Plasticity and repeatability of activity patterns in free-living Arctic ground squirrels

Abstract

Social interactions can shape daily activity patterns, and this is an area of growing research interest. The propensity for individuals to be active at certain times of day may structure interactions with competitors and potential mates, influencing fitness outcomes. Aspects of daily activity such as timing of activity onset and offset exhibit within-individual repeatability, which raises the possibility that they may be subject to natural and sexual selection. This study used a biologging approach in free-living Arctic ground squirrels, Urocitellus parryii , to test the hypothesis that interactions between the sexes shape temporal patterns of daily activity. We predicted that males would become active earlier than females during the mating period, consistent with sexual selection on activity timing. We also examined repeatability in activity timing and how repeatability estimates are affected by sampling methodology. We deployed collar-mounted light loggers on ground squirrels and used timing of light transitions to determine daily onset and offset of above-ground activity. We observed 87 animals over 3 years, collecting over 7500 observations of daily activity. We found that daily activity timing had moderate (onset) to low (offset) adjusted repeatability. The difference in daily activity timing between the sexes varied seasonally: males remained active later than females during mating, while females initiated activity earlier and extended activity later during lactation. Adjusted repeatability estimates were sensitive to sampling methodology: they varied seasonally and declined with increased behavioural sampling. These results suggest that offset, not onset, may be shaped by sexual selection and that life history differences shape sex-dependent activity patterns in ground squirrels. We propose that activity onset may be under more strict circadian control than offset. This study suggests that researchers must consider seasonality and sampling design when estimating behavioural repeatability. Biologging could transform studies of individual variation by enabling high-frequency sampling of free-living animals. • Arctic ground squirrels showed evidence of circadian chronotypes. • Activity onset and offset differed by sex, especially during mating and lactation. • Activity onset reflected life-history energetics rather than sexual selection. • Repeatability estimates were affected by sampling frequency and design.