Nocturnal movements of desert bighorn sheep in the Muddy Mountains, Nevada

Adequate management of a species requires complete knowledge of its ecology, including both nocturnal and diurnal behavior. Knowledge of the movement behavior of bighorn sheep can provide insight for understanding spatial population processes as the combined result of individual behavior, physiological constraints, and fine-scale environmental influences. However, because of past difficulties in tracking radio-collared animals in the dark, little is known about nocturnal movements of many ungulates, including desert bighorn sheep (Ovis canadensis nelsoni). The use of GPS collars that supply sequential location records over a 24-hour period has provided new opportunities for recording movements of animals. We report findings from a preliminary analysis of data on nocturnal movements of desert bighorn sheep wearing GPS collars in the Muddy Mountains, Nevada. The study is part of a larger research project on movement patterns aimed at assessing the connectivity of bighorn sheep populations that comprise the metapopulation in southern Nevada. We recorded nighttime locations at 2100 and 0400 PST and calculated a total of 8,758 nocturnal movement distances for 12 ewes and 16 rams. Nocturnal movements were characteristic of all individuals. Bighorn sheep varied in the magnitude of movement from night to night and there were significant differences among individuals in the mean distance moved. Females had a greater proportion of short movement distances when compared to males; males had the greatest proportion of long distance movements. Movement distances were affected by sex, season, and percent of the moon’s surface illumination. Mean movement distances of males were greater than those of females. For both males and females, mean movement distances were shortest during the late spring months (April-June) and greatest during summer (July–September). Mean movement distances were greatest when the moon was brightest for all seasons except late spring, when movement distances for both sexes were shortest. We also describe movement distances >1 km (19% ewes, 81% rams) relative to movements within or between 50% core use areas, and nighttime intermountain and intra-mountain travel.