Sleep Characteristics as a Comparative Indicator of Vigilance in Reindeer Populations (Rangifer tarandus)

During the daytime rest periods of reindeer, we recorded “looking around,” “waking,” “dozing,” and “sleeping.” Observations were made through a telescope from a distance sufficient to avoid disturbing the animals. We studied wild reindeer from the Snøhetta Mountains, Norway, and the Spitsbergen Archipelago populations, feral reindeer from the Forelhogna Mountains Norway, and the Wrangel Island populations and domestic reindeer from the Chukotka Peninsula. Reindeer from the Snøhetta Mountains, the Spitsbergen Archipelago, and the Forelhogna Mountains populations were studied in winter to early spring (during the snowy period). Reindeer from Wrangel Island and the Chukotka Peninsula were observed in June and July, during the snowless period. The duration of individual sleep episodes in the wild Snøhetta reindeer averaged about four minutes, compared to two to three minutes in domestic and feral reindeer. Highly reliable differences in sleep episodes were found between the Snøhetta Mountains reindeer and other populations, except for the Forelhogna reindeer, the differences in which, together with the Snøhetta Mountains reindeer, were only close to the reliable level (p = 0.076). The Wild Spitsbergen archipelago reindeer failed to differ from both the domestic Chukotka Peninsula reindeer and the feral Wrangel Island reindeer. For comparison, we used our data on the flight distance of reindeer when approached by humans in the same populations. Flight distances and the duration of sleep episodes in the Snøhetta Mountains reindeer differ from all other populations, and the characteristics of the wild Spitsbergen archipelago reindeer were close to those of the domestic Chukotka Peninsula reindeer. Domestic reindeer of the Chukotka Peninsula were studied in the summer, when reindeer gather in dense masses to rest (herd size more than 3000). Neighboring reindeer at rest often interfere with each other.