Climate warming is rapid in the Arctic, yet impacts to biological systems are unclear because few long-term studies linking biophysiological processes with environmental conditions exist for this data-poor region. In our study spanning 25 years in the Alaskan Arctic, we demonstrate that climate change is affecting the timing of freeze-thaw cycles in the active layer of permafrost soils and altering the physiology of arctic ground squirrels (Urocitellus parryii). Soil freeze has been delayed and, in response, arctic ground squirrels have delayed when they up-regulate heat production during torpor to prevent freezing. Further, the termination of hibernation in spring has advanced 4 days per decade in females but not males. Continued warming and phenological shifts will alter hibernation energetics, change the seasonal availability of this important prey species, and potentially disrupt intraspecific interactions.

From 1996 to 2021, we deployed implantable abdominal temperature loggers and/or collar-mounted light and temperature loggers on free-living male and female arctic ground squirrels at two field sites (Atigun River Bridge 2 and Toolik Field Station) on the North Slope of Alaska, 254 km above the Arctic Circle. Detailed descriptions of field sites, devices, and device deployments are provided in the published literature (Sherriff et al., 2011; Williams et al., 2011, 2014, and 2016). Briefly, squirrels were captured using carrot-baited Tomahawk live-traps. From 1996–2016, squirrels were anesthetized and implanted with abdominal loggers using aseptic surgical technique. In more recent years (2014–2020), squirrels were fitted with collar-mounted light and temperature loggers. Devices were recovered using similar methods. Over this 25-year period, we collected biologging records representing 306 hibernation seasons from 199 adult individuals. We manually determined phenological events from body temperature data including the beginning and end of heterothermy, onset of sub-zero (thermogenic) torpor, and parturition. See list below for definitions used to score phenological events:

Beginning of heterothermy: First date arctic ground squirrel body temperature drops below 30° and remains so for at least 48h.

End of heterothermy: First date arctic ground squirrel body temperature rises above 30° and remains so for at least 48h.

Onset of sub-zero (thermogenic) torpor: First date arctic ground squirrel body temperature drops below 0°C in autumn. Note different logger models deployed over time had different degrees of accuracy and precision, so we used a threshold of -0.4°C +/- 0.2°C for scoring thermogenic torpor. This date was not scored for collar-mounted temperature loggers. 

Parturition: Date approximately 3.5 weeks post-emergence on which female body temperature exhibits a characteristic rapid increase of 1–1.5 °C indicating parturition.

From Fall 1993–Spring 2019, we also deployed soil temperature loggers mounted in sand-filled plastic pipes one meter below the surface near arctic ground squirrel burrow locations. Logger models changed over time as devices failed and were replaced. Deployment locations remained the same, although we do not have data from all locations across all years due to device failures. For each soil temperature logger, we determined the date on which soils froze and thawed and identified the minimum temperature soils achieved over winter. Definitions of soil freeze and thaw are provided below:

Soil freeze: Date on which soil temperatures first dropped below -1°C after a distinctive multiday period in which temperature plateaued at approximately 0°C.

Soil thaw: Date on which soil temperatures first rose above 1°C after a distinctive multiday period in which temperature plateaued at approximately 0°C.