Den shifting behaviour of female wolverines (Gulo gulo) in Northern Sweden

Abstract: Many species of mammals make use of “den sites” for multiple purposes. Wolverine (Gulo gulo) females give birth to and rear their cubs in den sites during the denning season (February to June, hereafter denning season). Previous studies have focused on studying the den site habitat, den site characteristics and impact of reproduction on the condition of female wolverines. My thesis focusses on how many den sites are used, for how long the den sites are used and why wolverines shift den sites, which has not received much attention yet. I used GPS-collar data collected during 11 years (2004-2014), from 18 adult wolverine females that were intensively followed during a total of 33 denning seasons in and around Sarek National Park in the north of Sweden. Then the use of den sites was defined during this period and a total of 271 den sites were found. On average a female used 12 den sites during one denning season. The natal den site (i.e. where the cubs were born) was on average used for 43 days, but the variation was high ranging from 10 to 83 days. I used a survival analysis approach to examine the influence of spatial and temporal factors on the probability that the female would shift den site during the denning season. The results show a difference in daily hazard rate (the probability to shift den site) between the two habitats; alpine and forest. In the alpine habitat, the den shift probability increased with increasing cub age and was higher for den sites located on reindeer calving grounds. In the forest habitat on the other hand, the probability of shifting a den site decreased by increasing female experience and increased with increasing day-time temperature. Snow cover did not influence the probability of females to shift den sites, as snow is a crucial factor for wolverine ecology, this result was unexpected. However, the influence of temperature in the forest habitat could be related to snowmelt and poorer snow conditions. Calving grounds are especially important from the beginning of May, when the reindeer arrive to the area and give birth. Additionally, higher human activity levels will occur during this time of the year as reindeer herders patrolling the area to guard the reindeer herds. Consequently, the increase of den site shifts occurring on calving grounds in comparison to outside these areas, may be explained by the potential increase of food availability and/or the increase of human activity (i.e. disturbance). Previous studies on other species have shown that the experience of female individuals can been influential on the reproduction. Due to the more unstable denning conditions in the forest, female wolverines might need to learn to adapt their denning behaviour while denning in the forest region. Further research is encouraged to focus on the impact of climate change, especially on snow conditions, and the denning behaviour of females on lower latitudes in the boreal forest.