| Summary: | Biological rhythms provide a mechanism for scheduling activity in predictably cyclic environments. During summer above the polar circle, the primary timing cue, the geophysical light-dark cycle, is highly attenuated or absent, yet some animals will exhibit diel rhythms of behavior and physiology under these conditions. How and why diel rhythms persist under this ‘polar day’ remains to be fully explored. This work builds on previous investigations of the proximate and ultimate mechanisms for maintaining biological rhythms during polar day by using an Arctic breeding seabird, thick-billed murre (Uria lomvia), that has sex-specific timing of activity above and below the polar circle during summer. Thick-billed murres’ sex-specific behavior when breeding was rhythmic during polar day with a period length of 24 h, and each sex’s colony attendance was antiphase to the other, resulting in an almost complete segregation of the sexes across the diel cycle. Corticosterone was not associated with these rhythms: it was invariant across the diel cycle and not associated with activity. Melatonin did not rise at the beginning of each sex’s quiescent phase, but did have variation in its diel profile suggesting a rise when the vertical, ENE-facing cliff where breeding occurs became shaded at midday, indicating that their melatonin response may be flexible. The sexes of thick-billed murres foraged at different times of day and to different depths across the diel cycle, which, in the context of the West Greenlandic food-web, suggested that the sexes forage on different prey when rearing chicks. Interestingly, the indirect measurements obtained from bird-borne data-loggers indicated the possible presence of diel vertical migration of their prey during polar day. Together this body of work adds to our understanding of biological rhythms in the polar environment.
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