Baseline Corticosterone Is not An Indicator of Population Health during Non-El Niño Conditions in Galapagos Marine Iguanas

Abstract

Glucocorticoids are steroid hormones that are often used to evaluate vertebrate population health. However, glucocorticoid levels can vary widely among individuals and over time, making it challenging to unravel the factors influencing their expression. We examined the relationship between corticosterone – the primary glucocorticoid in reptiles – and various biological and ecological factors in Galapagos marine iguanas (Amblyrhynchus cristatus). Marine iguanas are ideal for studying drivers of physiological stress since they inhabit many isolated islands with varying ecological conditions. We found that baseline corticosterone levels differed among marine iguana populations throughout the Galapagos archipelago, and that females had significantly higher corticosterone values than males. However, we did not find strong relationships between corticosterone and other assessed factors, including body condition, food availability, and blood parasite infection. Our results differ from previous studies, which found that corticosterone is strongly linked to body condition and a good predictor of individual survival probability in marine iguanas. This discrepancy is likely because prior studies were conducted during periods of acute environmental stress, such as an El Niño or oil spill, whereas our work took place during typical non-El Niño conditions when food was abundant. Thus, baseline corticosterone in marine iguanas appears to be a good surrogate for individual health and survival during times of extreme stress but should be used with caution for routine monitoring. Our results add to a growing understanding of the applicability of glucocorticoids for assessing population health.

Keywords: Stress, Hormones, Parasites, Physiology, Reptiles