Twenty-four-hour feeding and lying patterns of wild reindeer Rangifer tarandus tarandus in summer

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Abstract
We tested whether sunlight and insect harassment were important proximate factors (Zeitgebers) controlling feeding and lying patterns of wild reindeer (Rangifer tarandus tarandus) in summer. With no insect harassment, feeding and lying each exhibited a polycyclic rhythm with, on average, 4 bouts at equal intervals during each 24-h period (unit-sum constraints in a polycyclic rhythm). When insects were absent, the activity percentages were 50 ± 4% feeding (mean ± SE) and 33 ± 4% lying for daytime and 44 ± 3% feeding and 47 ± 4% lying for nighttime, i.e., the percentage of time spent feeding was equal for daytime and nighttime (nychthemeral), while for lying it was not. With insects present, percentages of time spent feeding and lying were 18 ± 3 and 13 ± 3% for daytime and 45 ± 8 and 41 ± 8% for nighttime, respectively, i.e., neither feeding nor lying exhibited a nychthemeral pattern. During periods of insect harassment, 24-h feeding patterns had no cyclic rhythm, whereas lying remained cyclic, but the percentages of time spent lying during peak bouts (amplitude) and at peak duration (period) were unequal between night and day. Reindeer did not attune their activity to the set points of sunrise and sunset (not crepuscular), demonstrating that daylight is most likely not a powerful Zeitgeber for reindeer during summer.