ROMERO, L. Michael*; WIKELSKI, Martin; Tufts University; Princeton University: Daily Differences in the Capacity of Marine Iguanas to Respond to Stress

Recent data indicate that Galapagos marine iguanas (Amblyrhynchus cristatus) exhibit a distinct diel cycle in corticosterone release. Corticosterone concentrations are approximately twice as high in the day than at night. We tested whether lower nighttime concentrations might result from a decreased capacity to secrete corticosterone. Iguanas were captured during both the day and night and subjected to 60 min of handling and restraint. Serial blood samples collected over this 60 min period indicated a significant stress-induced rise in corticosterone concentrations during both day and night, but the response was higher during the day. We then tested the adrenocortical cells� sensitivity to an ACTH signal by injecting exogenous ACTH. Exogenous ACTH further augmented the stress-induced response during both the day and night, but stimulated more corticosterone release during the day. These data suggest that the lower endogenous corticosterone release at night is at least partially a consequence of a lower capacity to secrete corticosterone.