The physiological stress response is affected by both developmental and evolutionary exposure to stressors
Exposure to stress during development can have important lasting effects on physiology and behavior. In their native range, Eastern fence lizards (Sceloporus undulatus) are frequently attacked by invasive fire ants (Solenopsis invicta), and these encounters are stressful for the lizards. Because prolonged exposure to stress-relevant hormones (e.g. corticosterone, CORT) can have fitness costs, we expect that this stress response might be down-regulated either within a lifetime or across generations. Alternatively, since the stress response is critical for initiating escape from fire ant attack, this response may be heightened over time. We determined whether repeated exposure to predatory fire ants during a lizard’s development induced lasting changes to the physiological stress response. We collected gravid lizards from fire ant invaded and uninvaded sites. Resulting hatchings were exposed weekly to sub-lethal attack by fire ants, given a topical treatment of CORT (to mimic the stress of attack, without the physical stimuli), or assigned to a control group, until lizards reached maturity (~40 weeks). Several months later, we recorded baseline (resting). acute stress-induced, and ACTH-induced concentrations of plasma CORT.
While exposure to stress (CORT or fire ants) during development did not affect baseline physiological stress levels, lizards from high stress (fire ant invaded) sites responded more robustly to acute stress that did those from uninvaded sites. This was associated with elevated responses to ACTH (a pre-cursor to CORT), suggesting that, rather than perceiving fire ants as a greater threat, lizards from fire ant invaded sites are more responsive to stress. These results suggest that selection may favor elevated stress responses in high-stress environments.