title: Effect of tidal cycle and food intake on the baseline plasma corticosterone rhythm in intertidally foraging marine iguanas.
abstract: In most species, plasma levels of baseline glucocorticoids such as corticosterone (B) have a circadian rhythm. This rhythm can be entrained by both photoperiod and food intake and is related to aspects of energy intake and metabolism. Marine iguanas (Amblyrhynchus cristatus) offer a unique opportunity to better understand the relative importance of the light:dark cycle versus food intake in influencing the rhythm in baseline B in a natural system. Compared to other species, food intake is not as strictly determined by the phase of the light:dark cycle. Animals feed in the intertidal zone so feeding activity is heavily influenced by the tidal cycle. We measured baseline plasma B levels in free-living iguanas over several 24-h periods that varied in the timing of low tide/foraging activity. We found that baseline B levels were higher during the day relative to night. However, when low tide occurred during the day, baseline B levels dropped coincident with the timing of low tide. Whether the baseline B rhythm (including the drop during foraging) is an endogenous rhythm with a circatidal component, or is simply a result of feeding and associated physiological changes needs to be tested. Together, these data suggest that the baseline B rhythm in marine iguanas is influenced by the tidal cycle/food intake as well as the light:dark cycle.
authors.affiliation: Department of Biology, Arizona State University, Tempe, AZ 85281, USA. swoodley@bu.edu
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Woodley
authors.forename: Danika L
authors.initials: DL
authors.lastname: Painter
authors.forename: Michael C
authors.initials: MC
authors.lastname: Moore
authors.forename: Martin
authors.initials: M
authors.lastname: Wikelski
authors.forename: L Michael
authors.initials: LM
authors.lastname: Romero
mesh_terms: D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D003345:Corticosterone / Q000097:blood*; D004435:Eating / Q000502:physiology*; D004484:Ecuador; D007076:Iguanas / Q000502:physiology*; D010140:Pacific Ocean; D017440:Photoperiod
chemical_list: D003345:Corticosterone