Bear in mind! Bear presence and individual experience with calf survival shape the selection of calving sites in a long-lived solitary ungulate

The careful selection of ungulate calving sites to improve offspring survival is vital in the face of predation. In general, there is limited knowledge to which degree predator presence and prey’s individual experience shape the selection of calving sites. Predator presence influences the spatiotemporal risk of encountering a predator, while individual experiences with previous predation events shape perceived mortality risks. We used a multi-year movement dataset of a long-lived female ungulate (moose, Alces alces, n=79) and associated calf survival to test how predator presence (i.e. encounter risk) and females’ individual experiences with previous calf mortality events affected their calving site selection and site fidelity. Using data from areas with and without Scandinavian brown bear (Ursus arctos) predation, we compared females’ calving site selection using individual-based analyses. Our findings suggest two things. First, bear presence influences calving site selection in this solitary living ungulate. Females in areas with bears were selected for higher shrub and tree cover and showed lower site fidelity than in the bear-free area. Second, the individual experience of calf loss changes females’ selection the following year. Females with lost calves had a lower site fidelity compared to females with surviving calves. Our findings suggest that increased vegetation cover may be important for reducing encounter risk in bear areas, possibly by improving calf concealment. Lower site fidelity might represent a strategy to make the placement of calving sites less predictable for predators. We suggest that bear presence shapes both habitat selection and calving site fidelity in a long-lived animal, whereas the effect of individual experience with previous calf loss varies. We encourage further research on the relevance of female experience on the success of expressed anti-predator strategies during calving periods.

Methods
Female moose were immobilized with a CO2-powered dart gun (DANiNJECT, Kolding, Denmark) from a helicopter with a mixture of etorphine-acepromazine-xylazine or etorphine-xylazine (Evans et al., 2012; Græsli et al., 2020; Kreeger & Arnemo, 2007; Lian et al., 2014). Each female was equipped with a neck collar with a global positioning system (GPS) device, including a very high frequency (VHF) transmitter, a global system for mobile (GSM) communication, an ambient temperature recorder, and an acceleration sensor to monitor their movement over time (Vectronic Aerospace GmbH, Berlin, Germany, 2022). Using the GSM network or satellite, the tracking device sends continuous positions to the existing database Wireless Remote Animal Monitoring (WRAM; Dettki et al., 2014), which allows us to monitor females remotely in near real-time. The GPS provided half-hourly locations of the moose females, which we resampled for the habitat selection analysis to four times a day (00.00h, 06.00h, 12.00h, and 18.00h). Since moose are mostly active during dusk and dawn and females with calves move small distances during this period of the year (Neumann et al., 2012), we deemed four samples a day sufficient to capture female positions during both resting and active times to describe their major habitat selection. Moose are long-lived, mostly solitary-living, capital breeders with a life span of more than 20 years (Ericsson & Wallin, 2001). Females usually calve every year but their fertility decreases after the age of 15 years (Niedzialkowska et al., 2022). In our study, female age averaged 8.3 years (range 4-16 years) at their time of capture as indicated by their tooth wear (Ericsson & Wallin, 2001). All personnel handling the moose were certified according to the standards of the Swedish Animal Welfare Agency and the Swedish Board of Agriculture. The marking of moose has been approved by the Animal Care Committee in Umeå, Sweden (DNR A116-09, A12-12, A50-12, A205-12, A14-15, A3-16, A28-17, A11-2020).

For each female, we linked movement data to her age and reproductive success in a given year (i.e. calf survival, calf loss due to predation or an unknown cause), resulting in a dataset including the calving date, the number of calves born, their summer survival, and the movement and age of the female.

To study the effect of the female experience on her selection for habitat structures at the calving site during the first four weeks following parturition in a given year, we built step selection functions using five random steps for each observed step. We extracted habitat features at the end of the step to test for the selection of a given habitat feature (R package ‘amt’, Signer et al., 2019).

To test for site fidelity of female moose in relation to the bear presence and females’ experience over the years, we modeled the observed inter-annual distances between calving site locations in successive years calculated as Euclidean distances (km) separately for each female. For each female, we calculated this distance as the distance between the daily average GPS collar locations in successive years, for each day, starting from the birth date (day 0) through the end of the first week after calving (day 7). This allowed us to compare female selection (i.e. derived by the coordinates) both at the date of calving and during the first week among successive years.