Using the reactive scope model to understand why stress physiology predicts survival during starvation in Galápagos marine iguanas

Even though the term “stress” is widely used, a precise definition is notoriously difficult. Notwithstanding this difficulty, stress continues to be an important concept in biology because it attempts to describe how animals cope with environmental change under emergency conditions. Without a precise definition, however, it becomes nearly impossible to make testable a priori predictions about how physiological and hormonal systems will respond to emergency conditions and what the ultimate impact on the animal will be. The reactive scope model is a recent attempt to formulate testable predictions. This model provides a physiological basis to explain why corticosterone negative feedback, but not baseline corticosterone concentrations, corticosterone responses to acute stress, or the interrenal capacity to secrete corticosterone, is correlated with survival during famine conditions in Galápagos marine iguanas. Reactive scope thus provides a foundation for interpreting and predicting physiological stress responses.

► Only corticosterone negative feedback predicts marine iguana survival during famine.
► The reactive scope model can be used to interpret why.
► Famine conditions decrease the reactive scope – too narrow a scope leads to death.
► Weaker negative feedback exacerbates decrease in reactive scope.
► Reactive scope model thus explains why weaker feedback resulted in earlier death.