GPS monitoring reveals circadian rhythmicity in free-grazing sheep

Abstract

GPS collars are a technology that is used extensively to monitor livestock due to its versatility. In this study, the main objective was to confirm whether they can detect the circadian rhythmicity that modulates the behavior of free-grazing sheep. The Churra-breed flock that was monitored grazed an approx. 166-ha fenced area within a dehesa ecosystem in the northwestern Iberian Peninsula. Geolocations were recorded every 30 min for two years. Animal activities were categorized based on the speed; an animal was “moving” if the speed was > 0 m/s (the analyzed category), and “resting” if the speed was 0 m/s. Sheep grazing activity in terms of their speed, azimuth, and distance traveled, was subjected to a circadian adjustment derived from the online Cosinor tool. Results reveal that the flock activity, whether based on speed, distance traveled, or azimuth, fit a circadian rhythmicity (p < 0.05). In the summer, particularly July and August, sheep exhibited a significant advance in the acrophase (the time at which the peak of a rhythm occurs), which might have been caused by day length and temperature. In all seasons, flock activity was significantly higher in the diurnal period, while the lowest activity was found in all cases at night, although in the summer sheep activity was high at dawn. In addition, in the day, sheep activity was significantly higher in the fall than it was at other times of the year. The preferred grazing direction of the sheep was non-random, since it was modulated by the contour orientation and the limits of the grazing area. It could be concluded that GPS geolocations allow to demonstrate that free-grazing sheep activity is modulated by a circadian rhythmicity.