Figure 1.

Body temperature of a representative free-living male arctic ground squirrel in northern Alaska, USA over one year (upper panel). Sections of body temperature patterns (lower panels) shown in actogram form (double-plotted positive deviations from 24 h mean, time of day on x-axis, sequential days on y-axis, marked as a and b on upper panel). (a) A 10-day section from March, when the animal remains at high body temperatures within the continuously dark burrow for 2–3 weeks while showing no circadian rhythms. (b) A 10-day section from April, after the animal begins above-ground activity and displays robust, diurnal rhythms in body temperature. Details on methodology are provided in [20]. (Online version in colour.)