), which suggests an involvement of glucocorticoids in mediating fear behavior. A similar selection experiment in zebra finches generated lines with a strong corticosterone response, but selection for a weak response was not successful (Evans et al., 2006; Hodgson et al., 2007). Thus, the consequences of selection pressure on glucocorticoid levels are mixed, even if selection acts directly on this trait. Received 6 December 2019; Accepted 16 June 2020 1 Institute of Environmental Sciences, Jagiellonian University, 30-387 Krakó w, Poland. 2 Nencki Institute of Experimental Biology PAS, 02-093 Warszawa, Poland. 3 Department of Behavioral Neurobiology, Max Planck Institute for Ornithology, 82319 Seewiesen, Germany. *Author for correspondence (malgorzata.lipowska@doctoral.uj.edu.pl) M.M.L., 0000-0003-3550-4105; E.T.S., 0000-0003-1240-4814; U.B., 0000-0002- 6422-3815; W.G., 0000-0002-7553-5910; P.K., 0000-0003-0077-4957 1 © 2020. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2020) 223, jeb219865. doi:10.1242/jeb.219865 Journal of Experimental Biology