Where clocks are redundant: weak circadian mechanisms in reindeer living under polar photic conditions |
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Authors: | Bob E H van Oort Nicholas J C Tyler Menno P Gerkema Lars Folkow Karl-Arne Stokkan |
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Institution: | 1.Department of Arctic Biology and Institute of Medical Biology,University of Troms?,Troms?,Norway;2.Department of Biology,University of Troms?,Troms?,Norway;3.Centre for Sámi Studies,University of Troms?,Troms?,Norway;4.Department of Chronobiology,University of Groningen,Haren,The Netherlands |
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Abstract: | Biological rhythms are a result of interplay between endogenous clocks and the ambient light–dark (LD) cycle. Biological timing
in resident polar organisms presents a conundrum because these experience distinct daily LD cycles for only a few weeks each
year. We measured locomotor activity in reindeer, Rangifer tarandus platyrhynchus (SR, n = 5 and 6) and R. tarandus tarandus (NR, n = 6), ranging freely at 78 and 70°N, respectively, continuously throughout 1 year using data loggers. NR, but not SR, are
gregarious which enabled us to examine the integrated effects of differences in social organisation and the photic environment
at two different latitudes on the organisation of activity. In both sub-species, ultradian bouts of activity and inactivity
alternated across the 24-h day throughout the year. This pattern was modified by the LD cycle in NR but barely at all in SR.
Periodogram analysis revealed significant ultradian rhythmicity in both sub-species; the frequency of daily cycles of activity
increased from three per day in winter to nearly five in summer. We conclude that this increase, and a concomitant increase
in the level of daily activity, reflected the seasonal increase in the animals’ appetite and the quality of their forage.
Secondly, the combination, most evident in SR, of a weak photic response, weak circadian mechanisms and a weak social synchronization
reduces the constraints of biological timing in an environment which is effectively non-rhythmic most of the year and permits
expression of the basic ultradian pattern of ruminant activity. Third, the weaker 24-h rhythmicity in SR compared to NR indicates
a latitudinal decrease in circadian organization and photic responsiveness in Rangifer. |
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Keywords: | Activity pattern Actogram Arctic Biological rhythms Data logger Rangifer Svalbard |
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