Denning Phenology, Den Substrate, and Reproductive Success of Female Polar Bears (Ursus maritimus) in the Southern Beaufort Sea 1986-2013 and the Chukchi Sea 1987-1994

We used statistical process control methods to remotely identify maternal den entry, emergence, and duration using temperature sensor data from polar bear collars (as described in Olson et al. 2017). This process quantified the expected mean and variation in temperature for non-denning bears, and identified bears as denning when temperatures rose approximately 9°C above this expected range (referred to as a control limit; Olson et al. 2017) for a prolonged period. When compared to visual observations of denning, this temperature-based approach accurately classified denning and non-denning in 94.5% of 73 bears (Olson et al. 2017). Per the methods of Olson et al. (2017), we excluded potential denning events that lasted less than 34 days which may have eliminated some bears that entered dens, did not produce cubs, and abandoned the den early. We assumed that all denning events were likely maternal denning attempts because shelter denning (i.e., denning for reasons other than to give birth to cubs) is rare in the Chukchi and southern Beaufort seas (Olson et al. 2017). Den entrance was defined as the median between the last temperature observation within control limits and the first observation above control limits. Similarly, den emergence was estimated as the median between the last observation above control limits and the first observation to return within control limits. The total den duration was defined as the number of days between den entrance and emergence. This value most likely represented the time that a bear remained consistently within a den rather than the total amount of time a bear may have spent at the den location, but not in the den in preparation for or subsequent to denning. Because denning was defined as a consistent temperature above the expected temperature range of non-denning bears, den entry and exit estimates are affected by a bear moving in and out of a den for any reason. Dens were assigned as occurring on land, pack ice, or landfast ice using GPS and Argos locations as described in Olson et al. (2017). Location data were filtered as described in the Horizontal Positional Accuracy Report and if at least 1 observed location at the start of, during, or at the end of the denning period identified via temperature data occurred on land and locations preceding or subsequent to denning demonstrated a trajectory to or from that location, it was assumed the den occurred on land (Olson et al. 2017). Dens that did not occur on land were designated as pack ice or landfast ice dens.