Polar bear captures were conducted via helicopter darting using a Bell-206 or Airbus AS350 over the sea ice of the Beaufort Sea (spring and late autumn) or on the nearshore coastal plain and barrier islands between Utqiagvik, Alaska, and the Alaska and Yukon Territory (Canada) border (summer and early autumn). Darts contained the immobilizing drug Telazol (Warner-Lamber Co., NJ, USA) and was administered at doses estimated at 4–10 mg per kg based on visual assessments of body mass of the target bear. The duration of helicopter operations was calculated as the time elapsed between the minute of the initial sighting of the bear, the minute the bear exhibited symptoms of ataxia, and the minute the bear was observed to lay down. Events were recorded on standardized paper data sheets and later entered into a computer database. Rectal temperature of captured bears was measured with a digital thermometer (Syrvet, Inc., Waukee, IA, USA), usually within 10 minutes after immobilization. Bears were weighed by suspending them in a net from an electronic load cell scale that was connected to an aluminum tripod.
A total of 14 bears were fitted with accelerometers, temperature loggers, or both. Accelerometers (Actiwatch; Mini-Mitter Respironics, Bend, OR, USA), attached to an Argos-linked GPS radio collar (Telonics, Mesa, AZ, USA), were sensitive to motion in all planes and recorded a unitless count every 2 minutes, reflecting acceleration intensity. Body temperature was recorded by loggers surgically implanted in the rump (peripheral temperature) or in the abdomen (abdominal temperature). Peripheral temperature was recorded with Tidbit V2 loggers (every 5 or 10 minutes, resolution 0.02°C; Onset Computer Corporation, MA, USA). Abdominal temperature was recorded with iButton loggers (DS1922L, hourly measurements, resolution 0.0625°C; Maxim Integrated, CA, USA). For peripheral temperature, the logger was sutured to the surface of the gluteus maximus muscle, beneath the subcutaneous adipose tissue, slightly ventral to the base of the tail. For abdominal temperature, the logger was sutured inside the abdominal cavity, adjacent to the peritoneum, on the ventral mid-line and slightly superior of the umbilicus. On subsequent recapture of instrumented bears, temperature loggers were surgically extracted and radio collars with their accelerometer were removed. Logger and accelerometer data were downloaded as computer files for each instrumented bear.
For ambient air temperature of capture events, we used estimates of 2-m above-ground air temperature from the European Centre for Medium-Range Weather Forecasts ERA5 global atmospheric reanalysis (Hersbach et al. 2020) using the Env-Data tool (Dodge et al. 2013) implemented through Movebank (
https://www.movebank.org ). We prescribed the Env-Data tool to interpolate temperature values at the location and time of each polar bear capture (scale of 0.25° × 0.25°) by applying spatial and temporal inverse-distance weighting to the surrounding hourly, 31-km resolution gridded ERA5 outputs.