Telemetry reveals a stereotyped behavioral syndrome preceding hibernation onset in a wild rodent

Hibernation in mammals affects most living processes and structures the annual cycle, while transitions from euthermia to hypothermia require extensive physiological and behavioral adjustments. However, methodological constraints limit access to behavioral data during the final days before hibernation in free-living populations. Using telemetry, we examined activity and skin temperature patterns in yellow ground squirrels (Spermophilus fulvus) – desert-dwelling sciurids with prolonged hibernation – during the days immediately before and after immergence. We provide the first analysis of daily routine changes on the day of immergence in a mammalian hibernator. On this day, squirrels reduced aboveground time three- to fourfold, sometimes to only a few minutes, by retreating into their burrows unusually early. In contrast, they exhibited a 1.5-fold increase in motor activity, moving underground for several hours – a pattern absent on previous days. Intensive subterranean activity persisted during daytime for several days after immergence in all individuals. Behavior during hibernation onset was highly consistent across squirrels. Weather conditions, date, and individual age had little effect on immergence-day behavior, and individual behavioral characteristics also poorly predicted activity patterns on that day. We hypothesize that post-immergence underground activity may involve hibernaculum preparation. Our findings reveal a previously undescribed behavioral signature of hibernation onset, reflecting a possible trade-off between immediate energy costs and future survival benefits, and provide a framework for non-invasive monitoring of hibernation phenology in burrowing mammals.