| Summary: | The Svalbard rock ptarmigan (Lagopus muta hyperborea) is an arctic animal and lives as such in constant light and constant darkness for parts of the year. They are therefore an interesting subject for chronobiologists and have been thoroughly studied in terms of their biological rhythms. One rhythmic physiological factor, however, has received little attention in former studies: oscillation in core body temperatures (Tb). Keywords: Lagopus muta hyperborea, core body temperature, biological rhythms, locomotor activity, anticipation. In this experiment first-year birds were divided into two groups. While one group stayed permanently under a short photoperiod with 6 hours of light (SP), the other group went from SP to a long photoperiod with 16 hours of light (LP) and subsequently into constant light (LL). All birds under SP and LP showed clear cycles in Tb with high Tb in their active-phase during light phase and with decreased Tb during their rest-phase during dark. Furthermore, they showed rise in Tb in anticipation to light. Birds under LL, on the other hand, did not show convincing evidence for a sustained oscillation. Besides Tb, locomotor activity was recorded in male birds. Activity and Tb correlated in all studied bids under SP, LP and with one exception under LL. It was also shown that the anticipatory rise in Tb was, in some recorded cases, phase advanced to activity, indicating that the Tb cycle is an actively controlled mechanisms and not merely a side effect of activity cycles. Furthermore, it was shown that birds constantly held under SP showed a reduction in rest-phase Tb over 52 days. Increased heat loss during the rest-phase due to dropping ambient temperature and decreasing body mass are possible explanation for this observation.
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