Meeting Abstract
Ecologists frequently use baseline circulating corticosterone (CORT) concentrations as a physiological indicator of the state of wild animals. This reasoning is dependent upon the prevalent, yet unresolved, assumption of the Cort-Fitness Hypothesis that high baseline CORT concentrations signify individuals or populations with low fitness (reproduction and survival). In this study we employed a meta-analytical approach to evaluate the support for this hypothesis in birds, while also attempting to unravel the causes of discrepancies among studies. Our analysis of 30 studies across 8 taxonomic orders revealed a significant, yet weak, negative relationship between circulating CORT levels and fitness. This relationship was influenced by the species’ body mass, with heavier birds exhibiting a more negative relationship between CORT and fitness. However, when analyzing only the order Passeriformes, in which there is the greatest diversity of species studied, the opposite pattern was observed. No other species attribute (sex, age, breeding vs. non-breeding, latitude, baseline CORT concentration) or characteristic of the study design (fitness metric, scope of comparison, correlative vs. experimental approach) related to the pattern. This quantitative assessment moves the field forward by validating the negative relationship between CORT and fitness yet challenging the continued use of this weak bioindicator, although the sources of variation among studies remain largely enigmatic. We propose that an increase in the monitoring of CORT levels across the lifespan is needed to capture dynamic patterns within and between populations before this measure can be relied upon to indicate individual or population fitness.
