Corticosterone and nocturnal torpor in the rufous hummingbird (Selasphorusrufus)

Three experiments were designed to investigate whether corticosterone (CORT ), known to have a role in restoration of energy homeostasis, regulates noc turnal torpor, an energy conservation state used by some small mammals and birds to offset environmental challenges to energy balance. In two experime nts, one during autumn migration and one during early spring molt, captive rufous hummingbirds (Selasphorus rufus) were fed control and dilute (85% st rength) nectar on alternate days. In migratory birds, torpor occurred more frequently over all, and nectar dilution resulted in increased torpor durat ion and increased concentration of CORT in evening but not midday cloacal f luid (CF) samples. In molting birds, torpor occurred infrequently on both c ontrol and food dilution days, but, although there was a significant increa se in evening CF CORT on food dilution days, torpor duration did not increa se significantly in response and there was no correlation between torpor du ration and CF CORT at either time of day. Daily CF CORT patterns showed an increase from midday to evening during migration, but the reverse pattern d uring the molt. In a third experiment, CORT administered in the nectar elev ated the use of torpor and depressed food intake. The results of these thre e experiments support the hypothesis that CORT is involved in the regulatio n of torpor, but suggest that some feature of the CORT signal other than co ncentration per se may be required to fully explain seasonal changes in the relations among energy challenge, CORT, and nocturnal torpor in hummingbir ds. (C) 2000 Academic Press.