Wolverine denning behaviour and its implications for monitoring reproductive females

Knowledge about the number of reproductive females is important for monitoring population dynamics, and can be critical for managing human-wildlife conflicts. For wolverines (Gulo gulo), counts of reproductive females is the basis for estimates of population size in Scandinavia, as well as a key measure for compensation payments to Sámi reindeer-herders in Sweden. However, documenting wolverine reproductive events in the field is challenging and requires knowledge of female denning behaviour. Furthermore, females may shift den sites, presenting difficulties in determining whether two den sites belong to the same or two neighbouring females. In this study, we used data from 18 GPS-collared wolverine females, monitored intensively during the denning season (February 15 – May 31) in 2004–2014, to provide baseline information on denning behaviour. We documented reproductive events in 32 of 54 potential denning seasons and identified a total of 245 den sites. Females used 8.8±8.5 (mean±SD) den sites per denning season (median=6, range: 1–28). The number of den site shifts, the distance between subsequent den sites, and the time and distance females spent away from a den site increased during the denning season, while distances between neighbouring females' den sites remained constant. From late April, the distance between consecutive den sites used by the same female overlapped with distances between neighbouring females' den sites, resulting in increased uncertainty regarding whether two den sites belonged to one or two females. Using Bayesian modelling, we calculated probabilities that two den sites belonged to the same female or neighbouring females, conditional on the time of season and the distance between den sites. These findings will allow the monitoring program to adapt its methods for determining if multiple den sites belong to one or two reproductive events, using seasonally-dynamic threshold based on an understanding of wolverine denning behaviour.