Unifying adaptive stress and adaptive habitat selection hypotheses through movement ecology

Habitat selection is adaptive, but habitats where animals exhibit elevated glucocorticoid production are often associated with lower fitness. The perception of fitness costs persists in part because glucocorticoid samples rarely represent the internal state of individuals at the time of sampling, making it difficult to demonstrate a causal link between glucocorticoid production and habitat selection at fine temporal and spatial scales. We show how to test this fine-scale causal link by integrating movement ecology with stress physiology. We first synthesize physiological evidence that effect of glucocorticoid production on movement aligns with habitat selection theory. We distinguish whether habitat selection exposes individuals to stressors that elevate their glucocorticoid levels, or whether elevated glucocorticoids stimulate selection for better habitat. We then test the directionality of these two pathways using movement-based habitat selection analysis at the individual level. We capitalize on a combination of individual movement data and the predictable time required for glucocorticoids to peak in samples like hair, feathers, and feces from individuals. Our proof-of-concept shows how to distinguish when glucocorticoids stimulate adaptive habitat selection. Finally, we suggest a potential evolutionary role for glucocorticoids in habitat specialization, offering a mechanism by which populations might adapt to their environments over generations.