Collaborative Research: Persistence, entrainment, and function of circadian rhythms in arctic ground squirrels
The Earth's light-dark (LD) cycle is the strongest cue for entraining circadian rhythms and is considered the primary driver for the emergence and evolution of endogenous clocks. In polar regions, however, photoperiod exhibits extreme annual variation culminating with the sun remaining above or below the horizon for extended periods. Without a defined LD cycle, some arctic residents lose daily organization of behavior and physiology, and it has been hypothesized that the molecular clockwork that drives circadian rhythms may be weak or absent in arctic vertebrates. This study tests the hypothesis that persistence of circadian organization in some arctic mammals is adaptive because it allows organisms to minimize thermoregulatory costs. Specifically, the investigators will examine persistence, entrainment, and function of circadian rhythms in the arctic ground squirrel (AGS) during the continuous daylight of the active season and the continuous dark of hibernation. Four research objectives span from molecular and neurobiological mechanisms to physiological, behavioral and ecological adaptation: (1) determine when circadian rhythms are exhibited during the annual cycle of free-living AGS and establish if the onset of rhythmicity in spring coincides with first exposure to light, (2) determine if circadian rhythms persist in the master clock of AGS during hibernation by measuring patterns of clock gene expression in the suprachiasmatic nuclei (SCN) of the brain, (3) determine if AGS entrain circadian rhythms to daily changes in light quality or intensity, and (4) investigate patterns of re-entrainment and the function of rhythms in the arctic summer by experimentally phase-shifting free-living ground squirrels and measuring metabolic costs of nocturnal activity. Whether circadian rhythms persist during hibernation is contentious. Transcription and translation are thought to be globally suppressed during deep torpor and the only study to examine clock-gene expression within the SCN during multi-day torpor found no evidence of oscillations. However, circadian body temperature rhythms have been observed in some studies of captive ground squirrels during torpor and timing of arousals has been hypothesized to be controlled by a circadian clock. Persistence of oscillations in clock-gene expression in the SCN would support existence of tissue-specific mechanisms for translational control of a subset of genes relevant to survival during hibernation. In contrast, expression of daily rhythms may not be compatible with hibernation; SCN function and or output may be inhibited to prolong torpor. This study integrates research and learning by (1) training post-docs and students from three universities in ecophysiological studies in the laboratory and field, (2) inclusion of Alaska Natives in research through participation in the Alaska Native Science and Engineering Program, and (3) increasing K-12 students' engagement in research by participating in Teacher Research Experience programs. Results of research will be disseminated locally in classroom presentations and nationally through the University of Alaska media relations team.