Proximity-sensors on GPS collars reveal fine-scale predator-prey behavior during a predation event: A case study from Scandinavia

Although the advent of high-resolution GPS tracking technology has helped increase our understanding of individual and multi-species behavior in wildlife systems, detecting and recording direct interactions between free-ranging animals remains difficult. In 2023, we deployed GPS collars equipped with proximity sensors (GPS proximity collars) on brown bears (Ursus arctos) and moose (Alces alces) as part of a multi-species interaction study in central Sweden. On 6 June, 2023, a collar on an adult female moose and a collar on an adult male bear triggered on each other’s UHF signal and started collecting fine-scale GPS positioning data. The moose collar collected positions every 2 minutes for 89 minutes and the bear collar collected positions every 1 minute for 41 minutes. On 8 June, field personnel visited the site and found a female neonate moose carcass with clear indications of bear bite marks on the head and neck. During the predation event, the bear remained at the carcass while the moose moved back and forth, moving towards the carcass site about 5 times. The moose was observed via drone with 2 calves on 24 May and with only one remaining calf on 9 June. This case study describes, to the best of our knowledge, the first instance of a predation event between two free-ranging, wild species recorded by GPS proximity collars. Both collars successfully triggered and switched to finer-scaled GPS fix rates when the individuals were in close proximity producing detailed movement data for both predator and prey during and after a predation event. We suggest that, combined with standard field methodology, GPS proximity collars placed on free-ranging animals offer the ability for researchers to observe direct interactions between multiple individuals and species in the wild without the need for direct visual observation.

Methods
Bears and female moose were captured and collared via helicopter using established protocols (Kreeger and Arnemo 2007, Arnemo et al. 2012, Lian et al. 2014), which were approved by the Swedish Ethical Committee on Animal Research; Permits Dnr 5.8.18-03376/2020 and Dnr A11-2020. Moose capture efforts began in 2020, with the goal of collaring females near the 2018 burn and within the core study area (Fig. 1). Bear capture efforts began in 2022 and were focused on the area where moose had previously been collared to maximize temporal and spatial overlap between species and thus, the potential to observe interspecific interactions.

Captured bear and moose were equipped with GPS neck collars (Vectronic Aerospace, 2023). During the 2023 capture, a sub-sample of bears (n = 4; 2 adult males, 1 solitary female, and 1 female with cubs of the year) and moose (n = 18) were fit with GPS neck-collars that also had proximity sensors and UHF transmitters, i.e., GPS proximity-collars (Vectronic Aerospace GmbH, Berlin, Germany). Proximity collars are equipped with a UHF transmitter and receiver; the transmitter sends a weak UHF signal while the receiver scans for other UHF signals (see Table 1 for detailed settings). Once a signal was received by a collar, the collar reconfigured to a pre-determined fix schedule and logged the ID of the collar that it was triggered by. Once the signal was lost, the collar reverted to its original programming after a pre-scheduled amount of time. The range of UHF signal detection is based on terrain and cover but is usually about 100 m or so away. 

Bear proximity collars were programmed to take GPS positions every 30 minutes and increase to a fixed rate of 1 position every 1 minute for a duration of 15 minutes when they came within range of another UHF signal (Table 1). Moose proximity collars were programmed to take GPS positions every 30 minutes and increase to a fixed rate of 1 position every 2 minutes for a duration of 60 minutes when they came within range of a proximity-collared bear; the 2-minute setting was chosen to save battery life over the longer fix duration. Using the GSM-network or IRIDIUM satellite, the collars send continuously positions to the existing database Wireless Remote Animal Monitoring (Dettki et al. 2014) at the Swedish University of Agricultural Sciences, which allows us to monitor animals remotely in near real-time.