Meeting Abstract

9.11  Sunday, Jan. 4  Interpreting baseline corticosteroid measures: are they useful predictors of fitness? BONIER, F.*; MARTIN, P.R.; MOORE, I.T.; ROBERTSON, R.J.; WINGFIELD, J.C.; Queen’s University; Queen’s University; Virginia Tech; Queen’s University; University of California, Davis bonierf@queensu.ca

Baseline corticosteroid (cort) measures are routinely used to infer an individual’s stress level. The term stress has ambiguous meaning, and the significance of a high or low cort level for an individual animal is difficult to interpret. Nevertheless, researchers finding high cort levels in free-ranging animals generally interpret these levels as negative, reflecting poor condition and, presumably, reduced fitness relative to individuals with lower cort levels. The assumption that baseline cort accurately predicts fitness is questionable, and has rarely been validated. I will discuss two studies that look at natural relationships between cort levels and fitness components in free-ranging birds. Results suggest cause for caution in interpreting these hormone levels. First, in free-ranging white-crowned sparrows, I found sex-specific differences in the relationship between cort and a fitness component (number of genetic offspring fledged in one season) within one population: male birds’ cort levels did not correlate with fitness, whereas females with high cort had lower fitness. Second, in a study of free-ranging female tree swallows, I found changing correlations between fitness components and baseline cort levels across breeding stages in the same individuals. Early in breeding, I found a negative relationship with high cort levels associated with reduced fitness, but by late breeding cort was actually positively correlated with fitness. These two studies, along with others, emphasize the challenges of interpreting cort levels in the field, and point to several means of addressing these challenges.