Book Description

Understanding the consequences of stress is integral to predicting how organisms will respond to global environmental change. The stress response is generally adaptive, promoting physiological changes that allow an organism to deal with and recover from a threat. However, the stress response can lead to negative fitness outcomes because it diverts energy away from growth, reproduction, and immune function. My dissertation broadly investigates how traits of a stressor and of an organism affect the physiological outcomes of stress. To do so, I utilized populations of eastern fence lizards (Sceloporus undulatus) that co-evolved with different levels of stress associated with presence of invasive predatory fire ants (Solenopsis invicta). Stress is typically characterized by duration, with short-duration ("acute") stress assumed to be neutral or beneficial and long-duration ("chronic") stress assumed to have negative outcomes. However, the outcomes of stressors that result from repeated short-duration stress (e.g. frequent predator attacks) are not well understood. I first investigated the immune outcomes of these "repeated acute" stressors. Lizards from both high-and low-stress populations enhanced immune function following exposure to repeated acute elevations of exogenous corticosterone (CORT). This demonstrates that repeated acute stressors produce immune outcomes more typical of those expected from short-duration stress (i.e. immune enhancement). I then investigated the role of stressor duration, frequency, and intensity in determining the immune outcomes of stress. My results reveal that stressor intensity is a major driver of immune outcomes of stress in this system and suggest that the current duration-centric terminology is not adequate. Finally, I explored how exposure to stress within a lifetime and across generations affects stress physiology and immune function by raising offspring of lizards from high- and low-stress sites under high and low-stress conditions in the lab and measuring physiological outcomes in adulthood. Early life stress did not affect adult stress physiology, but offspring of lizards from high-stress sites had more robust stress responses than those from uninvaded sites. This suggests that, in this system, stress experienced by an individual's ancestors may be more important in shaping adult stress physiology than stress that an individual faces within its lifetime. By contrast, within- and across-generational factors interacted to affect adult immune function; CORT-elevation during early life suppressed adult immune function in lizards from low-stress sites but enhanced immune function in lizards from high-stress sites. Together, these results further our understanding of how traits of a stressor and those of an organism influence physiological outcomes of stress. This insight allows us to better predict how organisms will be affected by stress, including that imposed as a result of global change, and to better allocate resources to manage and mitigate these fitness-relevant effects.