DOI: 10.5061/dryad.pzgmsbcqq
title: Climate change is altering the physiology and phenology of an arctic hibernator

description: 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.
description: 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.