Invader danger: Lizards faced with novel predators exhibit an altered behavioral response to stress Whitney P. Trompeter, Tracy Langkilde ⁎ Department of Biology, Pennsylvania State University, PA 16802, USA abstract article info Article history: Received 19 October 2010 Revised 8 April 2011 Accepted 14 April 2011 Available online 29 April 2011 Keywords: Behavior Corticosterone Invasive Reptile Animals respond to stressors by producing glucocorticoid stress hormones, such as corticosterone (CORT). CORT acts too slowly to trigger immediate behavioral responses to a threat, but can change longer-term behavior, facilitating an individual's survival to subsequent threats. To be adaptive, the nature of an animal's behavior following elevated CORT levels should be matched to the predominant threats that they face. Seeking refuge following a stressful encounter could be beneficial if the predominant predator is a visual hunter, but may prove detrimental when the predominant predator is able to enter these refuge sites. As a result, an individual's behavior when their CORT levels are high may differ among populations of a single species. Invasive species impose novel pressures on native populations, which may select for a shift in their behavior when CORT levels are high. We tested whether the presence of predatory invasive fire ants (Solenopsis invicta) at a site affects the behavioral response of native eastern fence lizards (Sceloporus undulatus) to elevated CORT levels. Lizards from an uninvaded site were more likely to hide when their CORT levels were experimentally elevated; a response that likely provides a survival advantage for lizards faced with native predatory threats (e.g. birds and snakes). Lizards from a fire ant invaded site showed the opposite response; spending more time moving and up on the basking log when their CORT levels were elevated. Use of the basking log likely reflects a refuge-seeking behavior, rather than thermoregulatory activity, as selected body temperatures were not affected by CORT. Fleeing off the ground may prove more effective than hiding for lizards that regularly encounter small, terrestrially-foraging fire ant predators. This study suggests that invasive species may alter the relationship between the physiological and behavioral stress response of native species. © 2011 Elsevier Inc. All rights reserved. Encounters with threatening stimuli, such as aggressive conspecifics and predators, trigger the release of glucocorticoid stress hormones (Broom and Johnson, 1993; Sapolsky et al., 2000). Glucocorticoids act too slowly to trigger immediate behavioral responses to a threat, but elevated levels allow animals to adaptively respond to stressful situations by mobilizing energy and changing their behavior to survive subsequent threats (Sapolsky et al., 2000; Wingfield et al., 1998). Corticosterone (CORT) is the principal glucocorticoid in reptiles, amphibians, birds, and many rodents (Romero, 2004). Elevated levels of plasma CORT can affect behavior: initiating hiding and other defensive behaviors (Kalytnchuk et al., 2004; Thaker et al., 2009), suppressing or inducing locomotor activity (Belliure and Clobert, 2004; Ricciardella et al., 2010), and reducing or increasing thermoregulatory behavior (Belliure and Clobert, 2004; Preest and Cree, 2008). An individual's plasma CORT levels and behavioral response to elevated levels of CORT can be affected by factors including age, body condition and reproductive status (Wingfield et al., 1992; Woodley and Moore, 2002). Therefore, CORT modulation can vary within populations and even between populations due to environmental conditions such as rainfall, temperature, and food availability (Dufty et al., 2002; Romero, 2002), as well as perturbations including habitat loss (Homan et al., 2003; Suorsa et al., 2004), pollution (Norris et al., 1999; Wikelski, 2001), and the introduction of novel predators (Berger et al., 2007; Vitousek et al., 2010). Regardless of the levels of CORT production, an individual's behavior when CORT levels are elevated should vary depending upon the characteristics of prevalent threats in their environment (Wingfield et al., 1997), and thus may vary among populations (e.g., Robert et al., 2009). For example, hiding, may be adaptive when the prevalent threat in an area is a large and/or visual predator (Edmunds, 1974; Robinson, 1969), but maladaptive when the prevalent threat hunts by olfactory cues or is small enough to easily enter refuge sites, in which case fleeing may be a more appropriate response. Anthropogenic activities are increasingly disturbing natural populations, and impose novel selective pressures and stressors (Wright and Zamudio, 2002; Tull and Bussard, 2007; Phillips et al., 2006; Strauss et al., 2006; Martin et al., 2010). Therefore, we may expect populations exposed to these novel threats to alter their Hormones and Behavior 60 (2011) 152–158 ⁎ Corresponding author at: 208 Mueller Lab, University Park, PA 16802, USA. Fax: + 1 814 865 9131. E-mail address: tll30@psu.edu (T. Langkilde). 0018-506X/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2011.04.001 Contents lists available at ScienceDirect Hormones and Behavior journal homepage: www.elsevier.com/locate/yhbeh