House sparrows prioritize skin repair over constitutive innate immunity during long-term chronic stress.

The reactive scope model was created to address two major unanswered questions in stress physiology: how and when does the adaptive acute stress response turn into harmful chronic stress? Previous studies suggest that immunoenhancement should occur in reactive homeostasis (acute stress) and immunosuppression should occur in homeostatic overload (chronic stress). We used this dichotomy of immune function to further elucidate the transition from acute to chronic stress by treating house sparrows (Passer domesticus) with different intensities of chronic stress and then monitoring their immune function. By varying the number of stressors given per day and the length of chronic stress bouts over a period of 6 months, we produced four treatment groups: high, medium, and low stress, and captivity-only. We tracked immunity through the bacterial killing assay and monitored healing of a 4 mm skin biopsy punch. We hypothesized that higher-stress birds would repair their skin more slowly and have lower bacterial killing capacity. The opposite was true-high-stress birds initially repaired their skin fastest. Additionally, all birds dramatically reduced bacterial killing capacity after the biopsy and increased food-derived uric acid, suggesting increased energy acquisition and a shift in immune resources to a more immediate concern (healing). Once healing finished, only the high-stress birds were unable to recover circulating immune function, suggesting that the combination of high stress and an immune challenge pushed these birds into homeostatic overload. Prioritizing healing over other immunological processes might be the best defense for a bird in its natural habitat.