WOODLEY, S.*; PAINTER, D.; MOORE, M.; WIKELSKI, M.: Daily rhythm of basal corticosterone levels is not synchronized with feeding activity in Marine Iguanas

Basal plasma glucocorticoids (e.g., corticosterone (B)) levels follow a circadian rhythm. The rhythm can be entrained by photoperiod and food availability and functions in aspects of energy metabolism. Marine iguanas (Amblyrhynchus cristatus) provide a unique opportunity to better understand the rhythm of basal B levels because, unlike many vertebrates, feeding activity and photoperiod are uncoupled. Animals feed in the intertidal zone so feeding activity is determined by the tidal cycle. Since animals live at the equator, photoperiod remains relatively constant. If B modulates feeding activity (or vice versa), then B should cycle with feeding activity and not photoperiod. Basal B levels in free-living female Marine iguanas were measured every 6 hours over 24 hours. We sampled again 1 week later when the tide cycle was reversed, and hence, the feeding cycle was shifted. The pattern of B levels between the two weeks was identical, with a midday peak and a midnight low. Interestingly, the overall level of B was significantly higher the week when animals fed at midday, relative to the week when animals fed either early in the morning or late in the afternoon. To conclude, the rhythm of basal B is not entrained to feeding activity, but is instead entrained to photoperiod or some process in phase with photoperiod. Because B levels peak at midday, we suggest that B functions in energy metabolism associated with basking. Basking is critical to digestion and energy metabolism in ectotherms, and is most efficient at midday when the sun is hottest.