Meeting Abstract
Organisms continuously face environmental fluctuations, and allocation of metabolic investment to meet changing energetic demands is of fundamental importance to survival and reproductive success. Glucocorticoid (GC) hormones (e.g. corticosterone –CORT-) play an important role in energy balance and acquisition on the face of environmental challenges, by mediating metabolic processes involved in energy metabolism. Although fluctuations in GCs and metabolic rate are expected to covary, surprisingly few empirical studies have demonstrated relationships between GC concentrations and metabolic rate in wild, free ranging animals. We measured CORT (baseline and stress-induced) and metabolic traits (resting metabolic rate –RMR-, cold-induced VO2max, and aerobic scope) in female tree swallows (Tachycineta bicolor) during chick-rearing, and tested for their associations with several variables of reproductive performance. We found that only metabolic traits were associated with reproductive success; females with higher reproductive output showed higher resting metabolic rate and lower VO2max. Moreover, we found a positive relationship between resting metabolic rate and baseline CORT, but other associations between metabolic rates and CORT levels were not significant. This suggests that while baseline CORT may be a good indicator of an individual´s baseline metabolic investment, stress-induced CORT may be more stimulus-specific, and does not reflect aerobic scope or the upper limits of aerobic performance. Overall, our results suggest that metabolic traits may be better predictors of reproductive investment in tree swallows than CORT parameters. They further suggest the available energy budget may be more constrained in females investing more heavily in a current reproductive event due to elevated baseline metabolic costs.
