| Summary: | The existence and persistence of rhythmicity in animal activity during phases of environmental change is of interest in ecology and chronobiology. A wide diversity of biological rhythms in response to exogenous conditions and internal stimuli have been uncovered, especially for polar vertebrates. However, empirical data supporting circadian organization of large ruminating herbivores remains inconclusive. Using year-round tracking data of the largest Arctic ruminant, the muskox ( Ovibos moschatus ), we modelled rhythmicity as a function of behaviour and environmental conditions. Behavioural states were classified based on patterns in hourly movements, and incorporated within a periodicity analyses framework. We found that ultradian rhythmicity was prevalent when muskoxen were foraging and resting in mid-winter (continuous darkness). However, the probability of rhythmicity declined with increasing photoperiod until largely disrupted in mid-summer (continuous light). Individuals that remained rhythmic during mid-summer foraged in areas with lower plant productivity (NDVI) than arrhythmic individuals. We conclude that muskoxen may use internal time keeping when forage resources are low, but that the importance of this mechanism weakens once environmental conditions allow energetic reserves to be replenished. We argue that alimentary function and metabolic requirements are critical determinants of biological rhythmicity in muskoxen, which likely applies to ruminating herbivores in general. See ReadMe file for details. Funding provided by: 15. Juni Fonden Funding provided by: Miljøstyrelsen Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100007036 Funding provided by: AUFF Starting Grant Crossref Funder Registry ID: Award Number: AUFF-F-2o16-FLS-8-16
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