Daily rhythms of a diurnal hibernating rodent: insights from a telemetry study

Most studies regarding mammalian circadian rhythms have focused on nocturnal rodents under laboratory conditions, with far fewer papers on diurnal species. Here, we analyzed daily activity patterns of free-living yellow ground squirrels (Spermophilus fulvus) using telemetry. We decomposed their motor activity (rest/motion) and aboveground activity (in a burrow/on the surface) by combining data from light sensors and accelerometers. We confirmed that S. fulvus remains immobile underground during sunrise and sunset light transitions and at night. It remains unclear which environmental cues they use to synchronize their biological clocks. All squirrels exhibited decreases in both motor and aboveground activity duration as the season progressed. Circadian activity patterns were similar between the sexes but varied with age. Juveniles displayed longer aboveground and motor activity than adults, likely owing to greater energetic needs. On cooler days, motor activity declined and aboveground presence shortened, whereas on very hot days activity was not suppressed. On rainy days, both the onset and offset of aboveground activity shifted toward the evening, and squirrels spent more time underground during the daytime. Weather conditions increased variability of the light–dark and rest–activity rhythms in S. fulvus, thereby potentially affecting the entrainment of its internal clocks. We propose that weather-driven changes in activity in these folivorous rodents are related to their limited ability to cache food, and that food caching by burrowing mammals may modulate the effects of weather on their daily rhythms.